energies

Article Mechanical Characterization of Low Permeable Siltstone under Different Reservoir Saturation Conditions: An Experimental Study

Ayal Wanniarachchi 1 , Ranjith Pathegama Gamage 1 , Qiao Lyu 2,*, Samintha Perera 1,3 , Hiruni Wickramarathne 1 and Tharaka Rathnaweera 1 1 Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Building 60, Melbourne 3800, Australia; [email protected] (A.W.); [email protected] (R.P.G.); [email protected] (S.P.); [email protected] (H.W.); [email protected] (T.R.) 2 School of Geosciences and Info-physics, Central South University, Changsha 410012, China 3 Department of Infrastructure Engineering, The University of Melbourne, Building 175, Melbourne 3010, Australia * Correspondence: [email protected]



Received: 27 November 2018; Accepted: 18 December 2018; Published: 21 December 2018


Abstract: Hydro-fracturing is a common production enhancement technique used in unconventional reservoirs. However, an effective fracturing process requires a precise understanding of a formation’s in-situ strength behavior, which is mainly dependent on the formation’s in-situ stresses and fluid saturation. The aim of this study is to identify the effect of brine saturation (concentration and degree of saturation (DOS)) on the mechanical properties of one of the common unconventional reservoir rock types, siltstone. Most common type of non-destructive test: acoustic emission (AE) was used in conjunction with the destructive tests to investigate the rock properties. Unconfined compressive strength (UCS) and splitting tensile strength (STS) experiments were carried out for 78 varyingly saturated specimens utilizing ARAMIS (non-contact and material independent measuring system) and acoustic emission systems to determine the fracture propagation. According to the experimental results, the increase in degree of pore fluid saturation (NaCl ionic solution) causes siltstone’s compressive and tensile strengths to be reduced through weakening and breakage of the existing bonding between clay minerals. However, increasing NaCl concentration in the pore fluid generally enhances the compressive strength of siltstone through associated NaCl crystallization effect and actually reduces the tensile strength of siltstone through the corrosive influence of the NaCl ions. Moreover, results show that AE capture and analysis is one of the most effective methods to understand crack propagation behavior in rocks including the crack initiation, crack propagation, and final failure. The findings of this study are important for the identification of fluid saturation dependent in-situ strength conditions for successful hydro-fracturing in low permeable reservoirs.

Keywords: hydro-fracturing; siltstone; compressive strength; tensile strength; brine; stress–strain; acoustic emission

1. Introduction The rapid depletion of conventional energy resources is one of the major problems the world faces today, particularly given the continuously increasing energy consumption. Extensive research on potential new energy resources and production-enhancement techniques has therefore been initiated worldwide to effectively face this energy crises [1–3], and natural gas extraction is one effective solution. Although the extraction of conventional natural gas is generally much cheaper

