STUDIES OF HYDRATE COHESION, ADHESION AND INTERFACIAL
PROPERTIES USING MICROMECHANICAL FORCE MEASUREMENTS
by Erika P. Brown c Copyright by Erika P. Brown, 2016
All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Chemical Engineering).
Golden, Colorado Date
Signed: Erika P. Brown
Signed: Dr. Carolyn A. Koh Thesis Advisor
Golden, Colorado Date
Signed: Dr. David Marr Professor and Head Department of Chemical and Biochemical Engineering
ii ABSTRACT
The oil and gas production industry continues to innovate in new exploration and pro- duction techniques that allow the extraction of energy resources from increasingly extreme conditions. One consequence of this advancement is the increasing threat of hydrate plugs forming in the oil and gas production lines due to favorable thermodynamic conditions for hydrate formation. Complete inhibition of hydrates, which is traditionally the preferred method in hydrate treatment, can become prohibitively expensive or challenging due to en- vironmental regulations. As such, the industry has observed a shift in focus from hydrate avoidance to hydrate management. In this strategy, hydrates are allowed to form and flow as a slurry. The goal in hydrate management is to prevent the aggregation and deposition of hydrates so that the flowline can produce without impediment. In order to accomplish this, a sound understanding of the cohesive (particle-particle) and adhesive (particle-surface) forces of hydrates particles is needed. This thesis seeks to advance the knowledge available on hydrate cohesion and adhesion, especially in the presence of surfactant additives. Several models involve the cohesive/adhesive force in their calculations; the Capillary Bridge Theory uses interfacial variables to predict the inter-particle force, while the Camargo and Palermo Model balances the cohesion force with shear forces to predict the extent of aggregation in a hydrate-bearing system. Each of these systems was analyzed for the sensitivity of each variable and how the inter-particle force affects each system. A method for measuring the contact angle of water on the hydrate surface was developed to study changes in hydrate wettability in systems both without and with surfactants present. This method was verified using two apparatuses and three operators, and it was shown that the contact angle measurement was repeatable. Using this method, the contact angle of a water droplet on a cyclopentane surface was found to be 94◦±5◦. This contact angle is less hydrophilic than previous estimates, and it represents an important update to prediction
iii efforts using Capillary Bridge Theory. Based on the updated estimate of the contact angle, the embracing angle for a pure system was estimated as α =4.9◦. Studies were also conducted to determine whether the Micromechanical Force (MMF) apparatus could be used to rank anti-agglomerants (AAs) by their performance. Four AAs were used in a blind test and were ranked based on the reduction in cohesion force measured. The final ranking determined agreed closely with the results provided by an industrial lab us- ing a typical macro-scale method. The MMF, which focuses on interfacial-scale interactions, is attractive for ranking measurements due to the speed, precision, visualization capabilities and small sample size needed. The visual nature of the MMF measurements also provided insight into the mechanisms of the AAs and morphological changes that resulted from AA addition. Changes in the wettability of the particles were proposed as a mechanism due to a strong correlation between contact angle and force measured in the presence of the AAs. In addition to particle-particle interactions, particle surface interactions were studied in the presence of AAs. It was found that AAs decreased the adhesion force between a stainless steel surface and a hydrate particle, but that the forces may increase if the surface was coated with a petroleum wax. Forces also increased with the addition of dissolved wax for a system with no AA as well as an oil-soluble AA. A water-soluble AA exhibited no changes with the addition of wax to the system. Changes in the hydrate shell micro-structure were studied by measuring the force neces- sary to puncture a hydrate particle using a glass cantilever. It was found that the addition of model surfactants caused the force needed to puncture the shell to decrease. The reduced shell strength was compared to other phenomena in the system such as interfacial tension, growth rate and cohesion force. Model surfactant studies were continued by comparing the force reduction for several chemicals that were added simultaneously to the system. Three classes of interactions were identified based on these measurements: synergistic, antagonistic and those showing no interaction.
iv The effect of subcooling on cohesion was investigated based on improved measurement and error calculation techniques. It was found that the cohesion force increases significantly near the equilibrium temperature, but tapers to a near-constant value at high subcoolings. This trend agrees with measurements made on ice systems, as well as trends observed in the water layer on ice particles. The temperature dependence was found to persist for annealing times up to two hours. Finally, design of a high pressure Micromechanical Force apparatus was performed, in- cluding the identification of nano-manipulators which make the cell design possible. This system was tested using 3D printing for low pressure experiments before the final design was produced. Initial tests indicated reproducible experiments using gas hydrates and showed that CO2 was not an appropriate hydrate former for this system.
v TABLE OF CONTENTS
ABSTRACT ...... iii
LISTOFFIGURES ...... x
LISTOFTABLES ...... xxi
LISTOFSYMBOLS...... xxii
LISTOFABBREVIATIONS ...... xxiv
ACKNOWLEDGMENTS ...... xxv
CHAPTER1 INTRODUCTION ...... 1
1.1 ClathrateHydrateBackground ...... 1
1.2 AgglomerationMechanisms ...... 6
1.3 History of the Micromechanical Force Apparatus ...... 13
1.4 HydrateInterfacialandGrowthStudies...... 16
1.5 Thesissummaryandorganization ...... 17
1.5.1 Publicationsarisingfromthiswork ...... 20
CHAPTER 2 APPARATUS AND PROCEDURE ...... 21
2.1 MicromechanicalForceApparatus ...... 21
2.1.1 Cohesion/AdhesionForce ...... 23
2.1.2 ShellStrength...... 25
2.1.3 ShellThickness ...... 26
2.1.4 GrowthRate ...... 28
2.1.5 ContactAngle...... 28
vi 2.2 High Pressure Micromechanical Force Apparatus ...... 30
2.3 InterfacialTension ...... 32
2.4 NoteonProcedures...... 35
CHAPTER 3 SENSITIVITY OF PARTICLE COHESION ...... 38
3.1 CapillaryBridgeTheory ...... 38
3.2 CamargoandPalermoModel ...... 47
3.3 Conclusions ...... 54
CHAPTER 4 CONTACT ANGLE MEASUREMENTS ON HYDRATE SURFACES . 56
4.1 PureHydrateMeasurements ...... 56
4.2 Predictions Using Capillary Bridge Theory ...... 59
4.3 ContactAngleChangeswithSurfactants ...... 61
4.4 Conclusions ...... 66
CHAPTER 5 INDUSTRY AA RANKING STUDY ...... 68
5.1 CohesionTestsandIFT ...... 68
5.2 MorphologicalObservations ...... 74
5.3 ContactAngleMeasurementsofIndustrialAAs ...... 82
5.4 Conclusions ...... 87
CHAPTER 6 ADHESION MEASUREMENTS WITH WAXES AND ANTI-AGGLOMERANTS ...... 90
6.1 AABaselines ...... 92
6.2 WaxBaselines...... 95
6.3 Wax/AAInteractions...... 100
6.4 CrudeOilAddition...... 105
vii 6.5 Conclusions ...... 107
CHAPTER7 SHELLSTRENGTH ...... 108
7.1 Introduction...... 109
7.2 Materials ...... 110
7.3 ResultsandDiscussion ...... 111
7.4 Conclusions ...... 119
CHAPTER 8 COMPETITIVE EFFECTS OF CHEMICALS ...... 120
8.1 Introduction...... 121
8.2 Materials ...... 122
8.3 SingleChemicals ...... 123
8.4 ChemicalMixtures ...... 126
8.5 Conclusions ...... 129
CHAPTER 9 FUNDAMENTAL COHESION STUDIES ...... 130
9.1 TemperatureDependence ...... 130
9.2 Annealingtime ...... 133
9.3 GlassBeads ...... 134
9.4 Conclusions ...... 137
CHAPTER 10 HIGH PRESSURE MICROMECHANICAL FORCE APPARATUS . 139
10.1 ApparatusDevelopment ...... 139
10.2InitialTesting...... 142
10.3Conclusions ...... 145
CHAPTER 11SUMMARY AND CONCLUSIONS ...... 147
CHAPTER 12SUGGESTIONS FOR FUTURE WORK ...... 152
viii 12.1 Expansionofsurfactantstudies ...... 152
12.2 Highpressureapparatus ...... 155
REFERENCESCITED ...... 157
ix LIST OF FIGURES
Figure 1.1 Cage structures for Structure I and II hydrates, and numbers of each cage necessary to form a unit cell (the smallest repeating unit) of each structure. Modified from Grim with permission...... 2
Figure 1.2 Map of hydrate resources discovered around the world. (BSR = Bottom Simulating Reflector). Reproduced from Makogon . Used with permission. . 3
Figure 1.3 Conceptual picture of hydrate formation in a flowline, highlighting the majorphenomenaleadingtoaplug...... 4
Figure 1.4 Trace of a flowline depicting the amount of THI necessary to inhibit the system to conditions outside of the hydrate formation region...... 5
Figure 1.5 Coalescence of liquid droplets (top) and aggregation of solid particles (middle) represent two possible mechanisms of agglomeration. In addition, liquids and solids may agglomerate together (bottom), resulting in wetted solids, depending on the affinity of the liquid for thesolid...... 7
Figure 1.6 Charged particles agglomerating (a) and remaining dispersed after charge stabilization (b) from Hsu et al. . Used with permission...... 8
Figure 1.7 Conceptual figure showing the physical representation of the variables from the Capillary Bridge Equation. Reproduced from Aman et al. with permissionfromthePCCPOwnerSocieties...... 11
Figure 1.8 Conceptual picture of hydrate formation on a water droplet...... 12
Figure 1.9 Image of THF hydrate particles held in contact for (A) 20 seconds, (B) 32 minutes, (C) 7 hours and (D) 11 hours showing the process of sintering. Reproduced from Taylor et al. . Used with permission...... 14
Figure 1.10 Change in the spread of cohesion force data with the instigation of a 10 second waiting period after each pull-off measurement. Reproduced from Aman.Usedwithpermission ...... 16
x Figure 1.11 Conceptual picture showing how the research performed in each chapter of this work fits into the overall theme of hydrate cohesive and adhesive forces. The top flowline represents a system without any additives and may have large agglomerates and deposits. The addition of AA (middle flowline) reduces the agglomeration and the deposition. If waxes are present in the system, deposition can dominate over agglomeration in the bulk (bottom flowline), resulting in large deposits but small agglomerate flowing...... 19
Figure 2.1 Calibration for Tungsten cantilevers used for indirect calibration of glass fibers, calibrated using the process documented by Taylor ...... 22
Figure 2.2 Image of the MMF apparatus showing the microscope with recording equipment, the jacketed aluminum cell, and manipulators holding cantilevers...... 23
Figure 2.3 Configuration of the aluminum cell for cohesion (left) and adhesion experiments (right). For adhesion, the top particle is replaced with a surface...... 24
Figure 2.4 Procedure for pull-off measurements. (1)Particles begin at rest, separated. (2)The top particle is brought into contact with the bottom particle at a known pre-load force. (3)After ten seconds, the top particle is pulled away at a constant velocity. (4)The distance at which the particles separate is used to calculate the force between the particles. ReproducedfromAman. Usedwithpermission...... 24
Figure 2.5 Cell configuration used for shell strength experiments. The top cantilever holds only a glass fiber perpendicular to the bottom hydrate particle. . . . 25
Figure 2.6 Depiction of the distance used to measure the force needed to puncture thehydrateshell...... 26
Figure 2.7 Method of measuring the thickness, d, of a hydrate particle after it has beenpuncturedbyaglasscantilever...... 27
Figure 2.8 Illustration of how different positions in the Z-direction could cause an overestimateofhydrateshellthickness...... 27
Figure 2.9 Progression of hydrate shell growth as it spreads along the water/cyclopentane interface. Reproduced from Brown et al. with permissionfromthePCCPOwnerSocieties...... 28
Figure 2.10 Water droplet on a hydrate surface for contact angle measurements. . . . . 29
xi Figure 2.11 Photo of the High Pressure MMF apparatus with the pressure vessel on theleftandsaturationvesselontheright...... 30
Figure 2.12 Polycarbonate window mounted on the lid of the pressure cell. Screws around the perimeter of the lid are used to secure the lid in place...... 32
Figure 2.13 Image of the KSV Cam 200 apparatus used to measure interfacial tension andcontactanglesonsurfaces...... 33
Figure 2.14 Example of an IFT experiment with surfactant. The IFT value begins at a higher value and then decreases until a steady-state value is reached. The volume of the droplet must be stable over the course of the experiment...... 35
Figure 2.15 Graph of a typical MMF experiment showing that no significant trend exists over the time time taken to perform 40 pull-off trials. Trend line providedtoguidetheeye...... 37
Figure 3.1 Relationship between radius of the particle and the cohesion force...... 41
Figure 3.2 Relationship between interfacial tension and cohesion force predicted by theCapillaryBridgeTheory...... 41
Figure 3.3 Prediction of force values for varying contact anglevalues...... 42
Figure 3.4 Dependence of the cohesion force on the contact angle broken down into separate terms based on the Capillary Bridge Equation...... 43
Figure 3.5 Dependence of the cohesion force on the contact angle when H=500 nm and α=0.01◦ (broken down into two portions) based on Capillary Bridge Theory...... 44
Figure 3.6 Force prediction based on varying liquid bridge height...... 44
Figure 3.7 Relationship between force and the height of the liquid bridge for a neutral wetting angle (θ=94◦)...... 45
Figure 3.8 Sensitivity of the cohesion force to α,the embracing angle on the particle. . 46
Figure 3.9 Separate contributions from each portion of the Capillary Bridge Theory for the dependence of the force on the embracing angle...... 46
xii Figure 3.10 Contributions from the first and second term of the Capillary Bridge Theory (for a neutral contact angle of water on hydrate of θ =94◦ and a liquid bridge height of H=500 nm) describing the dependence of the force ontheembracingangle...... 47
Figure 3.11 Dependence of the relative viscosity on the diameter of the particles in the Camargo and Palermo model. Volume fraction of hydrate in the system is held constant, resulting in a decrease in relative viscosity with increasingparticlediameter...... 50
Figure 3.12 Relationship between the viscosity of the oil phase and the relative viscosity predicted by the Camargo and Palermo model...... 51
Figure 3.13 Relative viscosity as a function of shear rate for three difference force values...... 52
Figure 3.14 Fractal dimension sensitivity in the Camargo and Palermomodel...... 53
Figure 3.15 Sensitivity of the Camargo and Palermo model for the relative viscosity of an oil phase with hydrate to the volume fraction of the hydrate...... 53
Figure 3.16 Dependence of the relative viscosity on the maximum packing fraction, φMax intheCamargoandPalermoModel...... 54
Figure 4.1 Schematic showing how contact angle measurements were performed in the MMF apparatus (right) and a typical measurement from the IFT (left)...... 57
Figure 4.2 Comparison of the contact angles measured in the IFT and MMF apparatuses on glass cover slides. Air and cyclopentane were used as bulk phases for these measurements. Averages represent 5+ water droplets (error bars are ± onestandarddeviation)...... 58
Figure 4.3 Depiction of how small variations in the hydrate shell caused by surface roughness can alter the measured value of the contact angle...... 58
Figure 4.4 Investigation of possible value combinations for α, the embracing angle, and H, the height of the liquid bridge, which predict cohesion force based on Capillary Bridge Theory. Measured force for this system was 4.2 mN/m...... 60
Figure 4.5 Hydrate particle with the embracing angle, α =4.9◦ (on each side) visualized...... 61
xiii Figure 4.6 Force value predictions for different values of H and α for a system containing PVCap. Measured force value was F=4.01 mN/m...... 63
Figure 4.7 Hydrate particle showing the extent of an embracing angle α =9◦ (on each side) based on the predictions from CBT. This would indicate that the liquid layer spreads out very significantly for a PVCap-containing system...... 64
Figure 4.8 Predictions using CBT for the force in a system containing Arquad. MeasuredforcewasF=3.63mN/m...... 65
Figure 4.9 Model predictions using CBT to predict the force between hydrates in a systems containing DDBSA. Measured force was F=2.39 mN/m...... 65
Figure 5.1 Morphological changes observed in cyclopentane hydrate when formed with 2 vol.% of the Flowloop AA dosed in the water phase prior to hydrateformation...... 69
Figure 5.2 Hydrophobic interaction between a water droplet and a hydrate particle, bothwith2vol.%FlowloopAA...... 70
Figure 5.3 Morphological changes induced in hydrates formed using 1 vol.% of the Flowloop AA dosed in the water phase prior to hydrate formation. ....71
Figure 5.4 Cohesion force for hydrate particles created with varying concentrations ofAAdosedinthewaterphase...... 72
Figure 5.5 Cohesion force measurements in the presence of AAs dosed at 0.1 vol.% of the oil phase. The Flowloop AA showed no measurable force...... 73
Figure 5.6 (Top) Comparison of changes in the cohesion force and interfacial tension values for the AAs used in the ranking study. The cohesion force for the Flowloop AA was zero. (Bottom) Correlation between measured IFT and forcevalues...... 75
Figure 5.7 Hydrate particles before (left) and after addition of the Flowloop AA (right). Particles have a slightly darker color, and small water droplets can be seen on the exterior of the particles (circled in red). Note: the dots in the image are constant, due to dust inside the camera used to recordtheimages...... 76
xiv Figure 5.8 Morphological changes due to the addition of AA1. The top left image shows two hydrate particles prior to AA addition. After AA1 was dosed to the cell, cyclopentane was drawn into the hydrate particles (top right). As the hydrate shell filled with cyclopentane, unconverted water was excluded to the exterior of the hydrate shell, where it remained without convertingtohydrate(bottomimage)...... 77
Figure 5.9 Hydrate cohesion test when AA1 was present. Water droplets adhere to the exterior of the hydrate particle but do not appear to augment the liquidbridge...... 78
Figure 5.10 Hydrate particle reaction to the addition of AA2. No morphological changes were seen between the pure (left) hydrates and the hydrates with AA(right)...... 79
Figure 5.11 Pure hydrates (left) and hydrates which have excluded water after the additionofAA3...... 79
Figure 5.12 Gel-like layer that was observed to form on the surface of some hydrates when AA3 was present. The dark area near the end of the red arrow is the edge of the hydrate, while the translucent area above the arrow showsthegel-likelayer...... 80
Figure 5.13 Hydrate particles affected by AA3. Intact hydrate particles (left) became brittle and crumbled due to a small pre-load force (right)...... 81
Figure 5.14 Pure hydrates (left) darkened in color upon AA4 addition and small amounts of hydrate sloughed off of the particles (right)...... 82
Figure 5.15 Contact angle measured as a function of AA concentration for the FlowloopAAdosedinthewaterphase...... 83
Figure 5.16 Images showing the measured contact angles for each of the AA systems inthisstudy...... 84
Figure 5.17 Relationship between contact angle and cohesion force. Trend line providedtoguidetheeye...... 85
Figure 5.18 Structure of a quaternary ammonium salt, where R1 − R4 represent alkyl orarylgroups,whichcanbethesameordifferent...... 85
xv Figure 5.19 Hypothetical mechanism by which AAs could alter the hydrophobicity of the hydrate shell. Quaternary ammonium compounds with several hydrate-philic tails could adsorb onto the hydrate surface or incorporate into the hydrate cage structure. The remaining long-chain hydrocarbon tails would extend out into the bulk oil phase, reducing the local affinity for water and decreasing the wettability of the hydrate shell...... 86
Figure 5.20 Two proposed mechanisms describing how AAs facilitate slurry flow. (Top) AAs reduce the average droplet size in an emulsion, resulting in small hydrates that have converted the majority of the water into hydrate. Due to the lack of free water and the AA effects, the hydrates flow as a slurry. (Bottom) Hydrates form around relatively large water droplets, creating hydrates shells that surround unconverted water. Due to the morphological changes caused by AAs, the hydrates break down and release the unconverted water; however, the water and hydrates tend not to interact because of the hydrophobic hydrate shells caused by AAs. . 88
Figure 6.1 Morphological changes caused by the addition of AA3 to a pure hydrate particle (left) including water exclusion (middle) and breaking of a brittle hydrateshell(right)...... 93
Figure 6.2 Forces between a stainless steel surface and a hydrate particle with and without AAs present in the oil phase. AA1 showed zero force...... 94
Figure 6.3 Experimental setup for experiments between a hydrate particle and a water droplet deposited on the steel surface...... 95
Figure 6.4 Interactions between a steel surface with a water droplet deposited onto it and a hydrate particle without and with AA1 in various phases. Zero forces were observed when AA1 was present in both the droplet and the hydrate...... 96
Figure 6.5 Image of steel surface coated in candelilla wax submerged in cyclopentane. The area on the right which is submerged in the cyclopentane has a lighter color from the wax on the handle, which remaineddry...... 97
Figure 6.6 Baseline experiments showing the repeatability of measurements on a candelilla wax coated surface. Trials 3 and 4 show smaller errors due to thecreationofalongercantilever...... 97
Figure 6.7 Adhesion force measured for different amounts of saturation time for a candelilla wax coated surface soaking in cyclopentane. Each data point represents a single experiment with 40 pull-off trials...... 98
xvi Figure 6.8 Design of a synthetic wax to match the composition of wax from a flowline (blue). The base for the synthetic wax was paraffin, shown in red, and lighter hydrocarbon components (C17-C20) were added to createthesyntheticwaxshowningreen...... 99
Figure 6.9 Baseline measurements for the custom wax taken without changing out the cyclopentane bulk, resulting in increasing adhesion forces with increasingwaxconcentrations...... 99
Figure 6.10 Baseline measurements for the custom wax with new cyclopentane used for each trial to avoid wax buildup in the system...... 100
Figure 6.11 Dependence of the adhesion force on the amount of the custom-made petroleum wax dissolved in the cyclopentane bulk phase for systems containing either no AA, AA2 or AA3 (top). The bottom figure shows the relative change, using the force value with no added wax as a baseline. *Force tests not performed for AA3 at 1.3 wt. % wax. All measurements performed using a wax-coated surface...... 101
Figure 6.12 Comparison of adhesion forces between stainless steel surfaces (left-most data) and wax coated surfaces (right three sets of data) with and without AAs...... 102
Figure 6.13 Hypothesis for the increase in forces observed in systems containing waxes. Surfaces with dissimilar wettability tend to interact with higher forces than surfaces where there is a large difference in wettability. . . . . 103
Figure 6.14 Image of a hydrate particle formed with wax and AA3, where a portion of the hydrate broke off, exposing the interior water. Due to the presence of wax in the system, rather than causing a total failure of the particle, the shell was stable enough to continue the experiment...... 105
Figure 6.15 Hydrate particle and wax-coated surface in a 75/25 vol.% mixture of cyclopentaneandCrudeOilS...... 106
Figure 6.16 Adhesive forces between a wax-coated surface and a hydrate particle in thepresenceofvariousadditives...... 107
Figure 7.1 Chemical structures of DDBSA (top) and Tween 80 (bottom)...... 111
Figure 7.2 Hydrate particle before (left) and after puncturing (right). This illustrates an extreme example of the possible effects of lowering shell strength...... 112
xvii Figure 7.3 Force required to puncture the hydrate shell at three different temperatures for annealing times up to 90 minutes. Error bars represent the standard deviation of 4+ repeat measurements...... 113
Figure 7.4 Force required to puncture the hydrate shell with and without surfactants. All experiments were performed at 0.3◦C. Error bars are one standard deviation of 4+ repeat experiments...... 113
Figure 7.5 Growth rate of the hydrate shell with and without surfactants present (top) and typical distribution of values for growth rate measurements (bottom). Results shown in lower figure represent the values obtained for purehydrate...... 115
Figure 7.6 Thickness of the hydrate shell for surfactant and non-surfactant systems. Each data point represents a single experiment...... 116
Figure 7.7 Cohesive forces between two hydrate particles. Error bars are 95% confidence intervals based on 160+ measurements using more than 4 separateparticlepairs...... 116
Figure 7.8 Interfacial Tension measurements on systems with and without surfactant...... 117
Figure 7.9 Visual comparison of macroscopic hydrate morphology without and with surfactants present during growth, illustrating no significant morphologicalchangesforthesesystems...... 118
Figure 7.10 Conceptual picture showing how surfactants may cause steric hindrance and hence shell weakening during hydrate shell formation...... 119
Figure 8.1 Structures of the additives used in this work: Dodecyl Benzene Sulfonic Acid (top left), Arquad (bottom left) where n=13-15, and a monomer of PolyVinylCaprolactam(right)...... 122
Figure 8.2 Cohesion force and interfacial tension measurements for pure chemical tests. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments...... 124
Figure 8.3 Interfacial tension as a function of time for the three pure chemical species tested in a mineral oil bulk phase. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments. Pure mineral oil IFT baseline is 52mN/m...... 125
xviii Figure 8.4 Hydrate shell morphology when no additives are present (left) and when Arquad is present (right). Particles with DDBSA or PVCap (and no Arquad) appear similar to the particles on the left...... 126
Figure 8.5 Cumulative results for cohesion force and IFT measurements for pure and mixed chemical systems. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments. . . 127
Figure 8.6 Interfacial tension as a function of time for pure Arquad as well as mixtures containing Arquad. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments. PuremineraloilIFTbaselineis52mN/m...... 128
Figure 9.1 Temperature dependence of cyclopentane hydrate measured by Dieker and the predicted temperature dependence using the Camargo and Palermomodel...... 131
Figure 9.2 Cohesion force of ice crystals as a function of subcooling. Modified from Hosler et al. c American Meteorological Society. Used with permission. . 131
Figure 9.3 Temperature dependence of cyclopentane hydrate measured using improved techniques to increase accuracy and repeatability...... 132
Figure 9.4 Effect of annealing time on hydrate cohesive forces for three different subcoolings...... 133
Figure 9.5 Glass beads affixed to cantilevers. Glass bead diameter ranged from 100-200 µm...... 135
Figure 9.6 Inter-particle cohesion forces between glass beads at different vol% of cyclopentane and 200T mineral oil where each point represents the average of 40 pull-off measurements on 5 replicate particle pairs...... 136
Figure 9.7 Inter-particle cohesion forces between water-wetted glass beads with MEG addition. The line represents the dry bead cohesion force...... 137
Figure 10.1 Nano-manipulator used in HPMMF experiments...... 141
Figure 10.2 First-generation design for the high pressure apparatus. The left section housed the nano-manipulators, while the right section was used to visualizeexperiments...... 141
xix Figure 10.3 CAD design for another version of the pressure cell. The nano-manipulators would be attached to the lid in the large section on the right. The smaller section on the left was the visualization section usedtoobservetheparticles...... 142
Figure 10.4 Current high pressure cell (Sejin Co.). The window on the left is used to visualize the experiments, and the two large holes in the center are used for the electrical feed-throughs for the nano-manipulators (top). Interior of the cell, showing the nano-manipulator holding a cantilever (bottom). . 143
Figure 10.5 Cohesion force data for CO2 gas hydrates showing experimental repeatability for different operators. Previous data from Lee , new data byE.Brown...... 144
Figure 10.6 Corrosion on the nano-manipulator most likely caused by CO2 as the hydrateformer...... 145
xx LIST OF TABLES
Table 2.1 Composition of Mineral Oil 70T. Analysis performed by Weatherford Labs. 34
Table 3.1 Values used for variable parameters to perform a sensitivity analysis on theCapillaryBridgeEquation...... 40
Table 3.2 Base values used in the sensitivity analysis of the Camargo and Palermo model...... 48
Table 4.1 Measured and predicted values for cohesion force in the presence of surfactants. Forces predicted using the Capillary Bridge Theory and values measured for each system (contact angle and interfacial tension); the height of the liquid bridge and the embracing angle were assumed constant (H = 50nm and α =4.9◦,respectively)...... 62
Table 5.1 Phase solubility of each AA tested in the ranking study, determined throughdispersiontests...... 72
Table 5.2 Measured values for the contact angles for each AA ranked. Values of 180◦ indicated that the droplet would not interact with the surface, and no variation was observed over multiple measurements, leading to zero errorsforthesevalues...... 83
Table 6.1 Experimental matrix of the study of the interaction between waxes and AAs...... 91
Table 6.2 Summary of AAs used in this study. Dose is relative to the oil phase. AA3 was dosed at lower concentration due to morphological changes. . . . 92
Table 6.3 Cohesion force values measured for each AA used in this study and reductions from the force between hydrate particles with no AA present. . 93
Table 6.4 Composition of candelilla wax from Kuznesof ...... 96
xxi LIST OF SYMBOLS
General Nomenclature
D ...... CharacteristicLengthscale d ...... ImmersionDepth dA ...... AggragateDiameter dF ...... FiberDiameter dp ...... ParticleDiameter
E ...... ElasticModulus
F ...... Force(CohesionorAdhesion)
f ...... FractalDimension
H ...... HeightoftheLiquidBridge
k ...... SpringConstantofGlass Fiber
L ...... LengthoftheGlass Fiber
R∗ ...... NormalizedRadius
R1 ...... RadiusofCurvatureoftheIFTDroplet
R2 ...... RadiusofCurvaturePerpendicular to R1
RBottom ...... RadiusoftheBottomParticle
RTop ...... RadiusoftheTopParticle
Re ...... ReynoldsNumber
Teqm ...... HydrateEquilibriumTemperature
Tsys ...... SystemTemperature
xxii V ...... VolumeoftheLiquidBridge v ...... FluidVelocity
Greek Letters
α ...... EmbracingAngle
γ ...... InterfacialTension
∆D ...... SeparationDistance
∆P ...... PressureDifferenceAcrosstheLiquid Film
∆Tsub ...... Subcooling
θ ...... ContactAngle
µ ...... FluidViscosity
µ0 ...... ViscosityoftheContinuous Phase
µR ...... RelativeViscosity
ρ ...... Density
φ ...... VolumeFractionofHydrate
φeff ...... EffectiveVolumeFraction
φMax ...... MaximumPackingFraction
xxiii LIST OF ABBREVIATIONS
Anti-Agglomerant ...... AA
CapillaryBridgeTheory ...... CBT
DodecylBenzeneSulfonicAcid ...... DDBSA
FocusedBeamReflectanceMeasurement ...... FBRM
HighPressureMicromechanicalForce...... HPMMF
InterfacialTension(Tensiometer) ...... IFT
KineticHydrateInhibitor ...... KHI
LowDosageHydrateInhibitor...... LDHI
MicromechanicalForce...... MMF
Monoethyleneglycol ...... MEG
ParticleVideoMicroscopy ...... PVM
PolyVinylCaprolactam ...... PVCap
ThermodynamicHydrateInhibitor ...... THI
xxiv ACKNOWLEDGMENTS
There are many people without whom this thesis would not have been possible. Therefore, I would like to first extend my gratitude to my advisor, Dr. Carolyn Koh, not only for her kind words and steadfast support but for her patience and generosity during my hiatus from the group. She has managed to balance guidance and priorities with freedom in research by allowing students to explore their own ideas while also contributing research where it was needed. I would also like to thank Dr. E. Dendy Sloan for his advice and guidance from my time as an undergraduate through my graduate experience. My thesis committee, Profs. Tracy Gardner, David Wu and Luis Zerpa, have my thanks as well for their input, encouragement and expertise during the course of this thesis work. I have had the privilege to work with a large number of Hydratebusters during my tenure, and would like to extend my appreciation to all of the Busters, past and present for their support, company, and critical discussions that have led to great improvement of this work. In particular I appreciate, Chris Cabusao, Mike Jones, Gary Grim, Giovanni Grasso and Zach Ward for helping me to build a strong foundation in the group; Zach Aman for his MMF expertise and compelling discussion; Ahmad Majid, Jose Dapena, Sijia Hu, and Davi Salmin for their discussions and enjoyable company. I would also like to thank the doctors and nursing staff at Presbyterian St. Luke’s Medical Center, and in particular Dr. Richard Nash for their attention and dedication in care during my battle with leukemia. Though I don’t know her, I would also like to thank my bone marrow donor. Without her selfless donation, I would not be here to write this work. Finally, I would like to thank my family for their reliability, love and encouragement. To my mother, Bonnie Hansen, thank you for taking your time to help me improve this work, in addition to so much support and kindnesses you’ve given me through the years. For carrying me when I was weak (sometimes literally), for believing in me when I didn’t believe
xxv in myself, for his motivation, positivity and compassion, I would like to thank my husband, Logan Brown.
xxvi CHAPTER 1 INTRODUCTION
This chapter provides a brief background on clathrate hydrates, followed by a review of agglomeration and cohesion force mechanisms that apply to colloidal systems as well as hydrate-bearing systems. Portions of Chapter 1 will be submitted for publication in a peer-reviewed journal (PCCP: Physical Chemistry Chemical Physics) as a review article (invited). These parts of the chapter will be reproduced with minor changes.
Erika P Brown1, Carolyn A. Koh2
1Primary author
2Professor, Co-author and thesis advisor
1.1 Clathrate Hydrate Background
When water and appropriate guest molecules exist together at thermodynamically favor- able conditions, clathrate hydrates may stochastically nucleate and grow. Guest molecules are typically gas molecules and/or light hydrocarbons that are the correct size to stabilize the crystalline structure of the hydrate cages (e.g., methane, propane, cyclopentane) [1]. The structure of the hydrate phase formed will depend on which size of cage it stabilizes. Small molecules such as methane stabilize Structure I, while larger molecules such as propane and cyclopentane stabilize Structure II. These two structures are the most commonly formed, with many naturally occurring hydrates, such as those on or under the seafloor, comprised of Structure I, while hydrates that occur in oil and gas flowlines are usually Structure II. Other structures can also be formed using more specialized guest molecules (e.g. adaman- tane, methylcyclohexane). The cage structures and unit cells of each hydrate structure type are shown in Figure 1.1.
1 Figure 1.1: Cage structures for Structure I and II hydrates, and numbers of each cage necessary to form a unit cell (the smallest repeating unit) of each structure. Modified from Grim [2] with permission.
Naturally occurring hydrates represent a unique challenge that has great potential as an energy resource, as well as presents a great threat due to global warming and geological stability concerns. Hydrates typically form at high pressures and low temperatures, making subsea and permafrost environments ideal for formation. This has led to the formation of large naturally occurring deposits of methane hydrate around the world, representing a huge amount of natural gas (1016 m3) [3, 4]. The map in Figure 1.2 shows the locations of discovered natural hydrate deposits that could someday be tapped into as an energy resource. Because oil and gas production flowlines are increasingly moving toward more extreme environments, such as deep sea and permafrost areas, the temperatures and pressures will be well within the hydrate thermodynamic stability conditions. While natural hydrates contain mostly methane and form Structure I, oil flowlines frequently contain heavier components that can easily form Structure II hydrates. These hydrates are detrimental to production and pose a significant safety hazard. Hydrates that form in flowlines can agglomerate, bed and deposit, leading to increased pressure drop, viscosification of the fluid phases, and eventual blockage of the flowline. These hydrate plugs cause costly shutdowns and can become projectiles within the flowline if they are not remediated correctly.
2 Figure 1.2: Map of hydrate resources discovered around the world. (BSR = Bottom Simu- lating Reflector). Reproduced from Makogon [3]. Used with permission.
Hydrate formation in flowlines is a complex phenomenon that depends on thermody- namics, multi-phase flow, kinetics, and surface chemistry, among many others. Figure 1.3 shows a conceptual picture breaking the process of hydrate formation into smaller portions that can be more easily studied. First, the phases present mix together, creating emulsions or dispersions as well as dissolving gas into the liquid phases. Water entrainment into the oil phase may not be complete, leaving a free water phase present in the flowline. Hydrate nucleation and growth is most likely to occur near the water/oil or water/gas interface due to the higher concentrations of the hydrate formers at the interfaces. Because nucleation is stochastic, some water droplets may convert into hydrate while others remain liquid. Due to the turbulent nature of flowlines, the particles and water droplets collide and agglomerate, forming larger and larger hydrate aggregates. These aggregates can deposit onto the pipe wall, reducing the available area for flow within the flowline. The aggregates can also interact with existing deposits, resulting in plugging of the flowline. Hydrates in flow assurance have become the focus of significant study, and numerous methods for avoiding or controlling hydrates have been proposed and enacted. These solu- tions range in cost and effort, and include thermal methods, such as insulating the flowline so that it remains outside the hydrate formation region; hydraulic methods such as de-
3 Figure 1.3: Conceptual picture of hydrate formation in a flowline, highlighting the major phenomena leading to a plug. pressurization; process solutions like water cut reduction; and chemical methods such as Thermodynamic Hydrate Inhibition (THI) or Low Dosage Hydrate Inhibition (LDHI) [5]. Chemical methods are very common, especially in fields where thermal or process solutions are unfeasible, such as with rising water cut over the life of the field. Some fields operate outside of the hydrate range when they are new, but as the field matures, changing oil com- position and increasing amounts of produced water can lead to hydrate challenges later in the life of the field. In these cases, chemical inhibitors are often selected. The most com- mon methods of chemical treatments are THIs, such as methanol and mono-ethylene glycol. When added to a flowline, they shift the hydrate boundary, making it thermodynamically unfavorable for hydrates to form. These chemicals are dosed based on the water produced and can become very expensive for high water cut fields, with dosing rates that can exceed 50%. In addition, these chemicals are often toxic and environmentally unsafe. Figure 1.4 shows the trace of a flowline, which changes temperature and pressure with distance from the wellhead. The gray region shows the thermodynamically stable conditions for hydrate formation, as well as the amount of methanol needed to inhibit the system. LDHIs are another class of chemicals that are gaining more attention as they are typically dosed below 2 vol%. Two major classes of LDHIs are Kinetic Hydrate Inhibitors (KHIs) and anti-agglomerants (AAs). KHIs are typically water-soluble polymers which delay the nucleation and growth of hydrates [6]. These compounds allow hydrate formation to be delayed and may postpone hydrates for a long enough period to allow the fluid in the flowline
4 Figure 1.4: Trace of a flowline depicting the amount of THI necessary to inhibit the system to conditions outside of the hydrate formation region [1].
to leave a thermodynamically stable hydrate region. The driving force for hydrate formation is described in Equation 1.1.
∆Tsub = Teqm − Tsys (1.1)
Where ∆T sub is the subcooling, T eqm is the hydrate equilibrium temperature at a given pressure and Tsys is the system temperature. KHIs tend to work best at moderate to low subcoolings where the driving force is low. AAs function by allowing hydrates to form but reducing the forces between the hydrate particles so that they do not agglomerate. Solid hydrate particles suspended in the fluid bulk are then able to be transported as a slurry, which can result in a higher pressure drop but is clearly preferable to hydrate blockages. AAs have significant potential because they can be dosed at low rates (˜1 vol%), can operate with significant subcoolings, and can potentially be designed to be environmentally friendly. The purpose of this thesis work is to examine the agglomeration behavior of hydrate particles using a Micromechanical Force (MMF) apparatus for various physical variables as
5 well as in the presence of chemicals. The ability to understand and predict the agglomeration behavior of hydrates allows the correct hydrate mitigation strategies to be selected, as well as the development of more suitable AAs. Adhesion investigations (i.e. those between a particle and a pipe surface) can give insight into deposition phenomena and advancement of pipe coating technology.
1.2 Agglomeration Mechanisms
Agglomeration is the process by which the total number of particles in a system is de- creased but mass is conserved because individual particles are coalescing into larger particles. In the case of fluids, droplets can coalesce to form a larger droplet, while solids can form an aggregate as illustrated in Figure 1.5. In addition, solids and liquids that coexist in the same system can agglomerate. The wetting of the solid particles may differ in these cases based on the affinity of the liquid for the solid surface. Depending on the shape of the particles and the system conditions, fractal structures may be created as particles and aggregates interact [7]. There are multiple mechanisms by which particles can encounter one another in order to agglomerate; these are summarized below. Brownian When particles are suspended in a fluid medium, stochastic motion due to molecular collisions occurs. This process creates Brownian motion, whereby particles can encounter one another by chance movements of each particle. Despite its stochastic nature, Brownian motion is well-characterized mathematically [8, 9] and can be dependent on the concentration of the dispersed particles in the continuous phase [10]. Gravitational Aggregation driven by gravity occurs due to the differences in terminal velocity due to size. Larger particles tend to settle more quickly than smaller ones, and as they settle, the large particles can capture smaller particles that might otherwise stay sus- pended [11]. Often, this agglomeration mechanism is paired with another, such as Brownian, where particles grow by Brownian agglomeration, reaching a critical size before gravitational agglomeration becomes dominant [12]. Gravitational agglomeration is more dominant at larger particle sizes, generally greater than the micron size [13]. Gravitational agglomera-
6 Figure 1.5: Coalescence of liquid droplets (top) and aggregation of solid particles (middle) represent two possible mechanisms of agglomeration. In addition, liquids and solids may agglomerate together (bottom), resulting in wetted solids, depending on the affinity of the liquid for the solid.
tion may be a dominant mechanism in hydrate bedding phenomena, where particles grow by turbulent aggregation before settling to the bottom of a flowline [14]. Electrostatic Charged particles are an important topic in colloids. Electrostatic inter- actions can be used to stabilize colloidal solutions for longer periods of time. By giving the particles all the same charge (either positive or negative), the particles will repel one another rather than cohering, reducing agglomeration and extending the stable life of the solution as depicted in Figure 1.6 [15]. When short-range forces such as van der Waals forces dominate attraction, electrostatic repulsion can easily overcome attractive forces. Conversely, inducing a dipole in otherwise uncharged particles is a popular method of causing dispersed particle to begin to agglomerate [16–18]. Turbulent In flowing systems with a high enough Reynolds number, turbulent agglom- eration is likely to be a dominant mechanism of aggregation. The Reynolds’s number is defined in Equation 1.2 and is a balance between inertial and viscous forces [19].
Re = ρvD/µ (1.2)
7 Figure 1.6: Charged particles agglomerating (a) and remaining dispersed after charge stabi- lization (b) from Hsu et al. [15]. Used with permission.
Where ρ is the density of the fluid, v is the velocity, D is a characteristic length, often the diameter of the pipeline, and µ is the viscosity of the fluid. For low values of the Reynolds number, flow is laminar and is characterized by a smooth motion. However, for high Reynolds numbers, the flow becomes turbulent, which creates chaotic motion including eddies and vortexes within the flow. These random currents can carry particles, which can then collide and agglomerate. Turbulent agglomeration is a complex system to model due to the fluid dynamics, as well as the competing effects on particle size due to agglomeration and shear. Shear and collisions with walls can cause agglomerated particles to re-separate, causing a dynamic balance in the particle size distribution [20]. Multiple modeling efforts have been proposed but can vary based on the type of particles being modeled. For example, different models have been proposed for systems containing hydrates [21, 22], asphaltenes [23], or micro-particles [24]. In the case of hydrates, many measurements have been made in order to better under- stand how agglomeration occurs under turbulent flow. A hydrate-bearing system typically begins as an emulsion, with water droplets dispersed into an oil phase (although in some cases the water droplets may not be full dispersed in the oil phase). Nucleation is a stochastic process; therefore, these water droplets will nucleate at different times, leading to a system where both liquid water droplets and solid hydrate particles coexist. Thus, both coalescence and aggregation can occur as hydrates collide with other hydrate particles as well as wa- ter/hydrate and water/water collisions. Focused Beam Reflective measurements (FBRM)
8 and Particle Video Microscopy (PVM) imaging techniques have been used to measure the droplet size distribution in a turbulent emulsion [25]; however, these measurements may suffer in accuracy due to the reflectiveness of water droplets and typically underestimate the droplet sizes for emulsions [26]. Despite the shortcomings of PVM and FBRM, they are advantageous in that they can be used in situ at high pressure and low temperature condi- tions to directly measure hydrate particle sizes, for which they have higher accuracy. Other optical techniques such as image analyses have shown improved droplet sizing accuracy, but are slower [27, 28]. Results of some reflected light measurements have been more successful for solid particles such as glass beads, ice particles or hydrate particles, where the evolution of agglomeration has been measured [29, 30]. In the case of a cessation of flow, particles suspended by turbulent flow will often settle due to gravitational effects. This depends on the stability of the system and the degree of agglomeration that occurs between smaller particles under turbulent conditions. Once these particles have settled, re-suspension can be highly complicated [31], with re-suspension depending on the adhesion forces to the walls as well as the cohesion to other settled particles, also taking the fluid dynamic properties into consideration. Cohesion Each of the above agglomeration mechanisms relies on the existence of forces between the particles that cause them to adhere together. There are many types of forces that can cause particles to cohere, depending on the type of particle and the length scale in question. For the purpose of this work, cohesion is referred to when two particles in- teract, while adhesion refers to the interaction between a particle and a surface. Colloids are small particles that typically range from 1-1000 nm in diameter [32], and a number of forces can cause them to cohere. At the smallest end of the length scale, macromoleculu- lar colloids, hydrogen bonding or van der Waals forces can cause particles to stick to one another. Because hydrates are usually formed from emulsions where droplet sizes exceed 1 µm [30], the aforementioned forces are unlikely to explain the magnitude of the forces seen in hydrate systems. Solid-solid cohesion is a force that functions with soft spheres deform-
9 ing upon contact and may play a part in the agglomeration of hydrate particles; however, hydrate cohesion has been shown to have temperature-dependent behavior [33, 34], which solid-solid cohesion cannot account for [35]. Solid-solid adhesion may pay a larger role at significant subcoolings or long annealing times (especially when the particle size is small), as the temperature-dependent effects have been observed to diminish under these conditions; however, the magnitude of the cohesion forces measured indicate that solid-solid cohesion is unlikely to be the sole mechanism even under high subcooling/long annealing conditions. It is therefore most likely that capillary bridging is responsible for the forces observed in hydrate systems. Capillary attraction is caused by liquid bridges that connect two or more particles. The liquid bridge must be immiscible with the bulk phase, and the forces generated can be described by the Capillary Bridge Equation (1.3).
F 2πγ cos θ ∗ = H +2πγ sin(α)sin(θ + α) (1.3) R 1+ 2d
Where F is the force between the particles, R∗ is the normalized radius of the particles described in Equation 1.4, γ is the interfacial tension between the bridge and the bulk fluid, θ is the contact angle of the bridge fluid on the surface of the particle, H is the height of the liquid bridge, d is immersion depth, and α is the embracing angle.
1 1 1 1 ∗ = + (1.4) R 2 RTop RBottom ! The parameters described in Equation 1.3 can be visualized on particles, or between a particle and a surface as shown in Figure 1.7. A larger volume of liquid in the capillary bridge will increase the force between the particles, up to a certain point. It is therefore important to consider the origin of the liquid bridge on the hydrate particles. There are several possibilities, and it is likely that water layers are caused by a combination of mechanisms. Because hydrate cohesion shows
10 Figure 1.7: Conceptual figure showing the physical representation of the variables from the Capillary Bridge Equation. Reproduced from Aman et al. [34] with permission from the PCCP Owner Societies.
11 a dependence on temperature, it is possible that some of the liquid on the surface is caused by surface melting. Alternately, a thermodynamic water layer could be created in order to minimize the interfacial energy between the hydrate and the bulk phase, e.g., the energy at the hydrate/water and water/hydrocarbon interfaces is lower than the energy would be at a direct hydrate/hydrocarbon interface. Based on visual observations from the MMF, this water layer must be on the order of microns or smaller with no additives present. When hydrates form in a pipeline, they nucleate and grow primarily at the interface. After the initial growth, further thickening of the hydrate shell becomes mass transfer limited. If particles are larger than a critical radius of 20-50 µm [36], then unconverted water could remain in the center of the hydrate for a significant amount of time until annealing can be completed as shown in Figure 1.8. Liquid water from the core could also fuel a liquid layer, traveling through micro-pores in the hydrate shell which anneal and close over time. It has been observed that the water from the core of the particle is more mobile than that gas across the hydrate film that forms, even after pores in the hydrate shell have annealed closed [37]. The extent of mass transfer across the hydrate shell may also be influenced by the guest molecule, with some species migrating faster than others [36]. The amount of water is limited for this experimental study, so the water layer cannot be augmented by external water from elsewhere in the flowline. Free water or unconverted water droplets are both likely to exist under operating conditions, and the oil phase will be saturated with water (as well as gas). These could all augment the water layer on hydrate particles, leading to higher forces.
Figure 1.8: Conceptual picture of hydrate formation on a water droplet.
12 The existence of a water layer is supported by numerous observations. As mentioned previously, capillary bridging was determined to be the most likely cause of cohesion be- tween hydrate particles due to the magnitude of the forces and the observed temperature dependence. It has been observed that free-flowing hydrate slurries form at high subcoolings, whereas agglomeration dominates nearer the equilibrium temperature [33]. A water layer with variable volume depending on temperature fits this behavior well. In addition, AFM measurements on ice revealed a liquid layer tens of nanometers thick on the surface which grew at temperatures near the ice point and shrank at lower temperatures [38]. Because ice cohesion shows similar trends to hydrate cohesion [34], the mechanism may be common between the two systems.
1.3 History of the Micromechanical Force Apparatus
The Micromechanical Force apparatus (MMF) was designed and built by Kelly Miller and began using by Tetrahydrofuran (THF) hydrates, where THF and water were mixed at 20wt% THF to account for the volatility of THF before the temperature was reduced below the ice point to form hydrates (note: a stoichiometric mixture of THF + water is one in which the THF present is sufficient to fill all large cages of sII, and corresponds to around 19.05 wt.%). Using this method, Yang et al. [39] tested the temperature dependence of the cohesion for both ice particles and hydrates. Both systems were found to give higher cohesive values when the temperature neared the equilibrium temperature. This dependence on temperature supported the conclusion that capillary bridging was the dominant force behind cohesion, since neither solid-solid cohesion nor sintering forces could account for the magnitude of the forces as well as the temperature dependence. Taylor et al. [40] refined the MMF methodology by implementing the direct calibration of the glass cantilevers, rather than the previous method where the spring constant was calculated based on its dimensions and elasticity based on Equation 1.5.
3πEd4 k = F (1.5) 64L3
13 In Equation 1.5, k is the spring constant of the cantilever, E is the elastic modulus of
the glass (assumed to be 70 GPa), dF is the diameter of the fiber, and L is the length of the fiber. Direct calibration increased the repeatability of MMF experiments, allowing more detailed studies to be performed. He then followed with a scoping study that investigated the effects of contact time and pre-load force, as well as the first investigation of how Low Dosage Hydrate Inhibitors (LDHIs) affect cohesion [41]. When contact time was varied from 1 second to 15 hours, it was observed to increase exponentially; this behavior was attributed to the sintering of particles together. This phenomenon occurs due to hydrates growing across the liquid bridge that holds the particles together. Figure 1.9 depicts a series of images taken as particles sinter together.
Figure 1.9: Image of THF hydrate particles held in contact for (A) 20 seconds, (B) 32 minutes, (C) 7 hours and (D) 11 hours showing the process of sintering. Reproduced from Taylor et al. [42]. Used with permission.