Energies 2019, 12, 14; doi:10.3390/en12010014 www.mdpi.com/journal/energies Energies 2019, 12, 14 2 of 21 compared to unconventional gas [4], the amount of conventional gas available is quite limited. Therefore, unconventional natural gas extraction is becoming necessary. Recent advances in the petroleum industry, such as horizontal well drilling and hydro-fracturing, have significantly improved unconventional natural gas production from deep geological formations [5–11], particularly in countries such as the U.S., China, Canada, and Germany. Of the various reservoir productivity- enhancement techniques, hydro-fracturing is dominant and has been widely used. This fracturing process involves a number of steps: vertical well drilling, horizontal well drilling, injection of fracturing fluid, and fluid flow-back and gas production via the drilled production wells. Of these, the injection of fracturing fluid plays a major role in enhancing reservoir productivity. Although water with added chemicals is commonly used as the fracturing fluid, non-water-based fracturing fluids and gases (CO2, N2), foams, energized fluids, alcohols, and acids have also recently been tested in the field [12]. The creation of hydro-fractures in deep geological formations using any of these fracturing fluids depends on both the formation properties and the injection conditions of the fracturing fluid, including overburden stress, confinement, rock strength and composition, available pore fluids (water, brine, oil etc.), degree of saturation in the aquifer, and fracture fluid injection pressure [13–17]. According to existing research, with fracture fluid injection, fractures normally occur in the formation rock in a perpendicular direction to the minimum stress and failure can occur as tensile, shear or compression failure. However, it should be noted that rocks are generally highly brittle materials and therefore have much lower tensile strength than compressive strength (tensile strength is less than 10% of compressive strength). As a result, fractures are more likely to occur through tensile failure or shear failure or a combination of the two (hybrid failure). The strength of any reservoir rock depends on many other factors, including temperature, confining pressure (depth), mineral composition, pore size distribution, porosity, and degree of saturation [18–24] and the evaluation of the effect of strength is therefore a complex matter. On the other hand, in order to fracture formation rocks that exist under high confinements due to the greater depths of unconfined gas reservoirs, it is necessary to have higher injection pressure, because rock failure occurs only when the applied pressure goes beyond the effective stress applying on it (a negative effective stress condition). Apart from the reservoir rocks’ strength parameters, pore fluid concentration and its degree of saturation in the aquifer rock also play major roles in the hydro-fracturing process in deep unconfined gas reservoirs, because they indirectly affect the rock mechanical properties. With the injection of the fracturing fluid into a saturated (by water, brine, or oil) geological formation, the degree of water/brine/oil saturation around the injection well starts to decrease due to the forced migration caused by the fracturing fluid. This is depending on the injection pressure of the fracturing fluid and the ability of that fracturing fluid to move through the reservoir rock (its permeability) [25]. This pushing out of pore fluid (the forced migration) through the injecting fluid can have the effect of totally removing pore fluid from the well bore area. This pore fluid pushing influence reduces with increasing distance from the injection well due to the reduced pressure created by the injecting fluid on the pore fluid. Moreover, all of these pore fluid replacement processes through the injection fluid depend on the pore pressure of the fluid. Deep aquifers normally have greater pore pressure and therefore, the influence of the injecting fluid is less than in shallow aquifers [26,27]. Moreover, it should be noted that initiation and propagation of the primary hydraulic fracture have to be occurred very quickly for a minimal fluid leakoff, because it may demand high pumping rates. The situation is however different for the occurring secondary fractures, where initiation and propagation of them should be taken place over some considerable time duration to allow the fracturing fluid to leak into the reservoir rock. Therefore, the change of degree of saturation and its concentration has a direct influence on the rock mechanical properties, which therefore vary over time during the fracturing process, affecting the fracturing pressure required for hydro-fractures to be initiated. To date, many studies have therefore been performed to identify the effect of brine saturation (the salinity effect) on the strength of reservoir rocks, including sandstone, siltstone and limestone [25,28–31]. The studies have found a clear influence of brine saturation on reservoir Energies 2019, 12, 14 3 of 21 rock mechanical properties, including rock strength gain under certain circumstances, due to NaCl crystallization inside the rock pores. Although NaCl crystallization enhances rock strength by creating additional bonds between solid particles, brine saturation may cause the rock to be corroded by reducing its strength due to its high ionic strength [29]. Reservoir rock strength is therefore dependent on the dominant factor under each saturation condition. For example, according to Shukla, Ranjith [26], rock strength is highly dependent on the NaCl concentration, and it initially reduces, and then increases with increasing NaCl concentration. Although brine has some ability to enhance reservoir rock strength through NaCl crystallization, overall the saturation of any rock with brine causes it to soften in the wetting phase, reducing its strength, although this strength reduction can be reduced by the existence of ions in the saturation fluid. For example, rock strength reduction under water saturation is higher than the strength reduction which occurs under brine saturation, although both saturations reduce the rock strength. A number of studies to date have identified the effects of this fluid saturation and concentration on the mechanical properties of aquifer rock. However, most previous studies have been performed on sandstone and geo-polymer specimens, and only the effects of brine concentration/degree of saturation have been considered and the studies have been limited to unconfined compression strength (UCS) tests. However, as mentioned earlier, hydro-fractures are mainly initiated by the tensile failure of the rock due to the much lower tensile strengths of brittle rocks compared to their compressive strengths. The evaluation of the effect of saturation on rock tensile strength is therefore far more important than checking its influence on compressive strength for the hydro-fracturing process. Although siltstone is a common rock type in many unconventional reservoirs, to date little research has been conducted on strength variation with formation properties such as NaCl ionic concentration, composition and saturation, where the pore fluid is mainly composed of NaCl [29]. This paper therefore reports on a strength study of varyingly brine (NaCl) saturated siltstone.