Further supporting the capillary bridge theory’s role in hydrate cohesion, the effect of the interfacial energy of the bulk fluid was tested. The cohesive forces were found to be directly proportional to the interfacial energy, though not to the degree suggested by capillary bridging. This indicated that there must be other important interfacial parameters aside
14 from the interfacial energy that are influential to the magnitude of the cohesive forces. Cyclopentane was used as a hydrate former by Dieker [43] to replace THF hydrates. Cyclopentane offered several benefits over THF hydrates: because it is immiscible with water, it behaves more like gas hydrate formers that are common in oil and gas flowlines. It also forms a sII hydrate phase that eliminated the possibility of localized non-stoichiometry, which may have led previously to areas of ice on THF hydrates. Finally, the equilibrium temperature of cyclopentane hydrate is 7.7◦C, which allowed for a larger stable range above the ice point and completely eliminated the possibility of ice contamination in experiments. Using cyclopentane hydrates, Dieker was able to test the effects of different crude oils on the cohesion force [44]. Her experiments showed that naturally non-plugging oils reduced the cohesion force significantly, but the reduction was smaller in the oils that had been treated to remove the acid or asphaltene fractions. This indicated that the surface active components of the oil were most likely responsible for lowering the cohesive forces. These experiments were all performed at ≥10 wt% of crude oil, with the remainder of the bulk comprised of cyclopentane due to visibility and thermodynamic constraints in the oil phase. The MMF procedure was further refined by Aman [45]. After a pull-off trial was per- formed, a wait time of 10 seconds was introduced to allow the liquid layer on the surface of the hydrate particle time to re-equilibrate. Previous procedures used only 1-2 seconds of wait time in between trials. The result of the added waiting period was a reduction in the spread of values obtained over the course of 40 pull-off measurements. Figure 1.10 shows the effect of the change in procedure. Perhaps the most important effect of this change is that it reduced the large deviations seen in previous measurements and allowed MMF results to be reported with 95% confidence [45]. During his tenure, Aman studied the effect of many physical and chemical variables on cyclopentane hydrate particles and the corresponding effect on co- and adhesive forces. Among his studies were the effect of contact time and pre-load force, different bulk phases such as water and air, the relationship between interfacial tension and cohesion forces, ad-
15 Figure 1.10: Change in the spread of cohesion force data with the instigation of a 10 second waiting period after each pull-off measurement. Reproduced from Aman [45]. Used with permission hesion force studies and a wide array of surfactant tests [34, 46–49]. Many of the chemicals used in this thesis were first investigated and classified in these previous studies by Aman. The MMF has also been used to investigate adhesive forces, i.e., those between a particle and a surface. Aspenes et al. studied the adhesion force of cyclopentane hydrates as a function of the surface energy of different materials. She concluded that materials with higher surface energy tended to have higher adhesive forces [50]. Aman et al. investigated the effect of roughness on various mineral surfaces as well as several different surface treatments on a stainless steel surface [51, 52]. He found that changes in wettability of the surface can significantly influence adhesive forces, but that several coatings can also cause morphological changes to the hydrate particles.
1.4 Hydrate Interfacial and Growth Studies
While the MMF is a unique apparatus, with few other research groups performing particle cohesion/adhesion tests [53], there are many other studies that relate to hydrate interfacial
16 and surface properties. The physical properties of hydrates are not always well characterized, and investigations into physical variables such as the growth rate and mass transfer properties of the hydrate shell under various conditions are ongoing [54–57]. Also of interest from both a flow assurance and natural hydrate point of view are the interactions of hydrates with water or mineral surfaces. For flow assurance, the availability of free water to interact with hydrates significantly increases the forces present in the system [58, 59]. Water also fills the pore spaces in natural hydrates, and capillarity affects the formation of hydrates [60]. Mineral surfaces form these pores, and mineral deposits can also be present in flowlines due to sand or scale in the system [61, 62]. Understanding how these surfaces interact with hydrates will allow deposition and jamming phenomena to be better understood and predicted. Many hydrate agglomeration studies focus on the development and testing of LDHIs. It is important for both the effectiveness of different chemicals to be tested [63–66] as well as to advance understanding of the changes that the chemicals and hydrates cause to the oil/water/gas system [67–69]. The ultimate goal of many LDHI studies is insight into the mechanism by which they operate. Developing a structure-property relationship between chemical functional groups and the effect that they have on hydrates and slurries would allow the efficient development of new treatment protocols, or even specialized treatments that are designed based on the specific oil and water profiles of an individual well.
1.5 Thesis summary and organization
The work presented in this thesis is divided into twelve chapters. Chapter 2 describes the experimental methodology for the apparatuses used in the course of these studies, as well as discussing the importance of consistency in procedure for the repeatability of measurements. Chapter 3 focuses on models related to the cohesion force as predictive tools. The Capillary Bridge Theory (CBT) and the Camargo and Palermo model for the viscosification of the oil phase in the presence of hydrates are explored; the former, CBT, investigates the model for its potential in predicting the force for a hydrate system given physical variables such as interfacial tension and contact angle, while the latter (Camargo and Palermo) shows the
17 influence of the cohesion force on the model output as a function of the other variables in the system, such as oil properties. Chapter 4 expands on the modeling from Chapter 3 by introducing a novel method of measuring the contact angle of water on the hydrate surface. It has been hypothesized that the contact angle may be an important parameter in predicting the cohesion force via the Capillary Bridge Theory. This chapter discusses the measurements of contact angle for pure hydrate systems as well as those with model surfactants added, and uses these measurements as inputs for comparing predictions made using the Capillary Bridge Theory to the forces measured in the system. Chapter 5 explores the possibility of using the MMF as a rapid screening tool for AAs using a ranking study of AAs provided by industry. Chapter 6 focuses on hydrate particle interactions with surfaces both with and without surfactants. Chapter 7 deals with a study on the mechanical strength of the hydrate shell and the changes that occur when surfactants were present during hydrate formation. In Chapter 7, the strength of the hydrate shell is compared for systems with and without surfactants. Systems containing multiple surfactants and the interaction between different classes of chem- icals are presented in Chapter 8. Cohesion measurements without surfactant additives are the focus of Chapter 9. These studies include work on physical variables such as temperature and annealing time. Development and validation of a high pressure MMF apparatus is discussed in Chapter 10. Chapters 11 and 12 contain concluding remarks on the work performed in this thesis and suggestions for future research that would further the understanding of cohesion in hydrate systems. Figure 1.11 shows a graphic representing where each chapter fits into a conceptual picture of hydrate flow. In the top figure, no additives are present. There may be large agglomerates and deposits that are able to form. Once anti-agglomerants or surfactant additives have been added (middle figure), the particles are able to flow as a slurry and the size of deposits is
18 greatly diminished. However, if waxes have been deposited on the flowline walls, deposits can once again become larger, even though the bulk particles still flow as a non-aggregating slurry (bottom figure). Many of the studies detailed in this work involve measurements both for the pure system (top) as well as systems containing additives (middle).
Figure 1.11: Conceptual picture showing how the research performed in each chapter of this work fits into the overall theme of hydrate cohesive and adhesive forces. The top flowline represents a system without any additives and may have large agglomerates and deposits. The addition of AA (middle flowline) reduces the agglomeration and the deposition. If waxes are present in the system, deposition can dominate over agglomeration in the bulk (bottom flowline), resulting in large deposits but small agglomerate flowing.
The main hypotheses that are explored herein include:
• Understanding the influence of the cohesion force in models (Capillary Bridge The- ory/Camargo and Palermo) can increase the accuracy of predictions from these mod- els.
• Surfactants affect many aspects of the hydrate shell, including reducing co- and adhe- sion, altering growth, and diminishing the strength of the hydrate shell.
• Additives can behave differently when added together than when they are tested singly.
19 • Hydrophobicity/wettability of the hydrate shell may be a good indicator of cohesion force in systems containing AAs.
• The MMF can be used as a rapid screening tool for industrial anti-agglomerants (AAs).
• Gas hydrates may not have different characteristics from their atmospheric pressure counterparts. This will be tested using a high pressure system.
1.5.1 Publications arising from this work
Six publications have arisen from the work contained in this thesis, including one pub- lished article, two submitted articles, and three articles in preparation. Chapter 7 was published in Physical Chemistry Chemical Physics (PCCP), titled “Micromechanical mea- surements of the effect of surfactants on cyclopentane hydrate shell properties” [70], and data from Chapter 8 has been submitted for publication in Energy & Fuels. A manuscript has also been accepted for publication in the Journal of Natural Gas Science and Engineer- ing, JNGSE, containing work from Chapter 9.2 in conjunction with modeling work on the cyclopentane hydrate thermodynamic equilibrium. The work performed for Chapter 9.1 was submitted as part of a work describing the fundamental properties of hydrate cohesion titled “Interfacial mechanisms governing cyclopentane clathrate hydrate adhesion/cohesion” [34]. Future publications include a research article containing much of the literature review on agglomeration and cohesion from Chapter 1 to be submitted as an invited article to Physical Chemistry Chemical Physics (PCCP). Development and measurements on hydrate contact angle from Chapter 4 will also be submitted to a peer-reviewed journal for publication. Finally, the industrial AA ranking study from Chapter 5 will be developed into a manuscript to be submitted to Energy & Fuels for publication. Work from this thesis has also been included in poster presentations including the Inter- national Conference on Gas Hydrates (ICGH 8), the Thermophysical Properties Symposium, and the meeting of the American Chemical Society (ACS).
20 CHAPTER 2 APPARATUS AND PROCEDURE
Several apparatuses were utilized for the studies performed in this work. The Microme- chanical Force (MMF) apparatus was used for the bulk of the measurements in order to study the cohesive and adhesive forces for hydrates. Due to the nature of the visualization of the MMF, several other measurements are also possible, as described below. In addition, the procedure for experiments in a high-pressure MMF is detailed in this section. IFT measure- ments were also taken for these studies to contextualize cohesion and adhesion measurements using the Capillary Bridge Theory (Equation 1.3).
2.1 Micromechanical Force Apparatus
The MMF is a unique apparatus which allows the direct measurement of inter-particle forces. The apparatus consists of two cantilevers which are used to bring particles into contact with one another, then pull them apart to determine the force. The cantilevers can hold either a particle or a surface. Typically, a hydrate particle is created on a stationary cantilever that is held in place using a Narishige 3-axis manual micro-manipulator. The other cantilever is remotely manipulated using an Eppendorf Patchman micro-manipulator. Each cantilever holds a glass rod bent in such a way that the particles or surfaces can be submerged in a cyclopentane bath. Glass fibers at the end of the rod were indirectly calibrated using Tungsten wires, the calibration of which can be seen in Figure 2.1. An aluminum cell outfitted with a cooling jacket maintains the bath at the experimental temperature. This entire assembly is mounted on a Zeiss Axiovert S-100 inverted light mi- croscope equipped with recording equipment. A thermocouple is mounted on the microscope stage in such a way that it is submerged in the cyclopentane near the particles. One modification to the MMF for this work is the inclusion of a top-off line to the cell. Cyclopentane has a high volatility (vapor pressure of 0.164 bar at 3◦C [71]) and evaporates
21 Figure 2.1: Calibration for Tungsten cantilevers used for indirect calibration of glass fibers, calibrated using the process documented by Taylor [40]. over the course of an experiment. This can affect both the temperature of the cell as well as the concentration of any additives present. Rather than create a cyclopentane atmosphere to reduce evaporation as has been done previously, a gravity-driven drip line was added with a needle valve to control the flow. This is a safer option for the operator as it reduces the potential of inhaling cyclopentane vapors, which are hazardous [72]; additionally, less cyclopentane is used for each experiment. The apparatus is contained within a dry box to reduce interference from humidity and is placed on a vibration isolation table. Figure 2.2 shows an image of the MMF apparatus. A second modification to the previous procedure is an increase in the frame rate at which the results were recorded. Increasing the frame rate from 1 frame/sec to 10 frames/sec yields more accurate data because it gives the exact moment when the hydrate particles separate. In order to create hydrate particles, a water drop was deposited onto the end of a 35 µm glass cantilever using a pipette. The droplet was submerged in liquid nitrogen until the particle had frozen completely. The particles were then rapidly moved to the cyclopentane bath where they converted into hydrates. The annealing period was measured from the time when the ice particles were placed in the cyclopentane, and for a typical experiment the particles annealed for 30 minutes.
22 Figure 2.2: Image of the MMF apparatus showing the microscope with recording equipment, the jacketed aluminum cell, and manipulators holding cantilevers.
2.1.1 Cohesion/Adhesion Force
In order to perform cohesion force experiments, particles were created on both cantilevers. For adhesion, the top particle mounted on the remotely-operated manipulator is replaced with a surface. Figure 2.3 shows the configuration of the cantilevers within the aluminum cell for cohesion and adhesion experiments. The surface used for adhesion experiments can vary, but is typically smaller than 1 cm2. The average particle diameter ranged from 600-1000 µm. For additive experiments, the preferred method was to add the chemical to the phase in which it was most soluble prior to hydrate formation. This was preferable because the entire growth phase for the hydrate occurred in the presence of the chemical. However, some chemicals in this work were added after hydrate formation. These chemicals significantly destabilized the forming hydrate particles, causing them to fall off the glass fibers. Unless specified otherwise, it should be assumed that any chemical additives were added prior to hydrate formation.
23 Figure 2.3: Configuration of the aluminum cell for cohesion (left) and adhesion experiments (right). For adhesion, the top particle is replaced with a surface.
After the particle(s) annealed, forty pull-off measurements were performed. A pull-off measurement consists of four steps as shown in Figure 2.4. The top particle (or surface) was brought into contact with the bottom particle at a constant pre-load force. After a ten second contact time, the top cantilever was moved away at a constant velocity. The distance at which the particles separated was used with the spring constant of the glass fiber to determine the force between the particles according to Hooke’s Law (Equation 2.1). The measurement was repeated after a ten second waiting period with the particles separated.
F = k∆D (2.1)
Figure 2.4: Procedure for pull-off measurements. (1)Particles begin at rest, separated. (2)The top particle is brought into contact with the bottom particle at a known pre-load force. (3)After ten seconds, the top particle is pulled away at a constant velocity. (4)The distance at which the particles separate is used to calculate the force between the particles. Reproduced from Aman [45]. Used with permission.
24 Forty pull-offs were performed for each particle pair. At least three particle pairs were used to determine the average force for a system, resulting in a minimum of 120 pull-off trials to determine the average force. Due to surface roughness and changes in the alignment of the particle, there can be variations in the magnitude of the forces between each pull-off trial. Using a large number of trials, an accurate average can be calculated. Error bars reported for these measurements are 95% confidence based on all of the pull-off measurements.
2.1.2 Shell Strength
The procedure for shell strength measurements was based on qualitative experiments performed in a scoping study by Taylor [40]. To perform these measurements, only the bottom hydrate particle was created, as shown in Figure 2.5. The top cantilever held only a glass fiber. The hydrate particle was formed according to the procedure described above, and the annealing time was varied from 1 to 90 minutes after hydrate formation.
Figure 2.5: Cell configuration used for shell strength experiments. The top cantilever holds only a glass fiber perpendicular to the bottom hydrate particle.
After the specified annealing time, the top cantilever was brought into contact with the particle and pressed down at a constant velocity until it punctured through the hydrate shell.
25 The position of the particle just before it was punctured was used to determine the force needed to puncture the particle using Equation 2.1 as shown in Figure 2.6.
Figure 2.6: Depiction of the distance used to measure the force needed to puncture the hydrate shell.
All additives used for this study were added prior to hydrate formation in the phase in which they were most soluble. Averages for these measurements represent 3-5 separate trials under the same experimental conditions, and error bars reported are the standard deviation of all measurements taken at a particular set of conditions.
2.1.3 Shell Thickness
During the Shell Strength experiments described above, it was sometimes possible to measure the thickness of the hydrate shell. After the hydrate shell had been punctured, the cantilever was moved down until it made contact with the interior bottom surface of the particle. By measuring the distance between the bottom of the cantilever and the bottom of the particle, an estimate of the thickness of the hydrate shell could be made as depicted in Figure 2.7. It is important to note that these measurements were not possible for every particle. Often, the hydrate would adhere to the cantilever shortly after the particle was punctured, making it difficult to move. Alternately, when the particle was punctured, the cantilever would occasionally puncture straight through both the top and bottom of the particle.
26 Figure 2.7: Method of measuring the thickness, d, of a hydrate particle after it has been punctured by a glass cantilever.
These measurements represent a novel method of measuring the shell thickness for a spherical particle. Most measurements to date have been made using a lateral hydrate film. However, there is an intrinsic chance for error in these measurements. Because the MMF can only view the particles from the bottom up, there is an uncertainty in the location of the cantilever in the Z-direction. If the cantilever is not directly at the bottom of the particle, then the measurement will result in an overestimation of the thickness of the hydrate shell. Figure 2.8 shows a conceptual side view of the particles illustrating the possible introduction of error based on the Z-direction position of the cantilever.
Figure 2.8: Illustration of how different positions in the Z-direction could cause an overesti- mate of hydrate shell thickness.
27 2.1.4 Growth Rate
The growth rate of hydrates can be measured by recording the formation of the hydrate. Typically, the hydrate nucleated at several locations, and the crystals could move freely across the water surface of the particle once the ice had fully melted. These hydrate crystals tended to adhere to the glass cantilever. The hydrate then continued to grow along the water/cyclopentane interface; by recording this process, it was possible to determine the rate at which the hydrate grew to cover the entire particle. Figure 2.9 shows the progression of the hydrate shell growth as it extends from the glass cantilever on either side of the particle. An average of 70-100 measurements were taken to obtain an average growth rate for a set of conditions. Error reported is the standard deviation of these measurements.
Figure 2.9: Progression of hydrate shell growth as it spreads along the water/cyclopentane interface. Reproduced from Brown et al. [70] with permission from the PCCP Owner Societies.
2.1.5 Contact Angle
The contact angle of water/surfactant systems on glass was measured using a KSV Cam 200 apparatus. A glass cover slide was cut so that a square, approximately 1 cm x 1 cm, could be inserted into the glass cuvette filled with air or cyclopentane. A needle was used to deposit a drop of water onto the surface where the stationary contact angle was measured. Contact angle measurements on hydrate surfaces are not well-studied, and a novel method was developed in order to study them. Full details of the validation of this method are available in Chapter 4.
28 In order to perform these measurements, a large hydrate was created on the end of a glass rod using the same method as in the previous sections. The hydrate typically measured 1-2 mm and was allowed to anneal for 30 minutes. Large hydrate particles were used in this work to minimize the curvature at a small, local area to approximate a flat surface. After the annealing period, small water droplets (400-600 µm) were inserted into the cell at the end of the top cantilever. The top cantilever was used to gently bring the water droplet into contact with the hydrate surface at the top edge of the particle. Once the droplet had adhered to the hydrate, the cantilever was slowly withdrawn so that it did not drag or disturb the water droplet. Images were taken throughout the process. The earliest images after the removal of the cantilever were used to minimize the hydrate growth onto the water droplet, which varies in speed based on the subcooling and the presence of additives. On occasion, the cantilever could not be be removed without also removing the water droplet. When this occurred, the cantilever was positioned as much as possible to not affect the way the droplet rested on the hydrate surface. Figure 2.10 shows a water drop resting on a hydrate surface as well as an indication of the contact angle measured from this system.
Figure 2.10: Water droplet on a hydrate surface for contact angle measurements.
When surfactants were added in these experiments, they were added into the phase in which they were most soluble. If the surfactant was water-soluble, it was added to the water
29 used to form the hydrate as well as to the water droplet. If a surfactant was soluble in neither phase, it was used as a dispersion in whichever phase stabilized it best. Results are the average of 10+ measurements using 5+ droplets on hydrate surfaces. Error bars represent the standard deviation of these measurements.
2.2 High Pressure Micromechanical Force Apparatus
In order to study the differences between cyclopentane hydrate and gas hydrates, a High Pressure MMF (HPMMF) apparatus was developed, as discussed in Chapter 10. The HPMMF consists of a steel pressure vessel (Sejin Co.) which is shown on the left side of Figure 2.11 as well as a saturation vessel, which is shown on the right.
Figure 2.11: Photo of the High Pressure MMF apparatus with the pressure vessel on the left and saturation vessel on the right.
A chiller circulates a glycol mixture through a jacket in the pressure cell to maintain the cell temperature, separated from the pressure cell by a valve. The cell has a window atop a narrow section of the cell, which can be seen in Figure 2.11, as well as a pressure
30 gauge and thermocouple. Two couplings contain the feed-through wires that control and power a nano-manipulator. The nano-manipulator (from Klocke Nanotechnik) is a three-axis, remotely-controlled device with a total footprint of less than 10 cm per side. It is important that the size of the manipulator be minimized to reduce the volume needed for the pressure cell. The nano-manipulator holds one cantilever, which is moved in order to conduct exper- iments. A second, stationary cantilever is held by a mount on the lid of the cell. These cantilevers hold glass fibers in the same way the low pressure version does. Finally, the cell is equipped with a gas supply line with a needle valve and a gas vent line. In order to perform an experiment in the HPMMF, ice particles were first created by freezing drops on the end of the glass cantilevers. The droplets should be 1000 µm or larger to begin with due to shrinkage of the particles through the pressurization process. The chiller was set below the ice point. Just before the particles were added to the cell, the valve from the chiller was opened and the cell was allowed to cool. Once the cantilevers were mounted in their positions, the lid was placed over the cell and the feed-throughs for the manipulators were connected. Each screw in the lid was tightened in a star pattern. The screws securing the polycarbonate window should also be checked for tightness. This window should be periodically inspected and replaced if necessary to ensure that the repeated pressurization has not caused damage. Figure 2.12 shows the window as well as the screws that need to be tightened to secure the lid. Once the lid was secured in place, the cell was slowly pressurized using the needle valve to regulate the speed. The temperature in the cell should be monitored to ensure that the gas is not being added too quickly. Ideally, the ice particles should not melt through this process. After a pressure of 250-300 psi was reached, the cell was vented to remove any nitrogen and oxygen from the air, then re-pressurized. If the ice particles melted, venting should reduce the temperature enough to re-freeze them. If sufficient hydrate former remains in the cell, hydrates may also form upon venting. The venting procedure should be repeated 2-3 times
31 Figure 2.12: Polycarbonate window mounted on the lid of the pressure cell. Screws around the perimeter of the lid are used to secure the lid in place.
total before the cell is pressurized to the desired experimental pressure. After the experimental pressure was reached, the temperature was also adjusted to the desired level, and the annealing time began. Annealing times varied based on the experiment. After the annealing time, pull-off measurements were performed using the same procedure described in Section 2.1.1 (see Figure 2.4). More pull-off measurements are typically per- formed in the high pressure system than the low pressure system due to the higher variation in force values obtained. Measurements are taken for 30-40 minutes to avoid the effects of changing annealing time too severely and usually number from 50-80 pull-offs per experi- ment. Error bars reported for these measurements is 95% confidence of average values for multiple experiments.
2.3 Interfacial Tension
Interfacial tension (IFT) measurements were taken using a KSV Cam 200 apparatus. This apparatus consists of a camera facing a light source, as well as a mount for a cuvette and needle as shown in Figure 2.13. The camera is used to record well-defined images of
32 droplets extruded from the needle into the bulk fluid in the cuvette. The shape and curvature of the droplet are then regressed using the Young-Laplace Equation (Equation 2.2). 1 1 ∆P =γ + (2.2) R1 R2 Where ∆P is the pressure difference across the liquid film, γ is the interfacial tension,
R1 is the radius of curvature of the droplet, and R2 is the radius of curvature perpendicular
to R1 at the same tangent position. The pressure difference is solved for numerically using the difference in density between the two fluids used.
Figure 2.13: Image of the KSV Cam 200 apparatus used to measure interfacial tension and contact angles on surfaces.
In order to perform an experiment, a light phase and a heavy phase (which is typically the droplet phase) are needed. For the majority of the experiments in this work, the light phase was Mineral oil 70T, the composition of which is shown in Table 2.1. Mineral oil (ρ=0.8558 g/cm3) was used in lieu of cyclopentane for these measurements because the higher volatility of cyclopentane leads it to evaporate too quickly to take accu- rate measurements of low mobility surfactants. Measurements performed using cyclopentane tended to yield unstable results. Using mineral oil, results of IFT measurements with chem- icals present are compared as a decrease from the baseline of Mineral Oil 70T.
33 Table 2.1: Composition of Mineral Oil 70T. Analysis performed by Weatherford Labs.
Component Mass Fraction [%]
C16 0.09 C17 1.23 C18 5.22 C19 11.75 C20 16.04 C21 17.04 C22 12.20 C23 6.34 C24 4.23 C25 3.76 C26 3.29 C27 2.66 C28 2.27 C29 1.56 C30 12.34
The heavy phase used in these experiments was DI water (ρ =0.998g/cm3). Once again, surfactants were mixed into the phase in which they displayed the highest solubility. If the additive was soluble in neither phase, it was used as a dispersion in whichever phase held a stable dispersion. IFT measurements for pure systems reach a steady state essentially instantly, while those with surfactants begin at a higher value then decrease gradually to a steady state value as the interface saturates with surfactants. Some additives are more mobile than others; therefore the time that the system takes to reach steady state depends on the system being investigated. A trace of a typical IFT experiment is shown in Figure 2.14. It is important to note that the volume of the droplet must be stable as the steady-state IFT value is reached. While some decrease is natural due to a small solubility of water in mineral oil, a decrease of greater than 10% indicates that there is an error in the experiment and that the IFT result may be in error.
34 Figure 2.14: Example of an IFT experiment with surfactant. The IFT value begins at a higher value and then decreases until a steady-state value is reached. The volume of the droplet must be stable over the course of the experiment.
2.4 Note on Procedures
Particularly for the Micromechanical Force measurements, the procedure used is of ut- most importance. While procedural modifications are natural as an apparatus is developed and new technology becomes available, it is also important to note that minor procedural changes, such as those to annealing time or hydrate formation method, can have significant effects on the cohesive forces measured. In addition, additives may have different effects if added at different times. MMF measurements have proven to be highly reproducible with different operators as long as the same procedure is followed. Some parameters that require special attention are:
• Formation method - Historically, hydrates were formed by placing ice particles into a cyclopentane bath maintained under the ice point, then raising the temperature to the desired experimental temperature. For this work, hydrates were formed by immers- ing ice particles in a cyclopentane bath at the experimental temperature. In the new method, the particle was maintained during the growth and annealing times at the de- sired experimental temperature, while previously the particles nucleated and annealed at a lower temperature as the bath warmed. This change also helped standardize an-
35 nealing times; annealing was measured from the moment the ice particle was placed in the bath, rather than when the bath reached the experimental temperature. Hydrates formed with the previous method will have had a higher initial growth and longer an- nealing time, which may cause discrepancies for measurements at temperatures closer to the equilibrium point. High subcooling measurements show less deviation between the two measurements.
• Temperature - As the temperature dependence of hydrates increases steeply near the equilibrium temperature, small differences in the temperature may lead to large errors in the measured cohesion force. It is therefore important to use a properly calibrated thermocouple and place it as near to the particles as possible.
• Annealing time - As shown in Chapter 9.2, the cohesion force is not constant with annealing time. Annealing time for these experiments begins when the ice particle is placed in the bath and ends at the first pull-off measurement. A full set of 40 pull-off trials takes approximately 13 minutes. Figure 2.15 shows the pull-off values for a typical experiment. The trend line indicates that over the time taken to perform 40 pull-off measurements, the average force does not change significantly.