2. Experimental methodology

2.1. Rock Specimens Eidsvold siltstones formed in the Triassic, Jurassic, and Cretaceous periods were collected from the Eidsvold (Burnett Highway) basin outcrop in Queensland, Australia (Figure1a). Based on electron microscope (SEM) observations, this particular type of siltstone is nearly homogeneous with 0.01–0.05 mm grain size distribution. According to X-ray diffraction (XRD) (Bruker D8 Cobalt Bruker, Massachusetts, MA, United States) analysis, this siltstone is basically composed of quartz and kaolinite minerals (43% and 40% of quartz and kaolinite, respectively). Apart from these major minerals, it also has around 12% of tridymite, 3% of mica, 1% of anatase, 1% of other clay minerals and negligible amount of organic matter. Figure1a–b show SEM and computed tomography (CT) scanning images of a typical Eidsvold siltstone specimen. As can be seen from Figure1b, this siltstone has randomly distributed pore voids with an average size of around 800 nm and according to Figure1c, the pore voids are different in size and randomly distributed throughout the rock. Importantly, Figure1c illustrates how the pore voids in this siltstone connect to each other along the height of the specimen, creating a pore network, which significantly affects its mechanical properties, including strength, elastic modulus and Poisson’s ratio. Energies 2019, 12, 14 4 of 21