• Addition of surfactants - The time and order in which surfactants are added to the cell can affect the outcome of the experiment. Phase partitioning is not well understood for hydrate systems, and therefore the phase in which the chemicals are added affects the outcome of the cohesion/adhesion results. Adding chemicals after hydrate formation may be necessary at times in order to maintain particle stability. However, the resultant morphological changes and forces may not be the same for particles which were formed in the presence of an additive.
36 Figure 2.15: Graph of a typical MMF experiment showing that no significant trend exists over the time time taken to perform 40 pull-off trials. Trend line provided to guide the eye.
37 CHAPTER 3 SENSITIVITY OF PARTICLE COHESION
This section focuses on two separate models that involve the cohesion force. The Capillary Bridge Theory, discussed in Chapter 1, is a model that depends on several interfacial vari- ables to predict the magnitude of the cohesive force. Previous studies have shown that using the interfacial tension as the only experimental parameter in the Capillary Bridge Equa- tion to predict the cohesive force does not lead to accurate predictions. Therefore, other variables must also be considered in order to obtain accurate hydrate predictions. Chapter 3.1 examines the relative influence of the variables in Equation 1.3 in order to determine which physical variables have strong effects on the force value obtained. Predicting the force using physical variables that are easily measured is valuable because MMF measurements to determine force are not highly accessible to all parties studying hydrates. If there were a small number of parameters that could be measured and used to give a reasonable predic- tion for the hydrate cohesion force that would result from a given system, then the chemical development industry/flow assurance engineers would be able to screen chemical treatments far more quickly than current methods allow. The Camargo and Palermo model is currently being used in CSMHyK, a plug-in to OLGA that allows for multi-phase flow modeling in the presence of hydrate. The Camargo and Palermo model, presented in Chapter 3.2, relies on oil properties and hydrate parameters such as the particle diameter to determine the extent of viscosification of the oil phase as a result of hydrate [21]. It is important to understand the effect of each variable in this equation, and how the magnitude of the cohesion force changes the output.
3.1 Capillary Bridge Theory
The Capillary Bridge Theory, represented in Equation 1.3, can be used to predict the cohesive force between two surfaces connected by a liquid bridge comprised of a liquid sep-
38 arate from that of the bulk fluid. Rabinovich et al. proposed a model for the volume of the liquid bridge, V , which is used to solve for the immersion depth, d [73]. The equations for the immersion depth and volume of the liquid bridge are presented below in Equations 3.1 and 3.2.
H 2V d = −1+ 1+ (3.1) 2 s πRH2 V = πR∗2α2H +0.5πR∗3α4 (3.2)
Based on these relationships, the force resultant from the Capillary Bridge Equation is dependent on five independent parameters:
• R∗- Normalized particle radius (see Equation 1.4)
• γ - Interfacial tension between the bridge and the bulk phases
• α- Embracing angle of the bridge on the particle
• θ- Contact angle of the bridge on the particle
• H- Height of the liquid bridge
It is possible to measure several of these variables experimentally, while others must be estimated or solved for using a model. Using technology such as Particle Video Microscopy and Focused Beam Reflectance Measurements, particle size can be measured in a flowing system, such as a flowloop or autoclave cell [25]; particle size measurements are readily available from MMF data due to the nature of the video imaging/recordings. Interfacial tension measurements are commonplace and can be performed easily on a variety of different systems. Previously, contact measurements on hydrate surfaces had only been performed by Asserson et al. [74], though new developments have made these measurements possible for a wider variety of systems, which will be discussed in Chapter 4. The height of the liquid bridge has not been determined experimentally for hydrate systems. For the purposes of the sensitivity analysis in this Chapter, the thickness of the liquid layer was assumed to
39 be analogous to the liquid layer on ice particles, which was measured using Atomic Force Microscopy by Doppenschmidt et al. [38]. Using these parameters, Aman et al. [34] solved for the value of the embracing angle using a baseline cohesion force value. A sensitivity analysis was performed by varying each of these values one at a time and observing the effect that they had on the cohesion force prediction. The baseline values used in this analysis are tabulated in Table 3.1.
Table 3.1: Values used for variable parameters to perform a sensitivity analysis on the Capillary Bridge Equation.
Parameter Value Unit R∗ 300 µm γ 51 mN/m α 0.1 deg θ 29 deg H 50 nm
Based on the form of Equation 1.3, it is apparent that the force and the radius of the particles have a linear relationship. This is clearly confirmed in Figure 3.1, which shows that increasing the radius of the particle linearly increases the force predicted by the Capillary Bridge Equation. While somewhat simplistic, this parameter indicates that as agglomerates grow in a system, they are likely to cohere more strongly to each other. This compounding effect may play a part in the plugging of pipelines. Similarly, the interfacial tension also has a linear relationship with the cohesion force. This simple relationship and the ease of measuring the IFT led to a hypothesis that IFT measurements may be a reasonable indicator of the cohesion force. However, it has been found by multiple operators that when using surfactants, the IFT is not always a reliable indicator of cohesion force [48, 70]. It is clear, therefore, that adding surfactants leads to changes in the system more complex than simply lowering the IFT. Figure 3.2 shows the trend predicted by Capillary Bridge Theory for the relationship between the interfacial tension and the cohesion force.
40 Figure 3.1: Relationship between radius of the particle and the cohesion force.
Figure 3.2: Relationship between interfacial tension and cohesion force predicted by the Capillary Bridge Theory.
41 The contact angle has a much more complex relationship with the cohesion force than the variables examined previously (e.g., IFT, particle diameter). Because it occurs in both the first and second term in Equation 1.3, it can show different behaviors based on the other parameters in the system. Figure 3.3 shows the dependence on the contact angle for the parameters listed in Table 3.1. As can be observed from Figure 3.2, the contact angle only predicts a force for hydrophilic contact angles. Forces predicted for angles above 95◦ result in negative values, indicating a repulsive force between the particles, which is unlikely for this type of particle. Additionally, as will be presented in Chapter 4, forces can still be measured for systems where the contact angle is significantly hydrophobic (>150◦). In order to better understand this phenomenon, the Capillary Bridge equation was divided into two parts, shown in Equations 3.3 and 3.4.
Figure 3.3: Prediction of force values for varying contact angle values.
2πR∗cosθ F = H (3.3) 1+ 2d F =2πR∗sin (α) sin (θ + α) (3.4)
Breaking the Capillary Bridge Equation into these two portions will allow the competing effects of the contact angle to be seen; the cosθ term in Equation 3.3 will cause the first
42 term to be negative at any contact angle greater than 90◦. Therefore, the second half of the Equation represented by Equation 3.4 must dominate for hydrophobic contact angles. Separating the graph in Figure 3.3 into the contributions from each portion yields Figure 3.4, which clearly shows that for this set of parameters, the first term from the Capillary Bridge Equation dominates, and the second provides only a small influence.
Figure 3.4: Dependence of the cohesion force on the contact angle broken down into separate terms based on the Capillary Bridge Equation.
In order to shift the balance of the Capillary Bridge Equation to the second term in such a way that hydrophobic angles can still predict a force, other parameters from the equation must also change. Based on Figure 3.1 and Figure 3.2, changes in the particle radius or interfacial tension will affect both terms similarly. Therefore, the variable shift that occurs must be in either the height of the liquid bridge or the embracing angle, or a combination of both variables. Modeling a similar scenario using a liquid bridge height of 500 nm and an embracing angle of 0.01◦ and keeping all other parameters as listed in Table 3.1 shifts the dominance to the second term from the Capillary Bridge Equation as shown in Figure 3.5. It is important to note here that the values of the force measured for this set of parameters is very small. Multiple combinations of variables were analyzed, and all scenarios where hydrophobic contact angles were non-zero were low in magnitude compared to forces for
43 Figure 3.5: Dependence of the cohesion force on the contact angle when H=500 nm and α=0.01◦ (broken down into two portions) based on Capillary Bridge Theory. more hydrophilic contact angles. As will be discussed in Chapter 5, hydrophobic contact angles were often observed for the most effective AAs that were tested (i.e., AAs that had high contact angles reduced the force the most significantly). The height of the liquid bridge, H, also has behavior that is dependent on other pa- rameters in the model. When the parameters used in Table 3.1 were used to calculate the dependence of the force on the height of the liquid bridge, the force was observed to decrease with an increase in the height of the liquid bridge as seen in Figure 3.6.
Figure 3.6: Force prediction based on varying liquid bridge height.
44 This decrease could be due to the hydrophilic contact angle used in these measurements. On a hydrophilic particle, the water layer would spread out more and alter the curvature of the bridge, which can affect the forces. If the height of the liquid bridge is used to predict the force for a particle with a neutral wetting angle of 94◦ (which will be described in Chapter 4 in further detail), then the dependence follows a more logical trend, where a larger bridge equates to a higher force. This trend is illustrated in Figure 3.7.
Figure 3.7: Relationship between force and the height of the liquid bridge for a neutral wetting angle (θ=94◦).
Finally, α is another parameter that can have competing effects on the first and second term of the Capillary Bridge Equation. As described in Equation 3.2, the embracing angle is used in the calculation of the volume of the liquid bridge, which affects the parameter d in Equation 3.3. An increase in α causes an increase in the force obtained from the Capillary Bridge Equation, though the response in the force by changing α once again varied depending the hydrophobicity of the particle and the corresponding contact angle. Figure 3.8 shows the force prediction using the values from Table 3.1. As can be observed from Figure 3.8, the magnitude of the force is very sensitive to increases in the embracing angle, especially between 0◦ and 1◦. When the first term of the Capillary Bridge Equation is positive, an increase in the embracing angle causes both the first and second terms to increase. The more hydrophilic
45 Figure 3.8: Sensitivity of the cohesion force to α,the embracing angle on the particle. the contact angle (i.e., the closer to 0◦), the more dominant the first term is when determining the force. The two separated portions of the Capillary Bridge Theory are shown in Figure 3.9, which shows that the first term is contributing essentially to the entire force for this set of parameters. However, when a neutral contact angle of 94◦ is examined, along with a slightly higher value for H (H=500nm), the opposite trend is observed. The first term is negative, while the second term is growing larger. Figure 3.10 shows the force prediction for the less hydrophilic set of parameters.
Figure 3.9: Separate contributions from each portion of the Capillary Bridge Theory for the dependence of the force on the embracing angle.
46 Figure 3.10: Contributions from the first and second term of the Capillary Bridge Theory (for a neutral contact angle of water on hydrate of θ =94◦ and a liquid bridge height of H=500 nm) describing the dependence of the force on the embracing angle.
Based on this analysis, several conclusions become clear. Past experimental analyses have shown that the Interfacial Tension is often a poor indicator of probable cohesive forces in hydrate systems; this is also apparent in this analysis. The IFT has a very simple rela- tionship with the force and does little to capture the more complex behavior that is observed from changes in the contact angle or embracing angle. It is likely based on experimental comparisons that many parameters change in a hydrate system when surfactants are added. Contact angle appears to have a significant effect on many of the other parameters; the be- havior of the system appears to change significantly between hydrophilic, hydrophobic and neutral wetting regimes. A better physical understanding of these parameters can lead to a better mechanistic understanding of the capillary bridges that exist in hydrate systems as well as providing better estimates for the parameters used to predict the force via the Capillary Bridge Equation.
3.2 Camargo and Palermo Model
Camargo and Palermo proposed a rheological model that describes the viscosification of the oil phase based on the presence of hydrates [21]. Equation 3.5 is used to numerically
47 solve for the aggregate diameter, dA.
− 2 φ (3 f) (4−f) F 1 − dA dA φMax dP − (3−f) =0 (3.5) dp ! d2 µ γ 1 − φ dA P 0 dP Where dpis the particle diameter, f is the fractal dimension, F is the cohesion force, φ
is the volume fraction of the hydrate phase, φMax is the maximum packing concentration of randomly packed mono-dispersed spheres, µ0 is the viscosity of the continuous oil phase and γ is the shear rate. The aggregate diameter is then used with Equation 3.6 to determine the effective volume fraction, which is then used in Equation 3.7 to calculate the relative viscosity of the slurry, µr.
(3−f) dA φeff ≈ φ (3.6) dP !
1 − φeff µr = 2 (3.7) 1 − φeff φMax The cohesion force in the Camargo and Palermo model plays a large part, acting as a scaling factor that amplifies the effects of the other variables. The parameters used as constants for the sensitivity analysis are shown in Table 3.2.
Table 3.2: Base values used in the sensitivity analysis of the Camargo and Palermo model.
Parameter Value Unit
dP 1.5 µm µ0 60 cP γ 400 s−1 f 2.5 - φ 0.134 - φMax 0.571 -
In order to show the effect of the cohesion force on the model, three values of the force were used in each sensitivity analysis. The relative viscosity of each system was calculated using F=1.6, 4, and 50 mN/m. These three values were chosen as they are all used in different areas of literature and hydrate modeling. A cohesion force value of F=1.6 mN/m
48 was solved for in the original Camargo and Palermo model and used in order to fit the model to the experimental values [21]. In MMF measurements of cyclopentane hydrate, the average value of the force at 3◦C is about 4 mN/m [45]. Finally, in the CSMHyK model that runs within the OLGA plug-in, the force value used is 50 mN/m [75]. A value of 50 mN/m is not, however, arbitrary. Preliminary tests on gas hydrate (discussed in Chapter 10) have found that methane/ethane hydrate cohering in the gas phase have forces on this order. Relative viscosity was calculated as a function of each of the variables in Table 3.2 for each of the three force values to demonstrate the sensitivity to each variable at different forces. It is important to note that for certain parameters, the Camargo and Palermo model has difficulty accurately predicting the diameter of the aggregates that form. This occurs especially for lower force values at small particle radii, which do not provide enough cohesion to overcome the shear values, resulting in a model prediction of an aggregate diameter lower than the particle diameter. These values have been excluded from the graphs showing the sensitivity analyses, as they do not have a physical meaning. All of the forces herein are the normalized force, meaning that all force values have been normalized to the radius of the particles. The diameter of the particles in a system is strongly tied to the force, since the forces are all normalized. Figure 3.11 shows the effect of increasing particle radius on the relative viscosity. This trend is initially non-intuitive because the natural expectation is that larger particles would cause an increased relative viscosity. However, the sensitivity shown holds the volume fraction of the hydrate phase at a constant value, meaning that for hydrate particles of larger sizes, there would be fewer overall particles in the system. A large number of small particles would cause a larger increase in the relative viscosity of the system than just a few large particles. If the number of particles in the system were constant, rather than the hydrate volume fraction, then it would be expected that the viscosity would increase with an increasing particle diameter. In flowline systems, it is likely that the diameter of emulsified water droplets and therefore hydrate particles would exist in the range of 1-100
49 µm [25], and would exist in the very steep range on the left side of Figure 3.11.
Figure 3.11: Dependence of the relative viscosity on the diameter of the particles in the Camargo and Palermo model. Volume fraction of hydrate in the system is held constant, resulting in a decrease in relative viscosity with increasing particle diameter.
The relative viscosity of a system is defined here as the viscosity of a system after the formation of a hydrate slurry divided by the viscosity of the pure oil phase. An oil with a low viscosity is likely to experience a larger increase in the relative viscosity than a high viscosity oil, given the same amount of hydrate formation. In flowloop experiments, oils with high relative viscosity were also found to plug more readily and have lower transportability of hydrate than low relative viscosity oils [76], which is captured in Figure 3.12. Once again, the effect of viscosity on the relative viscosity is magnified by the cohesion force value, with a large force value causing significantly more agglomeration and a higher increase in the relative viscosity. The shear rate in a flowline is dependent on the velocity of the fluids, the roughness of the walls and the flow regime of a given system, among other variables. A system with laminar flow (based on the Reynolds number, Equation 1.2) will exhibit significantly different shear rates from a system with turbulent flow. Similarly, slug flow will display different shear rates from stratified flow, for example. It is therefore difficult to accurately estimate the shear rate
50 Figure 3.12: Relationship between the viscosity of the oil phase and the relative viscosity predicted by the Camargo and Palermo model. in a flowing system. This parameter is the subject of significant rheological study [77]. A higher shear rate will typically result in a smaller aggregate size in a system, as higher forces are necessary to stably bind large aggregates. Despite this effect, an increase in the shear rate still caused a decrease in the relative viscosity for this system, as shown in Figure 3.13 as expected intuitively. The fractal dimension is a complex parameter which describes the compact or spread-out nature of an aggregate comprised of smaller particles [78]. A very compact aggregate would have a fractal dimension closer to f=3, while an aggregate which has many loose “legs” protruding would have a lower fractal dimension [79]. Under shear conditions, it has been observed that aggregates are typically more compact, with fractal dimensions ranging from f=2-2.7, while perikinetic (diffusion-driven) aggregation corresponds to f=1.7 to 2.1 [80]. In the Camargo and Palermo model, the fractal dimension has been set to a constant value of f=2.5. For low values of the cohesion force, this has little effect on the relative viscosity (Figure 3.14). At higher forces, however, less densely agglomerated particles with a lower fractal dimension would have cause the relative viscosity to increase significantly. The fractal
51 Figure 3.13: Relative viscosity as a function of shear rate for three difference force values. dimension is an extremely difficult parameter to study, especially in hydrate systems which are kinetically active as well as under significant shear. The volume fraction of the hydrate phase is an indication of how much hydrate has formed at any given point in the flowline. This is an important parameter, as systems fail more frequently with larger amounts of hydrate present. The Camargo and Palermo model captures the viscosification of the oil phase. Additional phenomena that are no captured include bedding, deposition, plugging, or transitions from homogeneous to heterogeneous distributions of particles, all of which have been observed as important phenomena in hy- drate scenarios [76, 81]. Figure 3.15 shows that higher particle cohesive forces lead to more catastrophic effects at increasing amounts of hydrate in the system.
The maximum packing fraction, φMax, describes the density with which hydrate can pack a certain volume. The packing fraction used in this model is φMax = 4/7 (0.571), which corresponds to the maximum packing fraction of loose-packed mono-dispersed spheres. This parameter has been studied for many systems but can be very complex in the case of hydrate. The water droplets emulsified in the oil phase are unlikely to be mono-dispersed, and large ranges of particle sizes have been observed in hydrate systems. Poly-dispersity would
52 Figure 3.14: Fractal dimension sensitivity in the Camargo and Palermo model.
Figure 3.15: Sensitivity of the Camargo and Palermo model for the relative viscosity of an oil phase with hydrate to the volume fraction of the hydrate.
53 increase the maximum packing fraction because the smaller particles could fill the void spaces between the larger particles. Additionally, due to the large shear forces in flowline systems, the particles may pack more tightly than loose random packing in areas such as stationary beds and deposits. Free water in between particles can anneal into hydrate over time, given availability of the hydrate former. This annealing phenomenon would cause the gaps between the hydrates to slowly fill with hydrate, increasing the maximum packing further. However, aggregates of hydrate particles may not be spherical, which may also alter the maximum packing, depending on the aggregate shapes. Figure 3.16 shows the dependence of the relative viscosity obtained in the model on the maximum packing fraction. This parameter is not the most significant in its effect on the relative viscosity prediction even at high forces (maximum increase of only ˜10x versus 50-100x for other variables), but because it is a constant used across all systems, an accurate packing fraction is necessary to obtain realistic predictions.
Figure 3.16: Dependence of the relative viscosity on the maximum packing fraction, φMax in the Camargo and Palermo Model.
3.3 Conclusions
The parameter sensitivity analyses of the Capillary Bridge and Carmargo and Palermo models in this chapter show the importance of the cohesion force with respect to other
54 parameters used in modeling the effect of hydrate on multi-phase flow systems. In particular, it was shown that the wettability of the hydrate surface has a strong influence on how other variables affect the predicted force from the Capillary Bridge Theory. While the changes in relative viscosity predicted from the Camargo and Palermo model may be minimal for a given parameter at lower forces, many can cause increases or decreases of orders of magnitude when higher forces are present (such as those measured for gas phase systems in Chapter 10). Given free liquid in the system, the forces for a flowline system may increase significantly, magnifying the effects on relative viscosity even further. Obtaining accurate values for constant parameters such as the fractal dimension and maximum packing fraction should improve model predictions. Understanding the inter-particle forces in a system is of utmost importance if models such as Camargo and Palermo’s are to be used in a predictive capacity and further advanced for applicability and accuracy.
55 CHAPTER 4 CONTACT ANGLE MEASUREMENTS ON HYDRATE SURFACES
This chapter details a novel measurement of the contact angles on the surface of hydrate particles and the effect that surfactant additives have on these measurements and hence on hydrate particle wettability. These measurements become important when considering the predictive capabilities of the Capillary Bridge Theory. While a small number of these experiments have been tested on a Freon hydrate surface, [74] comprehensive study on the changes caused when surfactants are also present have not been conducted. Increasing the knowledge on how water droplets interact with the hydrate surface is of importance not only to capillary bridging but also influences the effect that free water present in the flowline will have on hydrate interactions. A measurement that can easily be made in the presence of surfactants and was reproducible was needed in order to begin examining this variable. Multiple measurement methods for contact angles exist, but few are adaptable for a solid surface that is both rough and reactive with water (i.e., the addition of water spurs further hydrate growth), such as the hydrate surface [82–84].
4.1 Pure Hydrate Measurements
The MMF apparatus is oriented differently than traditional contact angle measurements; therefore, it was first necessary to verify that it could report accurate contact angles. In order to do this, a system was needed that could be recreated on both the IFT and MMF apparatuses. Measurements began on a simple air/water/glass system and progressed to a cyclopentane/water/glass system. In the IFT, the apparatus was used to automatically measure the contact angle when a water droplet was deposited onto a glass cover slide (Fisherbrand Microscope Cover Glass). For the MMF, a special cantilever had to be created such that water droplets could be brought into contact with a glass cover slide using the micro-manipulators. Figure 4.1 shows the cantilever setup that was used in the MMF.
56 Figure 4.1 represents a top-down view of the particle, representing a 90◦ shift from the IFT measurements.
Figure 4.1: Schematic showing how contact angle measurements were performed in the MMF apparatus (right) and a typical measurement from the IFT (left).
The baseline calibration was necessary because gravity is acting differently on the water droplet in each case. For the IFT, gravity is a force perpendicular to the glass surface, while in the MMF gravity acts in a parallel direction. However, despite these differences, both apparatuses were in good agreement for each of the systems tested. Figure 4.2 shows the contact angle measurements for water on glass for each bulk phase on both apparatuses. From Figure 4.2, it is apparent that the MMF and IFT systems offer very comparable results. Based on these measurements, the MMF appears to have the ability to accurately report contact angles. Measurements in the IFT were slightly more repeatable than those performed in the MMF. For a hydrate surface, the procedure was similar to that for glass, but the glass surface was replaced with a large (1-2 mm) hydrate surface. Care was taken to deposit the water droplet on the edge of the hydrate so that a clear contact angle measurement could be obtained. A larger number of measurements (>10) was necessary for the hydrate systems in order to obtain an accurate average due to the variation caused by the surface roughness of the hydrates. The hydrate shell did not grow smoothly at the thermodynamic conditions used for these trials and resulted in local areas that may have very different topography than
57 Figure 4.2: Comparison of the contact angles measured in the IFT and MMF apparatuses on glass cover slides. Air and cyclopentane were used as bulk phases for these measurements. Averages represent 5+ water droplets (error bars are ± one standard deviation). the larger scale particle. Figure 4.3 shows an illustration of how the roughness of the surface can affect the measured value of the contact angle.
Figure 4.3: Depiction of how small variations in the hydrate shell caused by surface roughness can alter the measured value of the contact angle.
As such, each measurement reported here is the average of a minimum of 10 measure- ments on 5+ droplets. Error reported for contact angles are the standard deviation of all measurements at a particular condition.
58 Measurements of the contact angle of a water droplet on a cyclopentane hydrate surface yielded a value of 94.17◦±4.98◦. This value represents 70 measurements on over 35 individual droplets on hydrate surfaces conducted by multiple operators on two separate apparatuses. This result is somewhat non-intuitive; since the hydrate shell is predominantly composed of water, it was expected that the hydrate shell would have a lower contact angle, corresponding to a more hydrophilic surface [74].
4.2 Predictions Using Capillary Bridge Theory
Using this updated value for the contact angle, the Capillary Bridge Theory was re- examined to estimate the value of the embracing angle, α. This parameter was calculated as α = 0.1◦ by Aman et al. [34] using values for the contact angle by Asserson et al. [74]. However, the system used by Asserson et al. utilized Freon hydrate in a water bulk phase with a chloroform droplet phase. This system may differ greatly from the cyclopentane sys- tem from which Aman et al. took the remainder of the measurements. Using the contact angle for water measured directly on the cyclopentane hydrate surface should improve the accuracy of predictions for this system. The height of the liquid bridge, H, is also unknown for a cyclopentane hydrate system. Therefore, the effect of both the height of the liquid bridge and the embracing angle were examined using the updated contact angle. Using a contact angle of θ = 94◦, the force was calculated for α = 0.025◦ − 5◦ and H = 0.001µm- 5µm. The results are shown in Figure 4.4, where the red values are not physical, as they are negative; the yellow values are within one order of magnitude of the measured force of 4.2 mN/m; and the green values are of the same order of magnitude as the measured force. Based on the values in Figure 4.4, it is likely that either the height of the liquid layer or the embracing angle must be higher than the values originally used by Aman et al. Many of the values that predict the force most accurately use a liquid layer height of 5 µm, which is likely an unrealistic value since a layer of that size would likely be visible on the surface of the particles but has not been observed in experiments. Therefore, it is likely that the embracing angle is higher than 0.1◦ for this system. Values near α =5◦ gave values closest to
59 Figure 4.4: Investigation of possible value combinations for α, the embracing angle, and H, the height of the liquid bridge, which predict cohesion force based on Capillary Bridge Theory. Measured force for this system was 4.2 mN/m.
60 the measured force of F=4.3 ± 0.4 mN/m. Using the previous estimate of H=50 nm based on the height of a liquid layer measured on ice, α was solved using the Capillary Bridge Equation, resulting in α =4.9◦. Figure 4.5 shows how an embracing angle of this magnitude would appear on a hydrate particle.
Figure 4.5: Hydrate particle with the embracing angle, α =4.9◦ (on each side) visualized.