Figure 1. (a) Location of the Eidsvold basin (specimen location); (b) SEM analysis image of siltstone Figure 1. (a) Location of the Eidsvold basin (specimen location); (b) SEM analysis image of siltstone with pore size; (c) computed tomography (CT) scanning image with internal pore network. with pore size; (c) computed tomography (CT) scanning image with internal pore network. 2.2. Specimen Preparation 2.2. Specimen Preparation The collected siltstone blocks were first cored into 38 mm diameter specimens using the diamond coringThe machine collected available siltstone in the blocks Monash were University first core Deepd into Earth 38 Energymm diameter Research specimens Laboratory using (DEERL) the usingdiamond water coring as a lubricant machine (to available minimize in the the chemical Mona reactions),sh University and thenDeep cut Earth into two Energy heights, Research 76 mm andLaboratory 19 mm, (DEERL) using the using diamond water cuttingas a lubricant machine, (to forminimize the UCS the tests chemical and splitting reactions), tensile and strengththen cut (STS)into two tests, heights, respectively. 76 mm The and surfaces 19 mm, of theusing specimens the diamond were thencutting ground machine, using for the the diamond UCS tests grinding and machinesplitting availabletensile strength in the DEERL(STS) te tosts, comply respectively. with the The ASTM surfaces D3967 of standard.the specimens All the were specimens then ground were storedusing the in an diamond oven at 70grinding◦C for 4machine days to removeavailable moisture in the DEERL from the to specimens. comply with The the low ASTM temperature D3967 conditionstandard. ofAll 70 the◦C specimens was used towere avoid stored any in thermal an oven cracks. at 70 ℃ for 4 days to remove moisture from the specimens. The low temperature condition of 70℃ was used to avoid any thermal cracks. 2.3. Specimen Saturation 2.3. Specimen Saturation The oven-dried specimens were removed from the oven, weighed and then allowed to cool for 6 hours.The 10%,oven-dried 20%, and specimens 25% by weightwere removed NaCl brine from solutions the oven, were weighed then and prepared then allowed and initially to cool three for UCS6 hours. test 10%, specimens 20%, and (38 × 25%76 mm)by weight and three NaCl STS brine test solutions specimens were (38 × then19 mm)prepared were and fully initially immersed three in eachUCS brinetest specimens solution. Vacuum(38 × 76 desiccatorsmm) and three were STS used test to specimens saturate the (38 specimens × 19 mm) by were removing fully immersed the air from in theeach specimens. brine solution. Specimen Vacuum weights desiccators were measured were used over to timesaturate to monitor the specimens the saturation by removing process the until air thefrom weights the specimens. became constant. Specimen When weights the weightwere measur becameed constant, over time that to saturationmonitor the level saturation was considered process fullyuntil saturatedthe weights or 100%became degree constant. of saturation When the (DOS). weig Theht became weight ofconstant, absorbed that water saturation at the 100% level DOSwas considered fully saturated or 100% degree of saturation (DOS). The weight of absorbed water at the 100% DOS was calculated for both types of specimens and the water content for 25%, 50%, and 75% DOS was calculated. Three UCS specimens were then fully immersed in each brine solution and the Energies 2019, 12, 14 5 of 21 was calculated for both types of specimens and the water content for 25%, 50%, and 75% DOS was calculated.weight was Threemeasured UCS from specimens time to were time then until fully it achieved immersed the water in each content brine solutionrequired andto achieve the weight 25% wasDOS. measured The same from procedure time to was time repeated until it achievedfor other theDO waterS levels content (50% and required 75%) toand achieve for the 25% STS DOS. test Thespecimens. same procedure The saturated was repeated specimens for otherwere DOSthen levelsfully (50%covered and with 75%) polythene and for the and STS stored test specimens. in a fog Theroom saturated for one month specimens to equally were then distribute fully covered the brine with throughout polythene the and specimen stored in without a fog room evaporating for one monththe brine to in equally the specimen. distribute the brine throughout the specimen without evaporating the brine in the specimen. 2.4. Testing Procedure 2.4. Testing Procedure Three oven-dried specimens and 36 saturated specimens were selected for each UCS and STS Three oven-dried specimens and 36 saturated specimens were selected for each UCS and STS test. All the specimens were painted in a stochastic pattern using two colors (black dots on white test. All the specimens were painted in a stochastic pattern using two colors (black dots on white background) to enable the ARAMIS camera to capture images of specimen failure during loading. background) to enable the ARAMIS camera to capture images of specimen failure during loading. ARAMIS is a non-contact and material-independent measuring system with hardware and software ARAMIS is a non-contact and material-independent measuring system with hardware and software components developed by GOM, Brunswick, Germany, which has the capability to measure 3-D components developed by GOM, Brunswick, Germany, which has the capability to measure 3-D surface surface coordinates and surface strain values using two 5 megapixels 3.45 micron pitch cameras. An coordinates and surface strain values using two 5 megapixels 3.45 micron pitch cameras. An advanced advanced acoustic emission (AE) system was used to identify crack propagation and energy release acoustic emission (AE) system was used to identify crack propagation and energy release during UCS during UCS loading. Here, a passive AE technique was used to acquire the AE data, in which loading. Here, a passive AE technique was used to acquire the AE data, in which measurements are measurements are taken during the loading. Two AE sensors were attached to the top and bottom taken during the loading. Two AE sensors were attached to the top and bottom platens of the loading platens of the loading machine to capture the AE signals. A Shimadzu uniaxial testing machine was machine to capture the AE signals. A Shimadzu uniaxial testing machine was used for both the UCS used for both the UCS and STS tests, which is capable of applying a maximum axial load of 300 kN. and STS tests, which is capable of applying a maximum axial load of 300 kN. A 0.1 mm/min strain rate A 0.1 mm/min strain rate was selected to fail the specimens within 5–15 minutes, according to the was selected to fail the specimens within 5–15 min, according to the ASTM standards. The Shimadzu ASTM standards. The Shimadzu machine, data logger, AE system and ARAMIS camera (Figure 2) machine, data logger, AE system and ARAMIS camera (Figure2) were started simultaneously to start were started simultaneously to start the experiment and three specimens were tested for each DOS the experiment and three specimens were tested for each DOS and brine concentration. The UCS, elastic and brine concentration. The UCS, elastic modulus and Poisson’s ratio of the UCS specimens were modulus and Poisson’s ratio of the UCS specimens were calculated using the stress–strain diagram for calculated using the stress–strain diagram for each specimen and the splitting tensile strength was each specimen and the splitting tensile strength was then calculated using Equation (1) [32]. then calculated using Eqation1 [32]. 2P σσ=t = (1)(1) t πLDπLD where, σ is the splitting tensile strength, P is the maximum applied load indicated by the testing where, σtt is the splitting tensile strength, P is the maximum applied load indicated by the testing machine, LL isis thethe thicknessthickness ofof thethe specimenspecimen andand DD isis thethe diameterdiameter ofof thethe specimen.specimen.

Figure 2.2.Shimadzu Shimadzu testing testing machine machine with ARAMISwith ARAMIS (non-contact (non-contact and material and independentmaterial independent measuring system)measuring and system) acoustic and emission acoustic (AE) emission system. (AE) system.

3. Results and Discussion The degrees of brine saturation variation (calculated based on the accumulated weight over the time with respect to the 100% saturated specimen weight) over time in the specimens for UCS and Energies 2019, 12, 14 6 of 21

3. Results and Discussion The degrees of brine saturation variation (calculated based on the accumulated weight over the time with respect to the 100% saturated specimen weight) over time in the specimens for UCS and STS STStests tests for the for three the NaClthree concentration NaCl concentration conditions conditio consideredns considered are shown are in Figureshown3. Asin Figure the figure 3. shows,As the figurethe degree shows, of saturationthe degree is of clearly saturation dependent is clearl ony dependent the concentration on the ofconcentration the NaCl, and of the increasing NaCl, and the increasingNaCl concentration the NaCl from concentration 10% to 25% from causes 10% the to time 25% required causes tothe achieve time required a certain to degree achieve of saturation a certain degreeto be reduced. of saturation This is relatedto be toreduced. the lower This ionic is potentialrelated ofto thethe low lower concentration ionic potential NaCl thatof the leads low to concentrationa slower chemical NaCl interaction that leads with to thea slower rock. However,chemical interaction after sufficient with time the(10 rock. days However, in the present after sufficientstudy), the time degree (10 days of saturation in the present becomes study), similar the degree for all NaClof saturation concentration becomes conditions. similar for The all effectNaCl concentrationof this NaCl saturation conditions. at The various effect degrees of this ofNaCl saturation saturation and at at various various degrees concentrations of saturation on both and the at variouscompressive concentrations and tensile strengthon both of the siltstone compressive was then investigatedand tensile andstrength the results of siltstone are discussed was inthen the investigatedfollowing sections. and the results are discussed in the following sections.

(a) (b)