4.3 Contact Angle Changes with Surfactants
In order to investigate the changes that occur to the hydrate shell when chemical additives are present, contact angle measurements were also performed with several model surfactants present. Initially, three surfactants were selected based on the study detailed in Chapter 8. Many of the parameters for these chemical additives were already measured, such as the cohesion force and interfacial tension changes, so after the contact angle was measured, each system could be predicted using the Capillary Bridge Theory (CBT) using the value α =4.9◦. These results are shown in Table 4.1 and indicate that the predictions are off by large margins. This indicted that, like the contact angle and interfacial tension, the embracing angle and height of the liquid bridge may also vary upon the addition of surfactants. In order to investigate the possible changes according to the CBT, force values were predicted at various values of H and α to see what magnitude these parameters could have in order
61 to match the measured forces more closely.
Table 4.1: Measured and predicted values for cohesion force in the presence of surfactants. Forces predicted using the Capillary Bridge Theory and values measured for each system (contact angle and interfacial tension); the height of the liquid bridge and the embracing angle were assumed constant (H = 50nm and α =4.9◦, respectively).
Chemical Contact Angle F (measured) F (predicted) Error - Degrees mN/m mN/m % None 94±5 4.3 4.3 0 PVCap 0.5 wt.% 97±12 4.01 -7.74 293 Arquad 0.5 wt.% 156±4.5 3.63 -90.41 2586 DDBSA 10−8M 89±9 2.39 27.96 1067
For the PVCap system, the contact angle was very close to the value measured for pure hydrate. The force calculations using the CBT indicate that either the liquid layer must be very large or the embracing angle must increase even further from the value obtained for pure hydrate (Figure 4.6). A combination of both effects is also possible. The liquid layer could larger for systems containing surfactants than for pure systems, as some additional surface melting might be induced by the additive (especially in systems where methanol is a solvent). An embracing angle of α =8.8◦ (the angle for which F=4.01 mN/m and H=50nm) would indicate that the liquid bridge is spreading out very significantly beyond the point of contact for the hydrates as can be observed in Figure 4.7. Arquad gave a significantly hydrophobic contact angle when tested, though the force observed was reduced only slightly for this system (15.5% reduction). This quaternary ammonium salt shows that there must be more components in the mechanism of AAs than a simple increase in hydrophobicity (Arquad is not a significantly effective anti-agglomeration agent). In Chapter 3, it was observed that the baseline system predicted a negative force for hydrophobic contact angles. The force predictions in Figure 4.8 show that forces are only predicted as positive for very small embracing angles combined with large water layers. The forces predicted never reached the order of magnitude of the forces that were measured for this system. As is shown in Chapter 5, many AAs that have high contact angles are also
62 Figure 4.6: Force value predictions for different values of H and α for a system containing PVCap. Measured force value was F=4.01 mN/m.
63 Figure 4.7: Hydrate particle showing the extent of an embracing angle α =9◦ (on each side) based on the predictions from CBT. This would indicate that the liquid layer spreads out very significantly for a PVCap-containing system.
highly effective at reducing cohesion forces; however, there must be other criteria that need to be met simultaneously in order for these predictions to be accurate. DDBSA caused the hydrate shell to become slightly more hydrophilic than a hydrate without any additive. The forces predicted in Table 4.1 overestimated the magnitude of this force. As seen in Figure 4.9, there exists a wide range of conditions that yield forces close to the measured force for this system. The forces for this system are high even for low values of H and α, indicating that there must be a factor limiting the values of these parameters. Aman observed that for concentrations of DDBSA higher than 10−5M, morphological changes occurred that included the rapid conversion of the unconverted water from the center of the hydrate [45]. It is possible that a similar phenomenon occurs in this system, to a smaller degree. If the DDBSA caused the conversion of the free water on the surface of the particle such that only a minimal amount of water remained, the force would be reduced even though the particles were more hydrophilic. Increasing the concentration of the surfactant would also increase the effect, as well as possibly increasing the likelihood that a sufficient amount of surfactant could lead to macro-scale morphological changes.
64 Figure 4.8: Predictions using CBT for the force in a system containing Arquad. Measured force was F=3.63 mN/m.
Figure 4.9: Model predictions using CBT to predict the force between hydrates in a systems containing DDBSA. Measured force was F=2.39 mN/m.
65 4.4 Conclusions
The measurements here represent the development of a new method which has potential to aid future studies and may yield more understanding of a variable that has been mostly unexplored previously. Contact angle was shown in Chapter 3 to be an important param- eter influencing the cohesion force between particles, and has been mostly unexplored in hydrate studies. The method developed in this chapter represents a simple, rapid method for measuring changes in the contact angle for pure systems as well as systems containing surfactants. It was found that the contact angle of water on the hydrate surface in a cy- clopentane bulk phase was 94.17◦±4.98◦, which was a surprising result; since the hydrate is comprised of water cages, it was expected that the hydrate surface would be water-wet (low contact angle). Ideally, contact angle measurements could be used with other measured data such as the interfacial tension to predict the forces that would result from a given system. These predictions would be significantly improved by an understanding of the liquid layer on the surface of hydrate particles. If measurements such as atomic force microscopy could be conducted to measure the water layer on the particle under a variety of thermodynamic and chemical conditions (such as the measurements Doppenschmidt et al. took on ice [38]), then the validity of the Capillary Bridge Equation could be better tested, and improvements specifically for hydrate systems could be adopted, if necessary. The new data would be significant because if a model could be found that would accurately predict the cohesion force, oil systems and chemical additives could be rapidly tested to determine their efficacy. With rapid screening, specialized chemicals could be produced, including targeted chemicals based on well characteristics or green chemicals that abide by environmental regulations. Modeling efforts for parameters such as the water occluded in a hydrate bed (i.e., water that is in contact with hydrates, or contained within hydrates, but has not converted due to surfactant activity or mass transfer limitations) would benefit from an understanding of water/hydrate interactions under a variety of conditions. In addition, understanding the
66 behavior of the water layer would give significant insight into AA mechanisms, which have been shown to rely on a significant combination of variables.
67 CHAPTER 5 INDUSTRY AA RANKING STUDY
Studies performed on the MMF in the past have always focused on model surfactants and well-characterized chemical additives, with the exception of crude oil testing. However, the anti-agglomerants (AAs) used in industry are mixtures of many chemicals; they contain not only the active AA chemicals, but also are solubalized, stabilized and dispersed by a variety of other chemicals that also must be taken into account when examining the cohesion forces with AAs.
5.1 Cohesion Tests and IFT
Typically, AAs are evaluated using rocking cells [69, 85]. These measurements can be time-consuming and typically qualitative and may not offer all of the information that oper- ators need to know about the performance of AAs. The MMF is able to provide quantitative data about the cohesion reduction of a chemical as well as visual data that may give insight into the mechanism by which AAs work. Multiple tests can be conducted per day, making the MMF an attractive option for ranking AAs. In order to validate the MMF as a rapid screening method for AAs, it first needed to be demonstrated that the MMF could match results obtained in traditional testing methods. The first AA that was tested is referred to as the “Flowloop AA”. This AA was used in the 2014 Exxon-Mobil flowloop tests. The Flowloop AA was shown to significantly reduce the pressure drop increase due to hydrate formation as well as exhibiting demulsification effects upon hydrate formation. This AA was tested in the water phase, dosed prior to hydrate formation as well as in the oil phase after hydrate formation had occurred. When the AA was present during hydrate formation at 2 vol.%, it typically induced morphological changes in the hydrate particle, shown in Figure 5.1. The hydrates tended to nucleate in multiple locations; some crystals adhered to the glass fiber and helped to stabilize the particle so
68 that it didn’t fall off over the course of the experiment. Conversely, crystals which nucleated on other areas of the water droplet typically dropped to the bottom surface of the droplet due to gravitational effects. Rather than spreading along the water/hydrocarbon interface as is typical for hydrate growth, hydrates formed using 2 vol.% of the Flowloop AA grew directionally, extruding outward from the water droplet in a single direction (Figure 5.1). The growth was also significantly slower than hydrate growth when no AA was present.
Figure 5.1: Morphological changes observed in cyclopentane hydrate when formed with 2 vol.% of the Flowloop AA dosed in the water phase prior to hydrate formation.
The cohesion force between two hydrate particles formed using the Flowloop AA was unmeasureable, with forces smaller than the fibers used in the study could detect. Addition- ally, the interaction between a hydrate particle and water droplet (also containing 2 vol.% AA) was measured. The water droplet was observed not to interact with the hydrate par- ticle. Based on the discussion in Chapter 4, this corresponded to a contact angle of 180◦. Figure 5.2 shows a water droplet interacting with a hydrate particle formed in the presence of 2 vol.% Flowloop AA which did not undergo many of the morphological changes described previously.
69 Figure 5.2: Hydrophobic interaction between a water droplet and a hydrate particle, both with 2 vol.% Flowloop AA.
Hydrates were also tested in the presence of 1 vol.% Flowloop AA. While the morpho- logical changes were similar, they occurred to a different degree. While there was still some directional growth, where the hydrate grew away from the particle in a single direction, there was more conversion of the water droplet at the water/hydrocarbon interface, leading to shorter directional growth than for 2 vol.% AA as shown in Figure 5.3. The growth oc- curred much more quickly, taking about two-thirds of the time that the particles at 2 vol.% took to convert into hydrate. Cohesion forces between two hydrates formed in the presence of 1 vol.% of the Flowloop AA still exhibited zero hydrate forces. Based on these tests, it was important to find out whether the MMF could successfully measure changes in the force for effective AAs such as the Flowloop AA. As a screening tool, more sensitivity would be necessary than a simple pass/fail test. With this objective in mind, tests were performed using decreasing concentrations of the Flowloop AA. Forces were not observed for concentrations over 0.25 vol.%, but below that concentration, forces were observed to increase with a decreasing concentration of the AA. The results of this test are shown in Figure 5.4. From this, it was concluded that the MMF could provide more
70 Figure 5.3: Morphological changes induced in hydrates formed using 1 vol.% of the Flowloop AA dosed in the water phase prior to hydrate formation.
detail than simple pass/fail data, and the need for tests with more AAs was also recognized for further validation of this new method for potentially screening AAs. Several AAs were obtained through Nalco Chemical Company. These AAs had already been tested and ranked though Nalco’s internal procedures, but the information was held until the MMF ranking was complete. Thus, as a blind study, the results could not be influenced by any expectations. These AAs were numbered AA1 through AA4, and each underwent testing for cohesion force, interfacial tension, and contact angle; morphological changes were also observed for each system. The solubility for each AA, determined using a dispersion test, is listed in Table 5.1. Dispersion tests were conducted by observing whether a drop of the AA dispersed when injected into either water or mineral oil. For these studies, AAs were dosed into the cyclopentane phase after hydrate formation had occurred. The procedure was performed in this manner due to the interaction between the AAs and the water droplets. For some of the AAs tested, rather than forming a hydrate phase that adheres stably to the glass fiber, the hydrates nucleated at multiple points and did not interact with the glass fiber or with one another. This resulted in particles with no
71 Figure 5.4: Cohesion force for hydrate particles created with varying concentrations of AA dosed in the water phase.
Table 5.1: Phase solubility of each AA tested in the ranking study, determined through dispersion tests.
AA Solubility Flowloop Water AA1 Oil AA2 Water AA3 Water AA4 Water
72 solid shell; rather, multiple hydrates adhered to the water interface. Since these particles deformed on contact, cohesion experiments could not be performed. In other cases, as the hydrate began to form on the water droplet after the ice melted, the extremely low interfacial tension in the systems caused the water droplet to fall off the glass fiber before hydrates could form. From an experimental standpoint, it was therefore necessary to allow the hydrates to form partially in pure cyclopentane and inject AA into the solution afterward. While not ideal, this method allowed for cohesion measurements to be performed on all AAs tested. In order to obtain consistent results, all AAs were dosed after hydrate formation for this portion of the study. The results for the cohesion tests are shown below in Figure 5.5, where the baseline force for pure hydrate particles is also shown. All AAs in the figure were dosed at 0.1 vol.% relative to the oil phase. While hydrate dosing is typically measured relative to the water phase, the experimental cell in the MMF contains no free water except that in the droplets which form hydrates. Since the droplets have extremely small volumes relative to the oil phase in the cell (1x10−4ml water vs. ˜30mL cyclopentane), dosing based on the water phase resulted in minuscule AA doses that raised concerns about partitioning from the oil phase to the water phase. Based on advice from consortium members, it was concluded that a lower dose based on the oil phase would provide the most realistic results.
Figure 5.5: Cohesion force measurements in the presence of AAs dosed at 0.1 vol.% of the oil phase. The Flowloop AA showed no measurable force.
73 As can be seen from Figure 5.5, the Flowloop AA as well as AAs 3 and 4 all performed effectively, with reductions in force greater than 90% for each. AA1 and AA2 had poorer performances of 39% and 26% force reductions, respectively. This analysis agreed with the ranking performed by Nalco, indicating that the MMF can be used to capture the relative effectiveness of AAs. The true strength of this method will increase as more chemicals are tested. If a database can be built using many AAs from different producers, then as new chemicals are tested, they can be reported as a comparison to the database on hand. AAs could be ranked as “top 10%”, “top 25%”, “bottom 50%”, etc., giving chemical producers an idea of how it compares to other products on the market without revealing the manufacturers’ products to their competitors. IFT measurements were also taken for each AA system. IFT is typically a poor predictor of cohesion force, though the reductions of several of the AAs in this study are similar for both IFT and cohesion force, as shown in Figure 5.6. This once again highlights the complex relationship between the interfacial variables that govern cohesion. For example, IFT alone would predict that AA4 would be the most effective, performing significantly better than all other AAs. However, its cohesion reduction is less than the Flowloop AA and very similar to AA3, despite differences in the IFT changes measured.
5.2 Morphological Observations
The strength of the MMF testing procedure lies not only in its ability to screen chemicals; it also provides important information on morphological behavior of hydrates in the presence of AAs. For each of the AAs tested, videos were created showing any changes in shell structure or other morphological changes that the hydrates experienced when AAs were added. For the Flowloop AA, it was shown previously that the growth mechanism was altered significantly when the AA was present during hydrate growth (Figure 5.1). However, when the AA was added after the hydrate shell had already formed, no further growth similar to the hydrate growth with AA was observed. The hydrate shell darkened slightly after AA
74 Figure 5.6: (Top) Comparison of changes in the cohesion force and interfacial tension values for the AAs used in the ranking study. The cohesion force for the Flowloop AA was zero. (Bottom) Correlation between measured IFT and force values.
75 addition, as shown in Figure 5.7. Additionally, small water droplets were observed to be exuded from the interior of the particle, where much of the water remained unconverted. These water droplets were typically less than 10 µm in size and fell from the exterior of the particle rapidly despite the lack of shear in the system.
Figure 5.7: Hydrate particles before (left) and after addition of the Flowloop AA (right). Particles have a slightly darker color, and small water droplets can be seen on the exterior of the particles (circled in red). Note: the dots in the image are constant, due to dust inside the camera used to record the images.
AA1 also caused changes in the unconverted water on the interior of the particle. This AA was observed to first draw in cyclopentane from outside the particle, creating cyclopentane “bubbles” that can be seen in the top right image in Figure 5.8. As the particle continued to draw in cyclopentane, the unconverted water was displaced to the exterior of the droplet. The water droplets in this case were significantly larger than those observed for the Flowloop AA, with diameters usually ranging from 50-150 µm. The water droplets were observed to adhere to the exterior of the hydrate particle, which can be seen in the bottom image of Figure 5.8. Despite the prolonged contact, the water droplets were never observed to convert to hydrate. While the water droplets appeared to be stable on the surface of the hydrate particles, they did not seem to participate as a liquid bridge during cohesion tests. When a water drop
76 Figure 5.8: Morphological changes due to the addition of AA1. The top left image shows two hydrate particles prior to AA addition. After AA1 was dosed to the cell, cyclopentane was drawn into the hydrate particles (top right). As the hydrate shell filled with cyclopentane, unconverted water was excluded to the exterior of the hydrate shell, where it remained without converting to hydrate (bottom image).
77 was trapped between two hydrate particles it simply slid out of the way, remaining on the hydrate particle out of the way of the contact surface. Figure 5.9 shows two hydrate particles in contact where the water droplets remain apart from the cohesion interaction rather than contributing to a liquid bridge.
Figure 5.9: Hydrate cohesion test when AA1 was present. Water droplets adhere to the exterior of the hydrate particle but do not appear to augment the liquid bridge.
AA2 was the least effective in the MMF tests at reducing the cohesion force. It also displayed the least reaction to the addition of AA, morphologically. Figure 5.10 shows a hydrate particle pair both before and after addition of AA2. The “after” image appears slightly darker, but no changes were observed in the hydrate particles themselves. Water exclusion may be a common mechanism for AAs, as it was seen in three of the five AAs that were investigated in this study. Each AA presented the phenomenon in a slightly different manner. After AA3 was added to the hydrate particles, they were observed to rapidly darken in color and exclude a significant amount of water droplets which easily separated from the hydrate shell and fell into the surrounding bulk fluid. Figure 5.11 shows this progression. Water droplets appeared on the order of 5-50 µm. AA3 was highly effective at reducing cohesion force, showing that the water that had been excluded onto the hydrate surface did not increase the capillary bridge to increase cohesion. Water droplets remained for only a short time on the hydrate surface before falling into the bulk fluid.
78 Figure 5.10: Hydrate particle reaction to the addition of AA2. No morphological changes were seen between the pure (left) hydrates and the hydrates with AA (right).
Figure 5.11: Pure hydrates (left) and hydrates which have excluded water after the addition of AA3.
79 Though it was difficult to see in many experiments, when AA3 was added to a system, a thin, gel-like layer was observed to form on the surface of the hydrate. In order to observe this phenomenon better, a large hydrate particle was created as shown in Figure 5.12. The gel-like layer moved slowly along the hydrate surface if it was agitated using a cantilever. This gel tended to dissipate with time and did not appear to alter the cohesion forces observed.
Figure 5.12: Gel-like layer that was observed to form on the surface of some hydrates when AA3 was present. The dark area near the end of the red arrow is the edge of the hydrate, while the translucent area above the arrow shows the gel-like layer.
Another effect that AA3 had on hydrate particles became apparent near the end of cohesion trials. Even though the pre-load force is not a strong force, hydrate particles altered by the presence of AA3 tended to become very brittle near the end of an experiment. This led to several particles beginning to break apart as can be seen in the progression of images in Figure 5.13. This destabilization of the hydrate shell may be due in part to the exclusion of the water from the interior. Because the unconverted water escapes the hydrate shell very quickly, cracks and fissures may form in the hydrate shell. Typically, this sort of imperfection in the shell would be repaired over time as additional hydrate grows to fill the cracks. However, due to the interaction with the AA, no water is left available to form additional hydrate. AA4 displayed a different mechanism than others tested in this study. When added after the hydrate had formed, small chunks of hydrate began breaking off of the shell and falling
80 Figure 5.13: Hydrate particles affected by AA3. Intact hydrate particles (left) became brittle and crumbled due to a small pre-load force (right). away into the bulk phase. This AA has been described as a “true” anti-agglomerant based on this property; even hydrate that has already formed cannot cohere with other hydrate, triggering the sloughing of small hydrate particles away from the original mass as depicted in Figure 5.14. In order to perform cohesion tests on these particles, the pure hydrate had to be annealed for additional time. If additional annealing time was not used, the hydrates would slough off until only the unconverted water remained on the glass fiber. With increased annealing, a layer of hydrate remained intact, allowing measurements to be obtained. Based on the changes described, it appears that the AAs are able to infiltrate the hydrate shell much more quickly than one would expect based on mass transfer limitations. Each AA began to cause morphological changes within seconds of its addition to the experimental cell. If the AA could not penetrate into the interior of the shell, then it is unlikely that the significant effects on the water interior seen with the AAs in this study would be observed. The rapid morphological changes may indicate that the AAs move through pores in the hydrate shell rather than diffusing, or introduce defects into the shell that allow the AA to migrate rapidly through the shell.
81 Figure 5.14: Pure hydrates (left) darkened in color upon AA4 addition and small amounts of hydrate sloughed off of the particles (right).
5.3 Contact Angle Measurements of Industrial AAs
Based on the observations of the morphological changes caused by the Industrial AAs, it was hypothesized that changes in hydrate hydrophobicity may be a central mechanism by which these AAs operate. Excluding the water that existed, unconverted, on the interior of the particle indicated that the hydrate/water system was no longer energetically favorable as it was formed, and that the system might minimize its energy by limiting the water/hydrate contact. Based on the method developed in Chapter 4, contact angle measurements were performed on each of the AA systems to investigate how water was interacting with the hydrate shell. For the measurements where the AA was added to the oil phase, the water droplets used were DI water. For those measurements where the AA was dosed directly to the water phase, water with AA was used as the droplet phase. Recalling that the cohesion force was tested as a function of AA concentration for the Flowloop AA (Figure 5.4), contact angle measurements were taken for the same systems. Where the force was zero, the contact angle measured 180◦, but for concentrations where cohesion forces became measurable, the contact angle began to drop. Figure 5.15 shows the contact angles measured for each system.
82 Figure 5.15: Contact angle measured as a function of AA concentration for the Flowloop AA dosed in the water phase.
Contact angles were also tested for each system in the AA ranking study. Images from these tests are shown in Figure 5.16. The values measured for multiple repeat particles and water droplets are shown in Table 5.2.
Table 5.2: Measured values for the contact angles for each AA ranked. Values of 180◦ indicated that the droplet would not interact with the surface, and no variation was observed over multiple measurements, leading to zero errors for these values.
Additive Contact Angle (deg) Error N/A 94.17 8.47 Flowloop AA 180 - AA1 87.95 6.87 AA2 44.61 5.58 AA3 161.25 3.11 AA4 180 -
Based on the contact angle measurements and the cohesion force values presented in Fig- ure 5.5, it appears that a relationship between the cohesion force and the contact angle exists for the AAs tested in this study. This relationship, presented in Figure 5.17 as an aggregate of all the measurements on industrial AAs, supports the hypothesis that a dominant mecha- nism by which AAs operate is changing the hydrophobicity/ wettability of the hydrate shell.
83 Figure 5.16: Images showing the measured contact angles for each of the AA systems in this study.
Making a hydrate surface energetically unfavorable for water interactions would significantly limit the capillary bridge that could form between particles. Without knowing the chemical makeup of the AAs that cause these changes, it is difficult to deduce a structure-property relationship based on the mechanisms observed. It was observed in Chapter 4 that the model surfactants used did not cause changes that correlated well to the contact angle, though Arquad did cause an increase in the contact angle similar to those seen in the effective AAs tested in this study. Arquad is a quaternary ammonium salt, the structure of which is shown in Figure 5.18. Many AAs contain quaternary ammonium compounds as active anti-agglomerant com- pounds [63]. The R groups in these compounds can be varied widely and often contain one or more short-tailed groups as well as one or more groups with long hydrocarbon tails. One hypothesis for the mechanism by which the AAs tested in this study work could be based on the idea that quaternary ammonium compounds adsorb onto the surface of the hydrate due to hydate-philic groups, or groups that can be contained in one of the hydrate cages.
84 Figure 5.17: Relationship between contact angle and cohesion force. Trend line provided to guide the eye.
Figure 5.18: Structure of a quaternary ammonium salt, where R1 − R4 represent alkyl or aryl groups, which can be the same or different.
85 This could leave one or more long-chain hydrocarbon groups extending outward from the surface of the hydrate [1, 86]. Long-chain hydrocarbons are hydrophobic, so packing the hydrate interface would reduce the shell’s affinity for water and decrease the wettability of the surface. The degree by which the changes in the hydrate shell hydrophobicity change could be based on the specific long chain groups as well as the packing efficiency at the hydrate interface. Figure 5.19 shows a hypothetical interaction between a hydrate shell and quaternary ammonium groups with hydrocarbons tails.
Figure 5.19: Hypothetical mechanism by which AAs could alter the hydrophobicity of the hydrate shell. Quaternary ammonium compounds with several hydrate-philic tails could ad- sorb onto the hydrate surface or incorporate into the hydrate cage structure. The remaining long-chain hydrocarbon tails would extend out into the bulk oil phase, reducing the local affinity for water and decreasing the wettability of the hydrate shell.
It is important to note, however, that these AAs need to be tested as much as possible using a formation method where the AA is present during hydrate formation. As can be seen from the tests using the Flowloop AA, hydrates can display significantly different behavior when interacting with hydrate that is growing versus hydrate that has already had a chance to form. Investigating and understanding these differences may also offer further insight into AA mechanisms. Additionally, testing more AAs may show that contact angle does not always correlate well with cohesion force (such as with Arquad, shown in Chapter 4; note however that Arquad has been shown to have low effectiveness as an AA [87]). It is likely, given the breadth of chemicals used as AAs, that a single mechanism will not be sufficient
86 to describe all of the interactions. Based on AA mechanisms observed in this study as well as flowloop experiments per- formed in the University of Tulsa and Exxon Mobil [88, 89], two different mechanisms have been proposed for how AAs affect hydrates in a flowline. Figure 5.20 shows the proposed mechanisms; in the top image, the system begins with water emulsified in oil with relatively large water droplet sizes. After the AA is added, the emulsion becomes finer, resulting in small water droplets. The hydrate then nucleates and is able to convert a significant portion of the water into hydrate. The hydrates are then able to flow as a fine, non-interacting slurry where the capillary bridges are minimized due to the high water conversion and the AA effect. In the second mechanism (bottom of Figure 5.20), the AA does not influence the emul- sion characteristics of the system prior to hydrate nucleation. This results in larger hydrate particles that consist of hydrate shells surrounding unconverted water. Due to the AAs, the hydrates then break into smaller pieces and release the unconverted water through mech- anisms such as hydrate sloughing, shell embrittlement or water extrusion. A fine slurry consisting of both hydrates and water droplets results, but the hydrates and water do not agglomerate significantly due to the hydrophobic hydrate shells.
5.4 Conclusions
The studies shown here verify that the MMF can correlate with industry testing processes by matching the ranking of the AAs tested in a blind study. The MMF also offers advan- tages in visualization that many apparatuses cannot offer; observation of the morphological changes that different AAs have could yield better understanding of their mechanisms. It was also shown that for industrial AAs, both the interfacial tension and the contact angle showed a strong correlation for the force measured in each system. This deviates from the observations for model surfactant systems where neither IFT or contact angle correlated with the force values. Observations using the MMF as well as flowloop systems led to two different mechanisms proposed for the effect of AAs on a flowing system.