Figure 3. Variation of degree of saturation over time in specimens prepared for (a) unconfined Figurecompressive 3. Variation strength of (UCS); degree (b) of splitting saturation tensile ov strengther time (STS)in specimens tests for varying prepared concentrations for (a) unconfined of brine (10%,compressive 20%, 25% strength NaCl). (UCS); (b) splitting tensile strength (STS) tests for varying concentrations of brine (10%, 20%, 25% NaCl). 3.1. Compressive Strength of Siltstone Under Different Fluid Saturation Conditions 3.1. CompressiveThe variations Strength of of unconfined Siltstone Under compressive Different Fluid strength Saturation (UCS) Conditions of siltstone with NaCl ionic concentrationThe variations and degree of unconfined of saturation compressive were first checkedstrength and(UCS) the of results siltstone are shownwith NaCl in Figure ionic4. concentrationNote that triplicate and specimensdegree of weresaturation tested were for each firs NaClt checked concentration and the results and DOS are and shown the average in Figure value 4. wasNote considered that triplicate as the specimens UCS value were for atested particular for each saturation NaCl concentration condition. Table and1 showsDOS and the UCSthe average values valuefor each was tested considered specimen as the with UCS the value average for a values partic asular well saturation as the standard condition. deviations. Table 1 shows According the UCS to valuesFigure 4 forand each Table tested1, the UCSspecimen of the testedwith the siltstone average varies values from as 37.7 well MPa as to the 61.6 standard MPa, depending deviations. on Accordingthe NaCl ionic to Figure solution 4 and saturation Table 1, condition. the UCS of The the maximum tested siltstone strength varies of around from 37.7 61.6 MPa MPa to can 61.6 be MPa, seen dependingfor the 0% DOS on the (dry) NaCl specimen ionic solution and any saturation type of NaCl condition. ionic solution The maximum causes that strength strength of to around be reduced. 61.6 ThisMPa iscan due be to seen the waterfor the softening 0% DOS effect, (dry) throughspecimen which and waterany type molecules of NaCl weaken ionic solution the clay mineralcauses that and strengthrock bonding, to be resulting reduced. in This reduced is due overall to the strength water insoftening the rock effect, [33–35 ].through Further, which the attachment water molecules of water weakenmolecules the to clay rock mineral particles and breaks rock thebonding, solid-solid resulting bonds in by reduced creating overall new solid-liquid strength in bonds,the rock which [33–35]. are Further,much weaker the attachment than solid-solid of water bonds. molecules As a to result, rock theparticles increasing breaks number the solid-solid of such bonds bonds by causes creating the newrock overallsolid-liquid strength bonds, to be which reduced. are much Moreover, weaker since than the solid-solid tested siltstone bonds. has As high a result, quartz the content increasing (43%), porenumber fluid of saturation such bonds replace causes the the existing rock Si–O–Sioverall stre bondsngth with to Si–OH–OH–Sibe reduced. Mo withreover, creating since a the deficiency tested siltstoneto the quartz has mineralhigh quartz structure, content which (43%), in turnpore promotesfluid saturation the crack replace initiation the throughexisting weakeningSi–O–Si bonds the withcrack Si–OH–OH–Si tips [36]. This alsowith contributecreating a todeficiency reduce the to mechanical the quartz propertiesmineral structure, of rock upon which pore in fluidturn promotessaturation. the Furthermore, crack initiation adsorption through of weakening H+ ions into the the crack negatively tips [36]. charged This also clay contribute minerals in to the reduce rock thealso mechanical contribute toproperties the weakening of rockof upon the rockpore throughfluid saturation. expanding Furtherm the clayore, layers adsorption [37]. According of H+ ions to intoFigure the4, negatively a comparison charged of all clay the brine-minerals saturated in the ro specimensck also contribute indicates thatto th thee weakening 10% NaCl-saturated of the rock throughspecimens expanding exhibit the the lowest clay UCSlayers values [37]. and According the 20% NaCl-saturatedto Figure 4, a comparison specimens exhibit of all thethe highest brine- saturated specimens indicates that the 10% NaCl-saturated specimens exhibit the lowest UCS values and the 20% NaCl-saturated specimens exhibit the highest UCS values under any degree of saturation condition. The specimens saturated with the highest NaCl concentration (i.e., 25% NaCl) exhibit strength values between these figures. According to the strength classification of intact and jointed rocks (Table 2), dry specimens can be classified as class C (moderate strength). All the DOS Energies 2019, 12, 14 7 of 21

UCS values under any degree of saturation condition. The specimens saturated with the highest