87 Figure 5.20: Two proposed mechanisms describing how AAs facilitate slurry flow. (Top) AAs reduce the average droplet size in an emulsion, resulting in small hydrates that have converted the majority of the water into hydrate. Due to the lack of free water and the AA effects, the hydrates flow as a slurry. (Bottom) Hydrates form around relatively large water droplets, creating hydrates shells that surround unconverted water. Due to the morphological changes caused by AAs, the hydrates break down and release the unconverted water; however, the water and hydrates tend not to interact because of the hydrophobic hydrate shells caused by AAs.
88 If model AAs could be found that showed similar effectiveness to the industrial AAs (for which the compositions were unknown), then a structure-property relationship might also be investigated. For all the AAs used in the studies in this chapter, it was found that the contact angle of water on the hydrate surface was strongly correlated to the cohesion forces measured, with hydrophobic particles (oil-wet hydrates) corresponding to smaller forces. A possible mechanism where long hydrocarbon tails on quaternary ammonium salts reduce the hydrophilicity of the hydrate particle is one hypothesis by which this correlation could be obtained.
89 CHAPTER 6 ADHESION MEASUREMENTS WITH WAXES AND ANTI-AGGLOMERANTS
Hydrate agglomeration in the bulk fluid of a flowline can cause viscosification of the fluid phases, making it harder for the slurry/mixture to flow. However, flowline blockages due to hydrate can occur not only because of cohesion of hydrate particles in the bulk, but also adhesion of hydrate onto the flowline walls, leading to deposits that can slough or grow until they completely restrict flow. Adhesive behavior has been studied in the MMF previously [50, 51], but many facets of this complex phenomenon are still unexplored. This section presents a study focused on the interaction between waxes and AAs, with the hypothesis that waxes may alter the effectiveness of otherwise effective AAs. Waxes frequently deposit on the pipe surface in flowlines because the wall temperatures are low enough to precipitate the wax out of the oil. This represents a fundamental alteration of the flowline from a hydrate adhesion standpoint because the wall shifts from a typically hydrophilic steel surface to a hydrophobic surface coated with waxes. Wax surfaces have been tested previously using Candelilla wax, and it was found that a wax coating decreased the adhesive forces [45]. This study began with baseline force measurements using Candelilla wax and stainless steel surfaces as have been used in the past and progressed from there. Table 6.1 shows the breakdown of experiments performed for this study. AAs 1-3 in this study do not represent the same chemicals from the studies in Chapter 5, but a separate set of industrial AAs. As can be seen in Table 6.1, four general groups of experiments were performed. The first comprised baseline experiments; this study represents measurements that have not been extensively explored on the MMF before and required the establishments of proper proce- dures and baseline measurements to enable force comparisons later in the study. The second set of experiments involved testing the interactions between AAs, hydrates, and surfaces,
90 Table 6.1: Experimental matrix of the study of the interaction between waxes and AAs.
91 both for stainless steel and wax-coated steel. For the third set of experiments, a sample of crude oil was included in the bulk fluid to investigate whether crude oil further changed the surface interactions with AA. Finally, cohesion tests using the AAs were performed with wax dissolved in the bulk phase as well as in the presence of AAs.
6.1 AA Baselines
The three AAs used in these experiments are summarized in Table 6.2.
Table 6.2: Summary of AAs used in this study. Dose is relative to the oil phase. AA3 was dosed at lower concentration due to morphological changes.
Reference Solubility Dose (vol.%) AA1 Water 0.2 AA2 Oil 0.2 AA3 Water 0.1
AA1 and AA2 were both dosed at 0.2 vol.% of the oil phase. This dose was altered for AA3 due to significant morphological changes that caused the particles to destabilize at 0.2 vol.%. Similar to the method used in Chapter 5, AAs were dosed in the oil phase and added after hydrate formation had been initiated. It was found that AA1 and AA2 caused no morphological changes when added after the hydrate shell had been formed. AA3 caused the particle to exclude water and the shell to become extremely brittle. Figure 6.1 shows a progression of images taken just as the AA was added to the bulk fluid. The first image shows an unaltered hydrate particle, while the second image depicts the water being excluded from the interior of the hydrate shell in small droplets. In the third image, the shell is breaking apart, revealing the remaining unconverted water from the center of the droplet, which can be seen on the right side of the particle. Cohesion force values were measured for each of these AAs to determine whether they were effective at reducing agglomeration in the bulk phase before testing their effectiveness at reducing adhesion with the wall. All of the AAs tested for this study worked very effectively at reducing the cohesion force (Table 6.3). AA1 and AA3 reduced the force to a value lower
92 Figure 6.1: Morphological changes caused by the addition of AA3 to a pure hydrate particle (left) including water exclusion (middle) and breaking of a brittle hydrate shell (right).
than the limits that the MMF can measure, while AA2 reduced the cohesion forces in the system by 96%. The effective force reductions for cohesion indicate that each of these AAs would be effective at reducing the agglomeration of hydrates in the bulk phase, even when wax was present in the flowline. It also shows that waxes can naturally decrease the cohesive forces between hydrates even when AAs are not added.
Table 6.3: Cohesion force values measured for each AA used in this study and reductions from the force between hydrate particles with no AA present.
Additive Cohesion Force Reduction Cohesion Force (5 wt.% wax) Reduction mN/m % mN/m % None 4.3±0.3 - 2.53±0.26 41 AA1 0 100 0 100 AA2 0.17±0.013 96 0 100 AA3 0 100 0 100
Stainless steel surfaces were prepared for these experiments by soaking them overnight in a mixture of water and cyclopentane. This ensured that the surfaces were not too dry to conduct experiments; in the past, it had been determined that forces with stainless steel surfaces were too low to measure unless the surface had been pre-treated in this manner [45]. The wax-coated surfaces for this study were prepared by first melting the wax in a 60 ◦C oven in a closed container to prevent evaporation of the lighter components. A stainless
93 steel surface was dipped briefly into liquid nitrogen to cool it, then dipped into the wax for several seconds so that a smooth wax coating could deposit onto the steel surface. It was found that each of the AAs used in this study reduced the forces below those measured on bare steel. AA1 reduced the forces below measurable levels, while AA2 and AA3 showed more moderate decreases. Figure 6.2 shows these values.
Figure 6.2: Forces between a stainless steel surface and a hydrate particle with and without AAs present in the oil phase. AA1 showed zero force.
The zero values for AA1 presented a problem: force reductions based on the addition of wax would be impossible to measure from an already zero value. In order to probe the possibility of increasing the forces so that measurements would be easier, several variations were attempted for this AA. Dosing the AA into the water phase at 2 vol.% gave the same zero forces as 0.2 vol.% in the oil phase. To increase the forces further, water droplets were deposited onto the steel surface (Figure 6.3). When no AA was present, a significant increase in forces was observed, as shown in Figure 6.4. When AA1 was dosed in the water phase at 2 vol.% and a DI water droplet was used, the forces dropped significantly, but when AA1 was present in both the hydrate and the droplet, the forces were once again too low to be measured. Based on this, further experiments on AA1 were not performed. This leads, however, to important insights on
94 Figure 6.3: Experimental setup for experiments between a hydrate particle and a water droplet deposited on the steel surface. partitioning of AAs within a system. AAs are typically injected into an oil phase and expected to mix into the system through diffusion and turbulence in order to reach the areas in which they are needed. If the mixing or diffusion in the system is not sufficient, then the potential effect of the AA used is significantly decreased.
6.2 Wax Baselines
Candelilla wax was used to test the effect of a wax coating. Candelilla wax is a naturally derived product from the candelilla plant, a bush native to Mexico. The composition of candelilla wax is given in Table 6.4 [90]. It is notable that this composition differs greatly from the petroleum waxes generally found in flowlines. This wax was used initially to develop a procedure using waxes as well as to reproduce previous experiments performed with this wax [45]. An image of the steel surface coated in candelilla wax and submerged in the experimental cell is shown in Figure 6.5. Candelilla wax yielded a reproducible baseline, but the resulting forces were very low. This highlighted the need to increase the sensitivity of the cantilever used in these measure- ments to decrease the uncertainty. Figure 6.6 shows the results of four separate experiments, with Trials 1 and 2 performed using a shorter glass fiber with a higher k value. Once the new cantilever with a very long glass fiber had been created, the error on Trials 3 and 4 were
95 Figure 6.4: Interactions between a steel surface with a water droplet deposited onto it and a hydrate particle without and with AA1 in various phases. Zero forces were observed when AA1 was present in both the droplet and the hydrate.
Table 6.4: Composition of candelilla wax from Kuznesof [90].
Component Amount (wt.%) Hydrocarbons 42 Wax/resin/sitosteroyl esters 39 Lactones 6 Free wax/resin acids 8 Free wax/resin alcohols 5
96 Figure 6.5: Image of steel surface coated in candelilla wax submerged in cyclopentane. The area on the right which is submerged in the cyclopentane has a lighter color from the wax on the handle, which remained dry. reduced. This is important because these measurements are near the very low end of the MMF’s measurement capability, so having accurate pull-off trials builds more confidence in the measurements.
Figure 6.6: Baseline experiments showing the repeatability of measurements on a candelilla wax coated surface. Trials 3 and 4 show smaller errors due to the creation of a longer cantilever.
Due to the solubility of the candelilla wax in cyclopentane, the physical appearance of the wax-coated surface changed as it soaked in the cyclopentane. The wax became lighter in color, as can be seen comparing the wax in the cyclopentane in Figure 6.5 to the wax on
97 the cantilever above the cyclopentane. It also became softer and more pliable to the touch. The adhesion force for candelilla wax coated surfaces was therefore tested as a function of saturation time in cyclopentane to investigate whether the physical changes observed corresponded with changes in the adhesion force.
Figure 6.7: Adhesion force measured for different amounts of saturation time for a candelilla wax coated surface soaking in cyclopentane. Each data point represents a single experiment with 40 pull-off trials.
Figure 6.7 shows the force measured for the same particle pair as a function of saturation time up to 180 minutes. Over this time period, there were no changes observed for the adhesion force, despite the physical changes undergone by the wax. Based on feedback from industry professionals, development began on a wax that would better mimic the waxes found in production scenarios. A sample composition of a wax from an operational well was provided as shown in Figure 6.8. In order to match the sample wax composition, paraffin wax was used as a base [91]. Lighter hydrocarbon components from C17-C20 were added to the paraffin to create a wax with a composition closer to a realistic petroleum wax. This wax was then tested to obtain baseline measurements for the adhesion force. The first property observed from this wax was that the solubility of the wax in cyclopentane was much higher than the candelilla wax. While the candelilla wax would slightly change in color and texture over the course
98 Figure 6.8: Design of a synthetic wax to match the composition of wax from a flowline (blue). The base for the synthetic wax was paraffin, shown in red, and lighter hydrocarbon components (C17-C20) were added to create the synthetic wax shown in green. of hours in cyclopentane, the custom wax would fully dissolve in pure cyclopentane after approximately one hour. The adhesion forces measured for the custom wax system were also higher than for the candelilla system, which makes it an easier system to see changes with AA (Figure 6.9). However, as the baseline measurements were being taken, it was observed that the force value was steadily increasing. It was hypothesized that this increase may be due to an increase in the amount of dissolved wax that was accumulating in the cyclopentane with repeated measurements.
Figure 6.9: Baseline measurements for the custom wax taken without changing out the cyclopentane bulk, resulting in increasing adhesion forces with increasing wax concentrations.
99 In order to verify this phenomenon, baseline tests on the custom wax were repeated, replacing the cyclopentane bulk with fresh cyclopentane between measurements. As can be observed in Figure 6.10, this method resulted in very repeatable values for the adhesion force. It also confirmed the hypothesis that the adhesion force is affected by the amount of wax in the system for the custom wax. The dependence on wax concentration observed in the custom wax system but not the candelilla system implies that the wax composition may have a significant effect on the adhesion behavior observed in a flowline system. Testing varied wax compositions from a variety of geological areas and different wells in the future would allow further insight into the composition changes that may increase or decrease adhesion in the presence of waxes.
Figure 6.10: Baseline measurements for the custom wax with new cyclopentane used for each trial to avoid wax buildup in the system.
6.3 Wax/AA Interactions
The dependence of adhesion forces on the amount of dissolved wax in the system was investigated for pure cyclopentane systems as well as systems containing AA2 and AA3. Adhesion forces between a wax-coated surface were tested for multiple concentrations of wax up to 5 wt.% in the cyclopentane bulk phase. It was observed that both the system with no AA and the system with AA2 displayed an increased adhesion force with the initial addition of wax to the system, then the force plateaued with the addition of further AA (Figure 6.11). The system containing AA3 was tested at 0 wt.% wax and 5 wt.% wax, and
100 was found not to show a dependence on the wax concentration over this range. Adhesion forces were consistently highest when both dissolved wax and AA2 were present in the system, followed by the system with no AA. The AA3 system yielded the lowest forces. The bottom portion of Figure 6.11 shows the changes in each system relative to the system where no wax was dissolved into the bulk cyclopentane. This shows that the system with no AA experiences a larger increase than the system with AA2. The system containing AA3 shows a slight decrease with increasing wax concentration.
Figure 6.11: Dependence of the adhesion force on the amount of the custom-made petroleum wax dissolved in the cyclopentane bulk phase for systems containing either no AA, AA2 or AA3 (top). The bottom figure shows the relative change, using the force value with no added wax as a baseline. *Force tests not performed for AA3 at 1.3 wt. % wax. All measurements performed using a wax-coated surface.
101 It is important to note the differences in the adhesion measurements for a wax-coated surface versus a plain stainless steel surface. As can be observed in Figure 6.12, the force in the system with no AA decreases from a stainless steel surface to a wax-coated surface. This effect could be partially explained by a change in the roughness of the surface (becoming smoother with the addition of wax), which has been shown to cause a decrease in force [51]. Additionally, the surface becomes more hydrophobic for a wax surface than for a steel surface. The subsequent increase with the addition of dissolved wax indicated that the wax may be depositing onto the hydrate surface. Since waxes are typically hydrophobic, the interaction between a wax-coated hydrate and a wax surface may be greater than a more hydrophilic hydrate surface with the same wax-coated surface. Based on the plateau in the force increase between 1.3 wt.% and 5 wt.% wax, the surface of the hydrate may not require a large amount of wax to become saturated.
Figure 6.12: Comparison of adhesion forces between stainless steel surfaces (left-most data) and wax coated surfaces (right three sets of data) with and without AAs.
In contrast to the measurements without AA, AA2 begins with the lowest force observed for a stainless steel surface. When the stainless steel was substituted for a wax-coated sur- face, the adhesion force increased even when no dissolved wax was present; adding dissolved wax further raised the force. A possible hypothesis for this is the presence of long-chain
102 hydrocarbons in AA2 that caused the hydrate surface to become more lipophilic, reducing the interaction with a water-wet steel surface but increasing the interaction for a wax-coated surface. Further addition of dissolved wax to the system could amplify the oleophilic effect, resulting in further force increases as the wax packs the hydrate surface. This hypothesis is summarized in Figure 6.13, where low forces result from the interaction of surfaces with dissimilar wettability (e.g., a wax-coated surface with a hydrate particle or an AA-coated hydrate particle with a stainless steel surface). Conversely, when surfaces with similar wet- tability interact, the forces can increase, such as the interaction between a hydrate particle and a stainless steel surface, a wax-coated surface and a hydrate to which wax had adsorbed, or a wax-coated surface and an AA-coated particle.
Figure 6.13: Hypothesis for the increase in forces observed in systems containing waxes. Surfaces with dissimilar wettability tend to interact with higher forces than surfaces where there is a large difference in wettability.
103 AA3 is water-soluble, and therefore less likely to contain significant long-chain hydro- carbons, as these could impede dissolution into water. This hypothesis is supported by the trend in the adhesive forces with wax, which showed no significant changes either from a stainless steel to a wax surface, nor with the addition of dissolved wax into the bulk phase. As the wax was present in the cyclopentane prior to hydrate formation and the addition of AA, the lack of dependence on the presence of dissolved wax cannot merely be explained by steric effects (i.e., waxes and AA3 do not use the same binding site on the hydrate surface). AA3 may cause hydrophobicity changes in the hydrate shell that make the interaction with waxes less favorable. One notable change occurred when both AA3 and wax were present in a system: the brittle shell resulting from the AA addition was stabilized in the presence of wax. As shown in Figure 6.1, the hydrate shell was likely to become brittle and even break over the course of an experiment. Great care had to be taken during the experiments so that the particle remained intact over 40 pull-off measurements. The pre-load force used was very light; even so, out of seven experiments performed using AA3 with no wax, five failed at some point during the experiment. In contrast, when wax was also present in the system, out of seven experiments performed, only one failed to the point where continuing the experiment was no longer possible. The increase in particle stability with the addition of AA3 occurred despite the hydrate particles undergoing the same morphological changes described for AA3 previously. The hydrate shells still appeared to become brittle, but the shell failing did not result in the hydrate and water from the center falling off the cantilever. Instead, the hydrate persisted even when portions of the shell fell off. Figure 6.14 shows a hydrate particle created with the custom wax and AA3 where a large chunk of hydrate broke apart and fell from the main particle, exposing the water interior. Despite this large cavity, the particle remained stable throughout the course of the experiment.
104 Figure 6.14: Image of a hydrate particle formed with wax and AA3, where a portion of the hydrate broke off, exposing the interior water. Due to the presence of wax in the system, rather than causing a total failure of the particle, the shell was stable enough to continue the experiment.
6.4 Crude Oil Addition
To approximate a flowline environment, adhesion forces were also tested in the presence of crude oil. In order to dose the oil appropriately, the thermodynamic and visibility limits of the industry-provided crude oil (Crude Oil S) sample were tested in bulk cyclopentane. The oil provided was a light crude, and glass beads were used to determine how much oil could be added to cyclopentane before the light microscope could no longer penetrate the oil. Past experiments have limited oil addition to under 10% based on visibility limitations, but the crude oils used were typically heavier, darker oils [44]. The visibility limit for Crude Oil S was found to be around 60 vol.% in cyclopentane. However, using hydrate particles to test for thermodynamic limitations showed that the particles began to melt if more than 25-30 vol.% oil was added. The melting phenomenon was attributed to lack of sufficient hydrate former near the particle as well as changes in the thermodynamic equilibrium based on the addition of crude oil. Figure 6.15 shows a hydrate particle and a wax-coated surface in the presence of 25 vol.% crude oil.
105 Figure 6.15: Hydrate particle and wax-coated surface in a 75/25 vol.% mixture of cyclopen- tane and Crude Oil S.
Adhesion measurements were taken in the presence of Crude Oil S with and without AAs as shown in Figure 6.16. The increase in adhesion noted for the system without AA and the system with AA2 were eliminated when crude oil was present. However, when both the crude oil and AA2 were present, the AA appeared to offer little additional force reduction from the value when only crude oil was added. AA3 had similar force values between the case with 5 wt.% wax and the case with crude oil. These data are somewhat difficult to interpret because the crude oil that was added was drawn from an active well. As such, many compounds may be present in the oil which are unknown. It is likely that the oil has residual waxes in it, though many of the waxes may have precipitated as the oil was cooled after its extraction. It also likely contains other chemicals, such as treatments for corrosion or scale, or even other anti-agglomerants from the well production (which could effectively increase the concentration of AAs in the test environment significantly). It also likely contains natural surfactants, which could be better classified by their interactions with AAs if it were known that the oil did not contain other additives. Without a clear view of what is in the oil sample, it is difficult to know how these different factors may interact.
106 Figure 6.16: Adhesive forces between a wax-coated surface and a hydrate particle in the presence of various additives.
6.5 Conclusions
It was be concluded based on this study that waxes and AAs can and likely do interact in flowline conditions. This study showed that for the wax composition tested, the adhesion force increased for both a pure system as well as a system containing AA (though not for every AA tested). Based on some industry observations, the presence of wax typically indicates that hydrates will be less problematic. The results of this study directly contradict that observation, indicating that interactions caused by wax systems are more complex than previously thought. While particle-particle cohesion force reductions do not seem to be impacted negatively by mixtures of waxes and AAs, the coexistence of these components may affect the adhesive forces between hydrates and the pipe wall detrimentally for flow assurance. The effects observed were specific to the custom wax; candelilla wax behaved in a different manner, indicating that the composition of the wax could influence the magnitude of the interactions with AAs. Additionally, the AA used could have a significant effect on the wax interactions. While the oil-soluble AA2 showed increased forces when dissolved wax was added to the bulk phase, water-soluble AA3 had no dependence on the presence of wax.
107 CHAPTER 7 SHELL STRENGTH
The contents of this chapter are modified from a paper published in Physical Chemistry Chemical Physics entitled “Micromechanical measurements of the effect of surfactants on cyclopentane hydrate shell properties.” [70] Used with permission.
Erika P. Brown1 and Carolyn A. Koh2
1Primary reseacher and author
2Professor, co-author and thesis advisor
Investigating the effect of surfactants on clathrate hydrate growth and morphology, es- pecially particle shell strength and cohesion force, is critical to advancing new strategies to mitigate hydrate plug formation. In this study, Dodecylbenzenesulfonic acid and Polysor- bate 80 surfactants were included during the growth of cyclopentane hydrates at several concentrations above and below the critical micelle concentration. A novel micromechanical method was applied to determine the force required to puncture the hydrate shell using a glass cantilever (with and without surfactants), with annealing times ranging from imme- diately after the hydrate nucleated to 90 minutes after formation. It was shown that the puncture force was decreased by the addition of both surfactants up to a maximum of 79%. Over the entire range of annealing times (0-90 minutes), the thickness of the hydrate shell was also measured. However, there was no clear change in shell thickness with the addition of surfactants. The growth rate of the hydrate shell was found to vary less than 15% with the addition of surfactants. The cohesive force between two hydrate particles was measured for each surfactant and found to be reduced by 28% to 78%. Interfacial tension measure- ments were also performed. Based on these results, microscopic changes to the hydrate shell
108 morphology (due to the presence of surfactants) were proposed to cause the decrease in the force required to break the hydrate shell, since no macroscopic morphology changes were observed. Understanding the hydrate shell strength can be critical to reducing the capillary bridge interaction between hydrate particles or controlling the release of unconverted water from the interior of the hydrate particle, which can cause rapid hydrate conversion.
7.1 Introduction
The properties of the hydrate shell are important for determining the extent of agglomer- ation as well as events such as deposition and sloughing. Since hydrate particles can contain unconverted water, a breakage of the shell caused by compression or shear could release this water, leading to extremely high cohesion forces [58] and/or rapid conversion to further hydrate (due to the presence of additional nucleation sites generated from shell breakage). Therefore, further insight is needed into the properties of the hydrate shell and how they are affected by factors such as chemical additives. The strength of hydrate particles has been considered mostly in the context of natural hydrates cementing mineral deposits in deep ocean sediment conditions [61]. Little work has been performed in terms of the shell strength of hydrates in a production scenario. Previous reports of the thickness of the hydrate shell have been mainly inferred from lateral film growth measurements and found to be highly variable depending on the system conditions. Previous results of the hydrate film/shell thickness varied from tens of microns to several millimeters [92, 93]. The hydrate former (guest molecules), temperature and pressure conditions, and experimental procedures varied significantly in these previous experiments; therefore, there was little agreement among the measurements. Surfactants may also affect the growth of the hydrate shell; Karanjkar et al. [94] observed that the addition of Sorbitane monooleate to cyclopentane hydrates (Structure II) changed the growth geometry of the shell from a faceted shell when no surfactant was present to a hollow cone when 0.01 wt% of Sorbitane monooleate was present.
109 For this study, the MMF apparatus was used to measure the force that is required to puncture/disrupt the hydrate shell to determine the shell strength and hence potential for water leakage of the shell to enable further nucleation/growth of hydrates, as well as the cohesion force between hydrate particles both with and without the presence of surfactants. Interfacial tension measurements were also performed. A variety of parameters including annealing time, subcooling (the difference between the equilibrium temperature and exper- imental temperature), and the addition of surfactants were examined to determine which parameters had the strongest effect on hydrate shell growth, strength and cohesion.
7.2 Materials
Cyclopentane (Sigma Aldrich >99% purity) was chosen as the hydrate former for this study. Cyclopentane is able to stabilize Structure II hydrates (the same hydrate structure as that formed in flowlines) at atmospheric pressure. In addition, cyclopentane is non-miscible with water and forms a stable hydrate phase below 7.7◦C, such that experiments can be performed above the ice point to avoid ice contamination in the experimental results. Two surfactants were chosen for this study: Dodecylbenzenesulfonic Acid (DDBSA; 98% purity, Pilot Chemical Company) and Tween 80 (Spectrum Chemical Manufacturing). The struc- tures of these surfactants are shown in Figure 7.1. Mineral oil 70T (STE Oil Company, ρ = 0.8558 g/cm3) was used as described above to test the relative changes in interfacial tension without the high volatility of cyclopentane; using cyclopentane would make long measurements impractical. DDBSA is frequently used as an industrial dispersant, while Tween 80 has been studied as a model surfactant. Two concentrations of each surfactant were selected: 10−8 M was used for each surfactant as a value below the Critical Micelle Concentration (CMC). For Tween 80, a secondary concentration of 10−4 M was used to be above CMC, which for Tween 80 is approximately 10−5 M when measured in a water bulk phase [95]. Concentrations of DDBSA above 10−5M caused rapid morphological changes which are not the focus of this work [45], so 10−6 M was selected, which is still below the CMC of approximately 1 M in a bulk phase
110 Figure 7.1: Chemical structures of DDBSA (top) and Tween 80 (bottom). of Mineral Oil 70T [45].
7.3 Results and Discussion
The force necessary to puncture a hydrate shell may be an important parameter in high shear conditions. If the particle shell is weakened by the presence of surfactants, shear and collisions with other particles may be enough to break open the hydrate shell, releasing the unconverted water inside the shell. This water would be able to convert rapidly as it builds upon the existing hydrate and encounters multiple nucleation sites. Figure 7.2 shows a hydrate particle that was punctured using a glass cantilever. Upon breaking the hydrate shell, the water rapidly moved up the cantilever and converted into hydrate. The interior of the hydrate particle filled with cyclopentane as the water was displaced. While this phenomenon was not seen in all puncture force experiments, possibly because hydrates quickly grow to cover small cracks, it illustrates a possible effect of compromised hydrate shell integrity. Figure 7.3 shows the force required to puncture the hydrate shell as a function of annealing time at three different subcoolings (Equation 1.1). The force required to puncture the hydrate shell was shown to increase with annealing time, but it was only a weak function of subcooling. The growth rate of the hydrate shell is dependent on the subcooling [37]. However, the similar force/strength of the hydrate shell at various subcoolings indicates that the annealing period of the hydrate shell is more dominant than the subcooling. The
111 Figure 7.2: Hydrate particle before (left) and after puncturing (right). This illustrates an extreme example of the possible effects of lowering shell strength.
increase in shell strength as a function of annealing time indicated that this is a mass transfer limited process (Figure 7.3). In contrast to the minimal dependence of puncture force on subcooling, the puncture force showed a significant decrease when surfactants were added to the system (Figure 7.4). All of the surfactants, regardless of concentration, had a similar effect on the puncture strength, reducing it by 60% to 79% depending on the surfactant/concentration. This change in shell strength in the presence of surfactants indicates that the hydrate shell properties may be altered in some way; possibilities include slower growth leading to a thinner shell, or increased porosity compromising the integrity of the shell. In order to investigate further the decrease in shell strength with the addition of surfac- tants, the growth rate of the hydrate shell along the water droplet surface was measured for each surfactant system (Figure 7.5). The average values reported represent 8-10 separate experiments and a total of 70-100 measurements. A typical distribution of values is shown in the bottom of Figure 7.5. Variations of less than 15% were observed for the surfactant con- centrations studied and compared to the system without surfactant. These results indicate that the shell growth rate was not reduced significantly enough by surfactants to explain the decrease in puncture force. The thickness of the hydrate shell was therefore measured and compared for experiments with and without surfactants (Figure 7.6). The presence of
112 Figure 7.3: Force required to puncture the hydrate shell at three different temperatures for annealing times up to 90 minutes. Error bars represent the standard deviation of 4+ repeat measurements.
Figure 7.4: Force required to puncture the hydrate shell with and without surfactants. All experiments were performed at 0.3◦C. Error bars are one standard deviation of 4+ repeat experiments.
113 surfactants was found to slightly reduce the average hydrate shell thickness. However, this reduction is not statistically significant considering the scatter in the data. This scatter in the shell thickness values may be caused by variations in the z-direction positioning, as men- tioned in the section 7.2. The small change in shell thickness indicated in Figure 7.6 also does not appear to be significant enough to explain the large drop in shell puncture strength seen in Figure 7.4. The mass transfer of guest and water molecules across the hydrate shell is very slow [37, 96], with estimates ranging from 10-8 to 10-13 m2/s. Therefore, much of the initial shell growth would occur as a function of the concentrations of guest and water molecules at the interface. These experiments cover only short annealing times (up to 90 minutes), so very little shell thickening due to diffusion across the shell may occur. It is important to note that this effect can be dependent on the surfactant added. Other chemical additives may significantly affect the growth rate and shell thickness, such as Kinetic Hydrate Inhibitors, which slow the growth rate, or chemicals such as DDBSA (at higher concentrations), which accelerate growth [45]. However, the presence of surfactants may cause a more porous hydrate shell while si- multaneously altering the interfacial tension and the wettability of the hydrate shell. A higher porosity could increase cohesion by supplementing the water layer (i.e., increasing H in Equation 1.3), while the surface property changes would likely decrease cohesion (de- creasing γ and increasing θ). It is difficult to decouple the effect on the cohesion force from these competing factors, though changes in interfacial properties may dominate, since the cohesion force is reduced by all the surfactants tested. Figure 7.7 shows the cohesion force for hydrate particles with surfactants, compared to the baseline value of 4.2 mN/m without surfactant. The addition of surfactants had the most significant effect on the cohesion force, compared to other parameters measured (i.e., shell thickness, growth rate), with cohesion forces being reduced by 29% to 78%.
114 Figure 7.5: Growth rate of the hydrate shell with and without surfactants present (top) and typical distribution of values for growth rate measurements (bottom). Results shown in lower figure represent the values obtained for pure hydrate.
115 Figure 7.6: Thickness of the hydrate shell for surfactant and non-surfactant systems. Each data point represents a single experiment.
Figure 7.7: Cohesive forces between two hydrate particles. Error bars are 95% confidence intervals based on 160+ measurements using more than 4 separate particle pairs.
116 Interfacial tension (IFT) measurements were performed for each hydrate/surfactant sys- tem to investigate a possible correlation between the cohesion force, the shell puncture force and the interfacial tension. Figure 7.8 shows the IFT results for these systems. DDBSA likely shows no dependence on concentration because both values chosen are below the CMC [49]. The IFT results do not correlate with the cohesion force results; this difference may be due to changes in wettability that alter the term from Equation 1.3, or may be due to the presence of hydrates. Aman et al. showed that the apparent CMC for systems with hydrate may occur at lower concentrations than for systems not containing hydrate [49]. Tween 80 showed a significant decrease in IFT at the higher concentration, which is above the CMC [97].
Figure 7.8: Interfacial Tension measurements on systems with and without surfactant.
When examining the changes in shell strength, growth rate, shell thickness, cohesion and IFT, these data suggest the possibility of microscopic changes to the hydrate shell. It is possible that this change is caused by an increase in the porosity of the hydrate shell, though one might expect an increase in the cohesion force as a result of the increased water being transported from the unconverted center of the hydrate with increasing porosity. Changes in wettability caused by the surfactants could counteract this effect. Therefore, it seems that
117 the most likely change is an alteration of the shell’s microscopic structure itself, as observed in Karanjkar et al.’s study[94]. Although no significant differences in the macroscopic hydrate structure/morphology were observed, the microscopic structure may have been altered by the presence of surfactants in a way that compromised the strength of the hydrate shell. Figure 7.9 shows there is no significant difference in the macroscopic morphology of the hydrate crystals both without and with the surfactants added.
Figure 7.9: Visual comparison of macroscopic hydrate morphology without and with sur- factants present during growth, illustrating no significant morphological changes for these systems.
A hypothesis for the mechanism of this surfactant interaction with the hydrate shell is shown in Figure 7.10. Since these surfactants do not cause significant macroscopic mor- phological changes, it may be that rather than interact with the hydrate cages themselves, the surfactant molecules instead adsorb at the crystal interface as the hydrates are forming. These molecules can thereby provide steric hindrance for the growing hydrate shell, rather than form in the neatly ordered structure depicted on the left side of Figure 7.10. The hydrate shell may have microscopic irregularities associated with the hydrate crystallites growing around the surfactant molecules. This could account for a decrease in mechanical strength without the alteration of the thickness of the hydrate shell and lead to a simultane- ous reduction in the cohesion force due to the hydrate surface becoming oil wet rather than water wet due to the presence of surfactant molecules.
118 Figure 7.10: Conceptual picture showing how surfactants may cause steric hindrance and hence shell weakening during hydrate shell formation.
7.4 Conclusions
Surfactants can have a wide range of effects on hydrate particle structure and interactions, and these phenomena are just beginning to be investigated. The addition of DDBSA and Tween 80 to cyclopentane before hydrates are formed causes the hydrate shells to puncture with significantly less force than is required for pure hydrates. This indicates that the hydrate shell strength, in the presence of these surfactants, is reduced compared to pure hydrates. However, this change is caused neither by a change in the hydrate growth rate, nor by a difference in the thickness of the hydrate shell. The addition of surfactants was found from micromechanical measurements to reduce the hydrate particle cohesion force. Changes in the shell micro-structure or porosity of the hydrate shell could explain the changes in the shell strength. These insights allow a better understanding of the effect that chemicals can have when hydrates are formed during production. Advancing the understanding of the mechanics of the alterations in the hydrate shell with the addition of surfactants may allow development of more effective chemicals to prevent hydrate agglomeration and plugging.
119 CHAPTER 8 COMPETITIVE EFFECTS OF CHEMICALS
The contents of this chapter are modified from a paper submitted to Energy & Fuels entitled “Competitive Interfacial Effects of Surfactant Chemicals on Clathrate Hydrate Particle Cohesion”. Used with permission.
Erika P. Brown1 and Carolyn A. Koh2
1Primary reseacher and author
2Professor, co-author and thesis advisor
In oilfield production conditions, many chemicals are present in the form of natural surfactants that occur in the oil phase as well as additives that are injected to control the properties of the oil phase. These additives may include Low Dosage Hydrate Inhibitors such as kinetic hydrate inhibitors or anti-agglomerants, corrosion or scale inhibitors, dispersants, emulsifiers, etc. In order to achieve slurry flow in subsea oil pipelines, the agglomeration of hydrate particles must be minimized so that small, dispersed hydrate particles are trans- portable, rather than forming larger aggregates that may plug the pipeline. This study focuses on classifying the possible interactions of different classes of chemicals by measuring the cohesion force between hydrate particles. A Micromechanical Force apparatus was used to measure the changes in the cohesion force for cyclopentane hydrates when a dispersant, Dodecyl Benzene Sulfonic Acid, a kinetic hydrate inhibitor, PolyVinyl Caprolactam, and a model anti-agglomerant, Arquad 2HT-75, were added to the cyclopentane phase prior to hy- drate formation. The cohesion force was tested for each individual chemical as well as when combinations of the chemicals were present during the experiment. It was found that the interaction of specific chemicals can work synergistically (kinetic hydrate inhibitor + anti- agglomerant), where the cohesion-reducing effect is greater than either additive can produce
120 on its own; antagonistically, (dispersant + kinetic hydrate inhibitor) where the cohesion force is higher for the chemical mixture than for the separate chemicals; or show no interaction (dispersant + anti-agglomerant) where the cohesion force is similar to when only the more effective chemical is present.
8.1 Introduction
Cyclopentane hydrates have been used to study the effect of both physical and chemical variables on cohesion. Dieker et al. began studying the effect of chemical additives by investigating crude oils and their constituent components including acids and asphaltenes, which were effective at reducing cohesion [44]. Aman et al. continued this work by testing a variety of different additives to determine how they changed the cohesive forces [48]. In addition to the cohesion force, Aman et al. measured changes in interfacial tension [49]. By utilizing a range of concentrations, the packing density of several surfactants on both the water-oil and the hydrate-oil interface was estimated. It was found that reductions in the cohesive forces did not scale with changes in interfacial tension, indicating that other variables must also play a key role in altering the capillary forces. Studies have also determined that hydrate morphology changes can occur upon surfactant addition, which may be also a factor in agglomeration mechanisms [94]. However, despite several studies on the effects of chemical additives, cohesive forces have always either been measured for a single, well-characterized additive or an uncharacterized mixture of different chemicals, such as crude oil or asphaltenes. In actual flowline condi- tions, the chemicals present will represent a complex mixture comprised of both naturally occurring surfactants and chemicals that have been injected into the line to minimize flow assurance issues (e.g., corrosion, waxes, scale, etc.). These chemicals may interact with one another as well as with the hydrate particles that form. Understanding the influence of multiple chemicals and how they can change the effect that hydrate treatments may have on hydrate cohesion is necessary for developing more effective treatment strategies. For this study, chemicals are tested individually as well as in pairs, to investigate how the chemicals’
121 interaction with one another might affect the cohesive forces between hydrate particles. The chemicals chosen in this study represent broad classes of additives that may be found in flowlines and are used to illuminate the possible interactions between classes of chemicals.
8.2 Materials
Cyclopentane (Sigma Aldrich, >98%) was used as the bulk phase for these experiments. Since cyclopentane has a high volatility, a drip line was utilized to slowly replenish the cy- clopentane lost to evaporation, so that the concentration of the additives did not change over the course of the experiment. The chemicals used in these experiments represent different classes of chemicals that might be found in an oil production scenario. The structures of the chemicals for this work are depicted in Figure 8.1.
Figure 8.1: Structures of the additives used in this work: Dodecyl Benzene Sulfonic Acid (top left), Arquad (bottom left) where n=13-15, and a monomer of PolyVinyl Caprolactam (right).
Dodecyl Benzene Sulfonic Acid (DDBSA; Pilot Chemical Company, 97%) is an anionic surfactant used as a dispersant [98]. It may also have corrosion inhibition properties in certain systems [99, 100]. It has been shown previously [45] to cause morphological changes of hydrate particles at concentrations above 10−5 M. For this study, a concentration of 10−8 M was selected. DDBSA is oil soluble and was mixed into the cyclopentane. PolyVinyl Caprolactam (PVCap; Sigma Aldrich, 98%) is a kinetic hydrate inhibitor (KHI), which is used to delay the nucleation or growth of hydrates when operating at ther-
122 modynamically favorable conditions for hydrate formation. PVCap was used at 0.5 wt.%. PVCap is water-soluble and was dissolved into DI water prior to the creation of hydrate particles. Arquad 2HT-75 (Sigma Aldrich, 92%) is a quaternary ammonium salt. This class of compounds is frequently used as anti-agglomerants [63] (AAs) and as corrosion inhibitors [101]. Arquad has been studied previously as a model anti-agglomerant [87] and was used at 0.5 wt.% for these experiments. Arquad is not soluble in cyclopentane or water but was found to form a stable dispersion (over several days) in the water phase.
8.3 Single Chemicals
The cohesion forces between hydrate particles were tested first for each of the chemicals individually. DDBSA caused the largest reduction of the cohesive forces, causing the force to decrease by 44%. Arquad and PVCap were less effective, decreasing the cohesion force by 14% and 7%, respectively. These results are summarized in Figure 8.2. Arquad has been observed in autoclave tests to reduce agglomeration only in the presence of salt, i.e., when the surfactant is present singly. In these previous experiments, an autoclave system using crude oil only showed significant increases in motor current/torque that are analogous to a system plugging, but when Arquad and salt were both present, the torque increases were smaller and did not appear to plug the system [87]. This could indicate that Arquad does not effectively pack the interface unless another chemical species is present to stabilize it. PVCap is widely used in hydrate treatment as a KHI but has not been shown to be effective as an AA [102]. IFT measurements were also performed with each individual chemical (Figure 8.2). It has been shown previously that IFT measurements cannot be used to predict changes in cohesion force [49]. While the interfacial tension is a key parameter in the Capillary Bridge Equation, it is thought that changes in the wettability of the hydrate particle may accompany the interfacial tension changes caused by surfactants. This may cause changes to both the interfacial tension, γ, and the wetting angle of water on hydrate, θ, in Equation 1.3. Depending on the changes to wettability, these changes can sometimes
123 Figure 8.2: Cohesion force and interfacial tension measurements for pure chemical tests. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments.
124 have competing effects. Studies of the effect of wettability have been performed to determine the relative importance of each parameter on the cohesion force (Chapter 4). Figure 8.2 shows that DDBSA has the smallest effect on the IFT, decreasing it by only 16%. Arquad had the strongest effect, with a 68% reduction, and PVCap lowered the IFT value by 42%. Figure 8.3 shows a trace of a typical IFT experiment for each of the surfactants used. As Figure 8.3 shows, Arquad takes significantly longer to reach an equilibrium value than either of the other additives. It is therefore important to note that because the hydrate particles are given a 30 minute annealing time during cohesive force measurements, all of the surfactants should be able to reach an equilibrium interfacial concentration during the annealing time. No significant morphological changes were observed for the additives used; there was no rapid growth or dendrite formation as has been observed for chemicals that are known to cause morphological changes [45, 56, 103]. However, it was observed that the hydrate shell became more visibly textured and slightly darker in color for all experiments where Arquad was present. Figure 8.4 shows these differences in the hydrate shell.
Figure 8.3: Interfacial tension as a function of time for the three pure chemical species tested in a mineral oil bulk phase. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments. Pure mineral oil IFT baseline is 52mN/m.
125 Figure 8.4: Hydrate shell morphology when no additives are present (left) and when Arquad is present (right). Particles with DDBSA or PVCap (and no Arquad) appear similar to the particles on the left.
8.4 Chemical Mixtures
Following these baseline analyses, experiments were performed with two of the surfactants present at a time. It is important to note that for the experiments where multiple chemicals were present, the concentrations were not changed from the previous experiments. Therefore, the conditions where multiple chemicals are present represent a higher overall concentration of surfactants. Three different types of effects on the cohesive force were observed when two additives were present during an experiment. The first type of effect observed represented a situation where the addition of a second chemical had no observable effect on the cohesive force above that already observed for a single chemical. When DDBSA was tested singly, the reduction in force from that of pure hydrate was 44%. Figure 8.5 shows that when Arquad was also present, the force is reduced by only 45%. This may indicate that these two chemicals interact with the hydrate surface at the same sites and that the DDBSA was able to bind to these sites before the Arquad. Figure 8.6 shows IFT experiments for each mixture containing Arquad. Arquad took the longest amount of time to reach an equilibrium value when it was the only surfactant present, indicating that it had the lowest mobility of the surfactants tested. However, the IFT data show that when
126 Figure 8.5: Cumulative results for cohesion force and IFT measurements for pure and mixed chemical systems. DDBSA concentration was 10−8M, and Arquad and PVCap concentra- tions were 0.5 wt.% for all experiments. other species are present, the IFT reached an equilibrium value much more rapidly. This may indicate that the other species present were packing the interface more quickly than the Arquad could reach the surface. The second type of interaction observed was characterized as an antagonistic interaction. This classification indicated that the cohesive forces measured when a mixture of surfactants was added was higher than the forces for either of the surfactants used singly. When DDBSA and PVCap were present together in the hydrate system, the force returned to the levels seen for pure hydrates, despite a 44% reduction in force observed when only DDBSA was present and a 7% reduction for PVCap (Figure 8.5). This effect could be due to the preferential interaction of the surfactants with each other rather than the hydrate surface. It has been observed that PVCap will form coils and globules with Sodium Dodecyl Sulphate, which is similar to DDBSA [104]. These conformational changes can occur significantly below the Critical Micelle Concentration and may have caused the chemicals to migrate away from the hydrate/oil or water/oil interface in order to interact with one another. The third type of interaction was classified as synergistic. In this interaction, two chemi- cals which performed poorly when added alone reduced the cohesion force significantly more
127 Figure 8.6: Interfacial tension as a function of time for pure Arquad as well as mixtures con- taining Arquad. DDBSA concentration was 10−8M, and Arquad and PVCap concentrations were 0.5 wt.% for all experiments. Pure mineral oil IFT baseline is 52 mN/m. when used together. PVCap and Arquad only reduced the cohesion force by 7% and 15% respectively when each was tested separately. However, when tested together, the cohesion force was reduced by 57%, which was the largest reduction reported in this study. As men- tioned previously, Arquad was observed to have more favorable anti-agglomeration effects when it was used in the presence of 3.5 wt.% salt solution. It is possible that a similar inter- action between PVCap and Arquad occurs, increasing the packing of the surfactant at the interface. Research has been also performed investigating synergistic kinetic hydrate inhibi- tion effects between PVCap and quaternary ammonium salts [65, 105]. Though Arquad was not specifically investigated, it was found that several quaternary ammonium compounds increased the kinetic inhibition effect, delaying the onset of hydrate nucleation or growth more effectively than PVCap on its own. IFT measurements were also performed for each mixture of chemicals. The IFT trends found in these experiments were shown to be similar to the trends observed in the cohesion force value tests: the mixture of PVCap and Arquad reduced the IFT the most (95%), followed by the DDBSA and Arquad mixture (90%), and finally the PVCap and DDBSA mixture (39%). This may indicate that while the IFT is a poor indicator of cohesion force changes, it may be a reasonable indication of the inter-
128 actions between chemical additives. This study focused on three chemicals from different classes that might be found in a flowline. However, there are a significant number of other chemicals that should be also taken into account when studying the competitive effect of chemicals on hydrate cohesion, such as corrosion inhibitors, scale inhibitors, thermodynamic hydrate inhibitors, emulsifiers/demulsifiers, etc. Even within the classes examined in this work, there are a significant number of different functional groups and chemical types that can be used to achieve similar effects. More studies will be required to fully understand these chemical interactions and the contributing factors that determine whether the chemicals will be synergistic, antagonistic, or show no effect upon mixing.
8.5 Conclusions
When multiple additives were present in a system containing hydrates, the cohesion force was not altered in the same way as when a chemical was present separately. Some interactions were found to have no effect on the cohesion force, leaving the reduction in force the same as the force when only the more effective chemical of the pair was present. Other interactions were antagonistic, where a chemical that was effective at reducing the cohesion force performs poorly as a mixture, resulting in higher forces than when either chemical was used alone. Finally, the third type of interaction was a synergistic one. In this case, two chemicals that had little effect on the cohesive forces on their own caused a more significant reduction in cohesion when they were combined. These interactions may be explained by the alteration of the chemical interactions with the hydrate shell or by increased interactions of the chemicals with one another. Interfacial tension measurements were not a good predictor of changes in cohesive forces but may be a better indicator for the interactions between chemicals. Molecular experiments and modeling of the effects of a variety of other additives on hydrate particle interactions may be necessary in order to develop a clear mechanistic understanding of these complex interacting systems.
129 CHAPTER 9 FUNDAMENTAL COHESION STUDIES
Before understanding hydrate behavior in flowlines, it is important to build a compre- hensive picture of the effect of physical variables such as temperature, pressure, annealing time, etc. on hydrate particle interactions. In addition, it is important before investigating more complex systems, such as those involving surfactant additives, to understand the basic principles that govern hydrate cohesion. To that end, three studies of fundamental cohesion principles are presented below: a revised temperature dependence of hydrate particles using methods which have been refined since the temperature dependence of cyclopentane hydrate was first reported by Dieker [43], an estimate of the effect of annealing time on cohesion at different subcoolings, and a study where glass beads were utilized as a kinetically inactive hydrate substitute to examine the effect of THIs on cohesion.
9.1 Temperature Dependence
It has been shown previously that the cohesion force between hydrate particles is depen- dent on temperature [33]. Dieker observed a linear trend between temperature and cohesive forces as shown in Figure 9.1 [106]. However, the magnitude of the error bars in this first study suggest significant uncertainty of this trend. Additionally, measurements of the thickness of the liquid layer performed using Atomic Force Microscopy on ice particles by Doppenschmidt et al. [38] suggest that the volume of the water layer may increase exponentially as the temperature approaches the melting point. Measurements on ice by Hosler et al. also indicate that the adhesion between ice crystals increases exponentially as it approaches the equilibrium temperature [107]. Figure 9.2 shows ice cohesion data near the ice point. Since Dieker’s measurements, improvements have been made to the procedure, leading to more reproducible results in measuring the cohesive forces [45]. These improvements to the
130 Figure 9.1: Temperature dependence of cyclopentane hydrate measured by Dieker and the predicted temperature dependence using the Camargo and Palermo model [21, 106].
Figure 9.2: Cohesion force of ice crystals as a function of subcooling. Modified from Hosler et al. c American Meteorological Society. Used with permission. [107].
131 MMF procedure include an additional 10 second “resting time” where the liquid layers are allowed time after each pull-off measurement to re-equilibrate, and repairs to the vibration isolation table. The number of replicate experiments was increased from a single experiment to five replicate pairs of particles for each system measurement, leading to more confidence in each data point [45]. The result of these experiments is shown in Figure 9.3 with a trend line provided to guide the eye. This result is logical because the thermodynamic liquid layer that is hypothesized to exist on the exterior of hydrate particles should become exponentially larger as the particles approach their melting point [38]. Additionally, as the particles get colder, they approach a constant value where the liquid layer has reached a minimum and does not decrease with further cooling.
Figure 9.3: Temperature dependence of cyclopentane hydrate measured using improved techniques to increase accuracy and repeatability.
It was found that the uncertainty in the measurements was higher at lower subcoolings. This phenomenon has been observed throughout the experiments performed in this work, and was not limited to temperature. Higher forces tended to have a much larger spread in the force values obtained, while lower forces were typically more repeatable. The larger error for higher measurements may have to do with the larger liquid layer that may be associated with the stronger forces. A larger liquid layer may redistribute differently after
132 each measurement than a very small liquid layer would.
9.2 Annealing time
Based on the mechanism for hydrate formation proposed in Figure 1.8, hydrates formed in the MMF have a significant water layer remaining unconverted on the interior of the particle. It is likely that this water layer provides some of the liquid in the capillary bridge that causes cohesion. This supply may be through pores or cracks in the hydrate shell; in this case, increasing the annealing time may cause these pores to close, reducing the force. To test this theory, the cohesion force was tested at annealing times of 30, 60 and 120 minutes. Three different subcoolings were also tested. The results of these tests are shown in Figure 9.4.
Figure 9.4: Effect of annealing time on hydrate cohesive forces for three different subcoolings.
These results show that there is a decrease in the cohesion force with increased annealing times for all temperatures tested. It is notable that the decease for each temperature appears to be similar. Hydrate shell growth is typically temperature-dependent, with an increased driving force (i.e., a higher subcooling) causing faster formation of the hydrate shell. This
133 indicates that the annealing that occurs at longer times may be mass-transfer limited. In addition, the cohesion force values do not all converge onto a similar value, indicating that a temperature dependence still exists for these annealing times. It has been suggested that for significantly longer annealing times (>24 hours), the temperature dependence of hydrate cohesion may be significantly reduced [108]. However, these annealing times are not realistic, as hydrates that form in a flow scenario are likely to encounter other particles within a short period after their formation. A more pertinent study of completely annealed particles would focus on the cohesion between sufficiently small particles such that the majority of the water droplet is converted to hydrate during the initial growth period (50-100 µm in diameter).
9.3 Glass Beads
In order to investigate the importance of the water layer on hydrate cohesion, experiments were performed on glass beads. Glass beads were chosen initially as a simple analog to hydrate particles since unlike hydrate particles, which can be continuously reacting and converting and are therefore dynamic in nature, glass beads are an non-reacting system that could be used to isolate some of the phenomena observed in cohesion force experiments. Glass cantilevers were dipped in epoxy, then affixed to glass beads with diameters ranging from 100-200 µm as shown in Figure 9.5. The epoxy was tinted with orange coloration to allow visual assurance that the epoxy did not spread onto the experimental surface of the beads. The cantilevers were calibrated with the beads attached (for calibration details see [40]). Experiments were performed in a mixture of cyclopentane and mineral oil, with beads either dry or coated in water. Surfactants were also added at different concentrations to determine their effect on the surface water layer. For the dry beads experiments, the cantilevers were placed into an oven at 60◦C overnight. The wetted beads were soaked overnight in water or a mixture of water and surfactant. To test whether the beads were being adequately coated with water, experiments were run with varying amounts of cyclopentane in Crystal Plus 200T mineral oil (STE Oil Company, Inc.) which has a specific gravity of 0.856 at 25◦C and a viscosity of 39.9 cP at 40◦C. These data
134 Figure 9.5: Glass beads affixed to cantilevers. Glass bead diameter ranged from 100-200 µm.
are presented in Figure 9.6. When cyclopentane comprised more than 80 wt.% of the bulk phase, the wetted and dry beads performed the same, indicating that a liquid bridge was not able to form and increase cohesion between the particles. However, at lower concentrations of cyclopentane, the wetted beads took considerably more force to separate (e.g., at 20 wt.% cyclopentane, the wetted beads took 4x as much force to separate compared to dry beads), indicating that a significant liquid bridge existed under these conditions. This could be due to the higher solubility of water in cyclopentane compared to mineral oil, or a change in parameters such as the contact angle. If the contact angle of water on the glass beads was lower in mineral oil, corresponding to a more hydrophilic interaction, then the observed forces would also be higher. Adding 1 wt.% of Arquad, a model quaternary ammonium salt, caused the values to drop to the “dry” levels even at low concentrations (<40 wt.%) of cyclopentane. The addition of the surfactant to the system therefore appeared to disrupt the liquid bridge, weakening the forces between the particles.
135 Figure 9.6: Inter-particle cohesion forces between glass beads at different vol% of cyclopen- tane and 200T mineral oil where each point represents the average of 40 pull-off measurements on 5 replicate particle pairs.
A follow-up study was then performed using a solution of 20 wt.% cyclopentane and 80 wt.% mineral oil, with varying concentrations of monoethylene glycol (MEG). This concen- tration of the cyclopentane solution was chosen due to the fact that the beads exhibited different forces between the water-wetted and dry beads despite the increased size of the error bars. MEG was chosen due its ubiquity in pipelines as a thermodynamic inhibitor. A concentration range was tested to investigate the effects of under-inhibition on cohesion force. The line in Figure 9.7 represents the dry bead cohesion force using a 20/80 mixture of cyclopentane and mineral oil. After adding a small amount of MEG, the cohesive forces again dropped to the levels of the dry beads. These results show that the addition of chemicals to hydrate may decrease the volume of the liquid layer or alter its surface properties (such as surface tension), thereby reducing the cohesion force between particles. However, it is difficult to draw direct comparisons between glass beads and hydrates due to some important differences. Most notably, hydrates contain significant reserves of water, both in the hydrate and in the unconverted liquid interior contained within the hydrate outer shell. Therefore, the chemical additives may
136 Figure 9.7: Inter-particle cohesion forces between water-wetted glass beads with MEG addi- tion. The line represents the dry bead cohesion force. affect hydrates very differently from the glass beads because hydrates may have the ability to regenerate a liquid layer that has been affected by the presence of chemicals. In addition, there is likely a significant difference in the surface energies of glass beads and hydrates [109]. Thus, both the interaction between the water and the surface (hydrates or glass) and the interaction between chemical additives and the surface can differ. Finally, the quasi-liquid layer on hydrates is thought to be similar in magnitude to that of ice, 50-100 nm. It is uncertain what the size of the liquid layer is on glass beads. These factors can be significant, as indicated by Equation 1.3 which shows that the cohesion force is strongly dependent on the volume of the capillary bridge.
9.4 Conclusions
It was found in these studies that the cohesion force between particles decreased with increasing subcooling as well as increased annealing time. Subcooling effects persisted at annealing times up to two hours. The study of physical variables such as subcooling and an- nealing time is not only important for contextualizing the force trends observed in the MMF, but also for ensuring that experiments are reproducible by highlighting the importance of
137 consistency. Understanding the effect of physical variables is an important first step in the study of how hydrates behave in flowlines. In addition, these studies are able to be incorpo- rated into modeling packages such as CSMHyK, which highlights the importance of accurate measurements for values such as temperature and annealing time. Glass beads proved a difficult analogue to hydrate particles; while some of the kinetic variables (such as growth and annealing) were reduced by the substitution of glass beads for hydrate particles, other variables such as surface energy and hydrophilicity of the surface were also changed. Despite these shortcomings, glass bead experiments may still give insight into the effect of THIs on hydrate particles, which can be difficult to study in situ due to subcooling limitations. Experiments using glass beads showed that the addition of small amounts of thermodynamic inhibitors (THIs) reduced the bead interparticle forces of wetted beads to mirror the forces measured for dry beads. The water layer in under-inhibited systems (systems where not enough THI is present to completely inhibit hydrate formation) may interact preferentially with the THI to reduce the capillary bridges and therefore the interparticle force.
138 CHAPTER 10 HIGH PRESSURE MICROMECHANICAL FORCE APPARATUS
Cyclopentane hydrates have yielded many insights into hydrate behavior and have many benefits over other atmospheric pressure hydrate formers (e.g., Tetrahydrofuran). Similar to gas hydrate formers, cyclopentane is immiscible with water, and the hydrate equilibrium temperature is far enough from the ice point that ice contamination is not an issue. However, the question is always present as to whether cyclopentane hydrates actually provide an accurate view of how gas hydrates behave. It has been observed that methane hydrates (sI) grow much more quickly than cyclopentane hydrates (sII) [36], but other disparities between the two systems may also exist. In order to validate the data from the MMF, experiments must be performed on gas hydrates. The work in this thesis initiated the first design of a unique high pressure micromechanical force apparatus, including the identification of a nano-manipulator (the heart of the appara- tus) and initial prototypes of the cell system. Later cell designs, which have commonalities with the original design using the original nano-manipulators, led to the current system with the high pressure cell manufactured by Sejin Co. Using such an apparatus, gas hy- drate particle-particle cohesion measurements can be performed and compared with model cyclopentane hydrate systems. The initial studies indicate that the gas hydrate cohesion force is an order of magnitude larger than cyclopentane hydrate liquid phase measurements, and about three times the magnitude of cyclopentane hydrate gas phase measurements.
10.1 Apparatus Development
Adding high pressure to these experiments adds complexity to these experiments in sev- eral ways. All of the equipment must either be fully contained within the cell or connected through feed-throughs. Since visual observation is the basis of MMF experiments, viewing ports are needed to record the data and illuminate the cell. Perhaps the largest challenge in
139 this design lies in the necessary mobility of the particles. Accurate control of the particle is needed, and the velocity of the manipulator must also be controllable so that the measure- ments remain repeatable. The form factor of the manipulators must also be minimized so that the total volume of the cell is as low as possible, for safety reasons. Based on these criteria, the design of the high pressure cell began with the selection of nano-manipulators made by Klocke Nanotechnik, a German company. The manipulator is the crux of the micromechanical force apparatus. The manipulators selected were used in SEM imaging at near-vacuum pressures [110], so it was inferred that they would also stand up to higher pressures without issue [111]. The nano-manipulator consists of three stages which can each be controlled independently, one for each the x-, y- and z- directions. The x- and y- directional manipulators measure 3x5 cm when fully closed and have a stroke length of 2 cm. The large stroke length and small footprint are ideal, as the cell can be minimized in volume while still retaining a high versatility. As it was unknown how high the forces may be in various systems which would be tested in the high pressure MMF (HPMMF), allowing a large range of motion was an important consideration for this design. The z-direction module is 4 cm tall and mounts atop the x- and y- stages as shown in Figure 10.1. The stroke length in the vertical direction is 1 cm, which allows a hydrate particle to be brought into alignment with a second particle held by a stationary cantilever. The experimental cell was designed based on the dimensions of the nano-manipulators and went through several trial stages before the final design was created. Some of these designs were tested by 3D printing the cells out of ABS plastic in order to test how the components fit and moved before moving on to a steel construction. Figure 10.2 and Figure 10.3 show two of the preliminary designs. Figure 10.4 shows the pressure cell in its finished form (constructed by Sejin Co. [112]). Several features have been constant through each of the designs. The cell is surrounded by a jacket which circulates cooling fluids in order to maintain the temperature in the cell. Feed-throughs are necessary in order to accommodate the wires for power and data transfer.
140 Figure 10.1: Nano-manipulator used in HPMMF experiments.
Figure 10.2: First-generation design for the high pressure apparatus. The left section housed the nano-manipulators, while the right section was used to visualize experiments [113].
141 Figure 10.3: CAD design for another version of the pressure cell. The nano-manipulators would be attached to the lid in the large section on the right. The smaller section on the left was the visualization section used to observe the particles [114].
Finally, each cell has a mount where the stationary cantilever can be placed to perform experiments. The final version of the cell closely resembles the second re-design for the cell, inverted to improve the seals of the lid. The final version of the cell also contains several ports that are used for a pressure gauge and thermocouple, as well as gas feed and vent lines which were not needed in the printed preliminary designs. The maximum pressure for the cell is approximately 1500 psi.
10.2 Initial Testing
Initial tests were carried out using both methane/ethane and CO2 [115], and were verified repeatable by different operators as shown in Figure 10.5. It was observed that for gas phase experiments, the cohesion force was approximately one order of magnitude higher than liquid-phase cyclopentane measurements. Measurements of cyclopentane in the gas phase were approximately three times lower than the forces observed for gas systems. Based on the comparisons made in Chapter 3, this validates the use of a force value of F=50 mN/m made in modeling efforts for CSMHyK as a realistic assumption.
142 Figure 10.4: Current high pressure cell (Sejin Co.). The window on the left is used to visualize the experiments, and the two large holes in the center are used for the electrical feed-throughs for the nano-manipulators (top). Interior of the cell, showing the nano-manipulator holding a cantilever (bottom).
143 Figure 10.5: Cohesion force data for CO2 gas hydrates showing experimental repeatability for different operators. Previous data from Lee [115], new data by E. Brown.
Further experiments in a liquid phase will lead to a better understanding of the magnitude of the cohesion force for high and low pressure systems.
As validation using CO2 progressed, it was found that CO2 hydrates were not an ideal candidate for use with this system. Prior to the start of these tests, the outfitting on the cell
(such as the o-rings) was not checked for compatibility with CO2, and the repeated use of this hydrate former caused the o-rings to begin leaking. In addition, the nano-manipulators were mounted on an acrylic sheet which began to degrade, most likely due to the formation of carbonic acid in the cell. The corrosion caused the degradation of the acrylic, which then impeded the function of the nano-manipulators. Figure 10.6 shows some of the corrosion on the bottom surface of the nano-manipulator. This problem has been rectified, and other modifications have been performed on the cell to increase the repeatability and ease of experimentation. A pressure transducer was added to the cell, which will allow for continuous monitoring of the pressure throughout an experiment. Improvements were also made to the cooling line and saturation vessel, where
144 Figure 10.6: Corrosion on the nano-manipulator most likely caused by CO2 as the hydrate former. several supply lines were replaced using more apt equipment. Experimentation is ongoing, with initial studies focusing on reproducibility of measure- ments and the effects of physical variables such as annealing time, subcooling and particle size. Validating these parameters and comparing them to phenomena observed in the low pressure system will allow the first qualitative comparisons of high- and low-pressure systems to be made. After these validations, the most important update for the cell will be the in- corporation of a liquid cell. Once the measurements can be made in a bulk liquid phase, the comparisons between the two systems will be quantitative; surfactant-hydrate interactions will also be able to be studied in a system that contains a liquid bulk phase.
10.3 Conclusions
A new, high pressure Micromechanical Force apparatus was designed, with a focus on the nano-manipulators, which are crucial to the design of the cell. The nano-manipulators are capable of precise movement in three directions while only occupying a small footprint within the cell, allowing the inside volume of the cell to be minimized. Several iterations of the pressure cell were designed before the final cell was created by Sejin Co. Initial measurements on gas hydrates indicate that the forces are an order of magnitude larger than
145 liquid-phase cyclopentane measurements, which agrees with estimates made using CSMHyK. Initial measurements showed repeatability, but the cell was shown to be incompatible with
CO2 based on the degradation of the o-rings, adhesives and plastics in the cell. Experiments using a methane/ethane hydrate former are underway to validate the system and collect gas-phase data before the cell is modified to include the liquid phase.
146 CHAPTER 11 SUMMARY AND CONCLUSIONS
In order to properly predict and model hydrates in a flowline, accurate understanding of their agglomeration behavior is necessary. Agglomeration is a complex phenomenon that is affected by numerous factors including thermodynamic conditions, flow behavior, and the presence of surfactants (both natural components and synthetic additives) among other influences. This thesis work focuses on the role of cohesive and adhesive forces to highlight the effect of a variety of conditions on the inter-particle interactions. Much of the focus of this thesis was on the role of surfactants on hydrates and interfacial phenomena of the system; as industrial priorities shift from hydrate avoidance to hydrate management, understanding the role of different components in flowlines becomes of paramount importance. Below are brief summaries of the work performed and the conclusions drawn in each chapter of this work.
• Chapter 3 focused on sensitivity analyses of two equations that involve the inter-particle force. Capillary Bridge Theory can be used to predict the inter-particle cohesive force, given information on interfacial properties in the system. Using this equation, the effect of variations in each interfacial property on the force predictions was determined. It was found that the contact angle, representing the wettability of the hydrate surface, has a strong effect on the other variables in the model.
The Camargo and Palermo Model balances aggregation and shear forces to determine the size of aggregates in a flowline, which is used to predict the viscosification of the slurry in the presence of hydrates. The cohesive force between particles is an important variable in this model; sensitivity analysis of the other variables such as fractal dimension, oil viscosity and particle diameter show that the higher the cohesive force, the larger the effect on relative viscosity. This model highlights the importance
147 of understanding the cohesive forces in different systems. Better cohesion predictions should lead to more accurate model predictions, which will aid in hydrate treatment strategies.
• Chapter 4 developed a novel method for measuring the contact angle of a water droplet on the surface of a hydrate particle. Based on the analyses from Chapter 3, the contact angle of water on hydrate is an important parameter in the Capillary Bridge Theory. Measurements of the contact angle for pure systems as well as systems containing model surfactants showed that the addition of surfactants may cause variations in many interfacial properties related to the formation of a capillary bridge. The contact angle for a pure system was found to be 94.17◦±4.98◦, which was significantly less hydrophilic than was initially thought. The embracing angle, α, was estimated to be 4.9◦ for pure systems.
• Chapter 5 showed that in a blind study, MMF tests were capable of matching vendor tests ranking the effectiveness of AAs. These MMF tests have significant advantages over typical tests in terms of cost, sample volume required, quantitative ranking, mea- surement ease and time. In addition, the MMF was able to provide important visual data on the changes caused by these AAs to the hydrate shell morphology. Three ma- jor morphological changes were identified: water exclusion, shell embrittlement, and hydrate sloughing. Contact angle measurements taken on the hydrate surface in the presence of these AAs showed a strong correlation between the hydrophobicity of the hydrate surface and the cohesion force measured between the particles. A hypothesis involving adsorption of AA molecules onto the hydrate shell which leave hydrophobic hydrocarbon chains protruding into the bulk was proposed to explain the method by which AAs can alter hydrate wetting characteristics. Mechanisms were also proposed for the effect of AAs in flowlines based on observations from the MMF as well as flowloop systems.
148 • Chapter 6 detailed a study in which the interactions between waxes and AAs on the adhesive force (i.e., between a hydrate and a surface) were studied. It was found that interactions depended on the composition of the wax used; candelilla wax exhibited low forces and no saturation effects. Conversely, a petroleum wax created to mimic waxes found in flowlines showed dynamic adhesion force behavior with the concentration of wax dissolved in the bulk. The force reduction caused by AAs was also altered in the presence of waxes, for an oil-soluble AA. Using the petroleum wax, it was observed that for a system with no AA and for a system with an oil-soluble AA (provided by industry), the adhesive forces increased with an increase in the amount of dissolved wax in the system. However, a water-soluble industry AA did not show any effect with dissolved wax concentration. Cohesion force measurements indicated that all AAs tested were highly effective at reducing cohesion force, but that they may not be as effective at reducing adhesion when wax was present in the system. These observations challenge traditional industry knowledge which suggest that systems with waxes suffer fewer hydrate issues. The composition of the wax, the composition of the AA, and the other compounds present in a flowline may influence the wax/hydrate interactions.
• Chapter 7 involved using a cantilever to puncture the hydrate shell and measure the force necessary to break through the hydrate shell. It was found that the addition of surfactants during the hydrate formation process led to hydrate shells that were significantly easier to break, suggesting that the surfactants may lead to changes in the hydrate shell micro-structure, which then lead to a weaker hydrate shell. A novel measurement technique for spherical hydrate particles indicated that the shell thickness after 30 minutes of annealing for cyclopentane hydrate may range from 30-50 µm.
• Chapter 8 showed that additives may have different effects when they are present in conjunction with other additives than if they were added to the system singly. Three types of interactions were observed in this study, where a quaternary ammonium salt,
149 a KHI and a dispersant were used both alone and in pairs. The interactions were clas- sified as synergistic, antagonistic, or no reaction. Synergistic interactions caused the cohesion force to decrease more than either of the chemicals added alone. Antagonistic interactions caused less cohesion reduction than the more effective chemical, and addi- tives that caused no interaction showed the same force reduction when both chemicals were present as when only the more effective chemical was added. These data indicate that the interaction between chemicals that are often added to the flowline together, or between additives and natural components, may have a significant influence on the performance of anti-agglomerants (AAs).
• Chapter 9 explored how physical variables affect hydrate behavior as well as the effect of THIs on the cohesion between glass beads. It was found that, using improved methods to collect more accurate MMF data, the temperature dependence of hydrate cohesion increases significantly near the hydrate equilibrium temperature, displaying a logarithmic trend with subcooling. This supports the existence of a liquid bridge on the hydrate surface based on capillary bridging, as well as agreeing with published literature on the water layer on ice particles.
Annealing time was tested at a variety of subcoolings, and it was found that cohesion force decreased with longer annealing. The data also suggest that the dependence on subcooling persisted at annealing times up to two hours.
Glass beads were used to test the effect of THIs on cohesive forces, where it was found that the addition of a small amount of Monoethylene Glycol (MEG) would reduce the cohesion between wetted glass beads down to the cohesion value observed for dry beads. This indicated that the THI likely drew the water away from the surface of the beads, but analogies to hydrate were difficult to draw due to large differences in the glass bead and hydrate systems.
150 • Chapter 10 outlined the development process for a high pressure MMF apparatus. Initial designs of the apparatus included identification of a nano-manipulator and were tested using 3D-printed versions of the cell to determine the needs and capabilities of a high pressure version of the low pressure MMF apparatus. Initial experiments confirmed the repeatability of high-pressure measurements. However, it was found
that CO2 was not an appropriate hydrate former for this apparatus, as it caused degradation of the o-rings, the acrylic mount for the manipulator, and the adhesives used in the manipulator.
151 CHAPTER 12 SUGGESTIONS FOR FUTURE WORK
Future work on the MMF and related apparatuses can take many directions, based on the large amount of work done in the past as well as the topics presented in this thesis. Many of the topics that are selected for study are shaped and influenced by the scientific and indus- trial/technological knowledge gaps at a given time, so ongoing research must also reflect the priorities and struggles that are faced in the field. Below are listed three areas where research could be expanded to significantly further the understanding of agglomeration/aggregation phenomena in flowlines.
12.1 Expansion of surfactant studies
Each of the studies detailed in Chapters 4 - 8 could easily be expanded in scope to improve the mechanistic understanding of the phenomena involved.
• Contact Angle Measurements – Contact angle measurements on the surface of hydrates may vary based on the surface roughness, as discussed in Chapter 4. In order to study this effect, particles could be created at different subcoolings; slower-growing particles tend to have less roughness than particles which grow at high subcooling. Contact angle measurements could then be conducted on each particle set (after returning them to a constant experimental temperature) to determine whether the surface roughness pays a significant role in contact angle measurements. Additional experiments could also be performed with different components in the bulk fluid to investigate the effect of the bulk fluid on the contact angle measured.
Contact angles were poor predictors of force for the model surfactants used in Chapter 4. However, the AAs tested in Chapter 5 showed a strong correlation between the contact angle of water on the hydrate surface and the cohesion force measured in the
152 presence of the AA. This highlights the need for better understanding of the composi- tion of AAs in order to advance insight into the differences between model systems and AAs used industrially. For example, Arquad (shown to have poor AA performance) caused a significant increase in the hydrophobicity of the hydrate particles, but was not effective at reducing the cohesion force. Future studies could focus on investigating whether there exist other quaternary ammonium salts that are effective at reducing the cohesive force or if there are synergists that are necessary to promote the desired effects. Testing a wider range of well-characterized chemicals could give greater insight into which functional groups or synergists have greater effects changing the hydrate cohesion force and the contact angle, and would allow the hydrophobic hydrate particle mechanism for anti-agglomeration to be better understood.
In addition, measurements of the liquid layer on the hydrate surface would give sig- nificant insight into the predictive relationship between the Capillary Bridge Theory and the cohesive forces measured in the MMF. These measurements would be best obtained using cryo-atomic force microscopy or a similar technique. Measuring the magnitude of the water layer on the hydrate surface would strongly support or refute the assumptions made using the Capillary Bridge Theory. Advancing understanding of the natural water layer on hydrates could drastically improve modeling efforts as well as help develop new insights into “cold flow” in production scenarios (i.e., hydrates are allowed to form in the bulk where they neither agglomerate nor deposit and move through the flowline as a slurry).
• Industrial AA Ranking – The true strength of the study from Chapter 5 may lie in acquiring a large library of data. With a large number of AAs tested, the ranking study could be used to classify new AAs based on their relative effectiveness compared to other AAs in use. Since these measurements can be taken rapidly and also provide visual data on the effects of AAs on hydrate particles, this method could be used during chemical development to determine the relative effectiveness of a new additive. Having
153 a number of chemicals from different manufacturers would also promote anonymity in the comparisons.
Just as the AAs tested in this thesis work gave insight into the wettability changes that accompany AA addition, testing other AAs may introduce other mechanisms of ag- glomeration interference, such as rapid water conversion that leaves the particles with only minimal water layers to cause cohesion. Identifying and exploring new mecha- nisms would also allow better modeling efforts and predictive capabilities. One goal of these tests would be to find a small number of easily obtainable parameters that might give a reasonable first pass indication of the predicted performance of an AA, such as IFT, contact angle, or similar measurements. Furthermore, coupling the inter- facial measurements with molecular modeling would provide further insight into the interactions between model AA and hydrate systems.
• Adhesion Studies – There are many available paths forward when studying adhesion. Similar to the contact angle study, the study from Chapter 6 could be improved using model AAs, where the chemical makeup of the AA is known in order to discern the mechanisms by which waxes and AAs may interact. This would allow variation of the hydrocarbon groups on the quaternary ammonium salts (or other compounds) to investigate whether phase solubility or functional groups are responsible for the wax/AA interactions observed.
Another possibility that has gained attention from industry members is coatings for flowlines. A “hydrate-phobic” coating could be applied onto the inner surface of the pipe, causing a reduction or elimination of deposition of hydrates onto the pipe wall. The MMF would be ideal to test such coatings, as it does not rely on shear forces which may not always give clear results on the hydrate affinity for a pipe surface.
• Shell strength – These measurements could be improved not only by testing a wider array of chemicals, but by altering the measurement method slightly to make the
154 measurements more applicable to actual flow scenarios. It is unlikely in a flowline that a thin, sharp instrument similar to the cantilever used will be encountered by the hydrate particles. However, as these systems are typically at high shear rates, collisions with other particles or the wall could occur with significant force. As detailed in Chapter 7, the addition of surfactants reduced the force needed to puncture the hydrate shell. Measurements using a surface to apply force onto the hydrate shell until it fractured or collapsed would give an indication of the shear forces needed to compromise the integrity of a hydrate shell.
• Competitive Chemicals – The three chemicals used in the study detailed in Chapter 8 were effective in highlighting several different types of interactions that could be observed in the cohesion force when chemicals were added in tandem. However, sig- nificant study remains to determine the mechanism by which the effects are brought about. Varying the dosing rate of each chemical would be one approach to begin in- vestigating these complex interactions. Measuring the critical micelle concentration of each chemical and mixture would also help contextualize the changes observed in the cohesion force.
After the mechanisms are better understood, the study could be expanded using differ- ent classes of chemicals such as asphaltenes, waxes, other naturally occurring compo- nents in oil, emulsification agents, corrosion inhibitors, etc. Since flowlines are highly complex systems, there are many options for interactions that could be investigated.
12.2 High pressure apparatus
Gas phase measurements are necessary in order to contextualize atmospheric pressure measurements as well as to highlight any differences that may exist between the two systems. Before experiments in the high pressure apparatus can be relied on, it must first be verified that results obtained are repeatable. This will also involve developing an efficient procedure. As noted in Chapter 2, MMF results are dependent on the use of a consistent procedure; this
155 will also likely be the case for the high pressure apparatus. Initial tests on gas hydrates should focus on physical variables similar to those in the early atmospheric MMF system, such as temperature, annealing time, contact time or pre-load force. These tests will begin to show the differences and similarities between the high- and low-pressure apparatuses. However, in order to make accurate comparisons as well as in order to perform AA experiments, the apparatus will need to be upgraded to include the presence of a liquid phase. It would also be interesting to perform high pressure gas hydrate/surface adhesion force measurements to compare against the previous corresponding low pressure measurements. If a second-generation version of the apparatus were to be constructed, several changes to the current design should be incorporated. The location of the feed-throughs should be revised such that they do not interfere with the stroke of the nano-manipulators, and the manipulators themselves should be attached to the lid of the cell. The manipulators were designed to work best in this position, and mounting them to the lid would allow the assembly to remain intact, rather than being connected and disconnected between each experiment, which may diminish the apparatus lifespan. Additionally, mounting the manipulator to the lid would keep it away from any fluids that either spill within the cell or condense on the cold cell walls, protecting it. Future versions of the cell should also incorporate both pressure and temperature transducers that are capable of tracking these variables over the course of the experiments.
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