NaCl concentration (i.e., 25% NaCl) exhibit strength values between these figures. According to the strengthlevels (i.e., classification 25%, 50%, of75%, intact and and 100%) jointed in 10% rocks and (Table 25%2), NaCl dry specimens can be classified can be classifiedas class D as (medium class C (moderatestrength). strength).However, Allin 20% the DOSNaCl, levels 25% (i.e.,and 50% 25%, DO 50%,S levels, 75%, and specimens 100%) in belong 10% and to class 25% NaClC (moderate can be classifiedstrength) asand class 75% D (mediumand 100% strength). DOS levels, However, specimens in 20% belong NaCl, 25%to the and class 50% DOSD (medium levels, specimens strength). belongHowever, to class the Cclass (moderate C classification strength) andis marginal 75% and 100%for 20% DOS NaCl levels, specimens specimens and belong they to can the classalso Dbe (mediumclassified strength). as class D. However, Increasing the si classltstone C classification strength with is marginalincreasing for brine 20% concentration NaCl specimens is related and they to canthe alsoNaCl be crystallization classified as class effect, D. as Increasing the formation siltstone of NaCl strength crystals with increasingin rock pores brine can concentration offer an added is relatedstrength to to the the NaCl rock. crystallization This can be effect, confirmed as the by formation using the of NaClSEM crystalsimages inof rockthe saturated pores can offerand dry an addedsiltstone strength specimens to the (Figure rock. This 5). This can has be confirmed been observ byed using in other the SEMtypes images of rocks of such the saturated as sandstone and dryand siltstonegeo-polymer specimens in the (Figureresearch5). literature This has beenand seems observed to be in otherapplicable types to of siltstone rocks such [29,30]. as sandstone On the other and geo-polymerhand, NaCl insolution the research reduces literature the diffusion and seems double to be la applicableyer (DDL) to of siltstone the existing [29,30 clay]. On minerals the other in hand, near NaClminerals solution surface reduces that thein diffusionturn weaken double the layer rock (DDL) [38,39]. of theFurthermore, existing clay as mineralsmentioned in near earlier, minerals ionic surfaceconcentrations that in turn can weakenhave a corrosive the rock [influence38,39]. Furthermore, on reservoir as rock mentioned by reducing earlier, its ionicstrength concentrations [40,41], and canthis have may aalso corrosive reduce influence the UCS onstrength reservoir characteristics rock by reducing at very its high strength NaCl [concentrations40,41], and this (25% may NaCl also reduceconcentration the UCS here). strength This characteristics also can be seen at veryin the high Figure NaCl 5, concentrationswhere at 25% NaCl (25% NaClconcentration, concentration some here).areas Thisof the also rock can matrix be seen have in thecorroded Figure 5by, where reducing at 25% the NaCloverall concentration, strength of the some siltstone. areas of According the rock matrixto the results, have corroded it can be by concluded reducing thethat overall when strengthsiltstone ofis thesaturated siltstone. with According 10% NaCl to concentration, the results, it can the beionic concluded strength that of the when NaCl siltstone ionic so islution saturated is not with sufficient 10% NaCl to have concentration, any influence the on ionic siltstone strength strength. of the NaClThe strength ionic solution of the is notrock sufficient is reduced to have mainly any influencedue to the on quartz siltstone mineral strength. dissolution The strength and of softening the rock iseffect reduced created mainly by duethe towetting the quartz phase mineral (water) dissolution molecules and in softening the pore effect fluid, created and byincreasing the wetting the phaseconcentration (water) moleculesof NaCl ionic in the solution pore fluid, to 20% and causes increasing the creation the concentration of a considerable of NaCl number ionic solution of NaCl tocrystals 20% causes among the the creation pores in of the a considerablerock. Although number further of increasing NaCl crystals the NaCl among concentration the pores in causes the rock. the Althoughcrystallization further processes increasing to the accelerate, NaCl concentration the simultaneously causes the crystallizationincreasing ionic processes corrosion to accelerate, and clay theminerals’ simultaneously diffusion increasingdouble layer ionic reduction corrosion effects and goes clay beyond minerals’ the diffusioncrystallization double effect layer at reduction25% NaCl effectsconcentration. goes beyond the crystallization effect at 25% NaCl concentration.

Figure 4. Variation of compressive strength of siltstone in 10%, 20%, and 25% NaCl. Figure 4. Variation of compressive strength of siltstone in 10%, 20%, and 25% NaCl. Energies 2019, 12, 14 8 of 21

Table 1. Mechanical properties of dry and brine-saturated siltstone.

NaCl STS (MPa) UCS (MPa) Poisson’s Ratio E (GPa) DOS (%) % Value Avg. SD Value Avg. SD Value Avg. SD Value Avg. SD 0 DOS-1 7.35 60.78 0.31 16.44 0 0 DOS-2 7.35 61.32 0.29 15.68 (Dry) 7.33 0.03 61.59 0.79 0.30 0.01 16.00 0.32 0 DOS-3 7.30 62.66 0.30 15.87 25 DOS-1 6.00 41.76 0.33 12.50 25 DOS-2 6.00 6.20 0.29 40.67 40.75 0.80 0.35 0.34 0.01 12.50 12.65 0.22 25 DOS-3 6.61 39.82 0.33 12.96 50 DOS-1 5.60 38.32 0.39 15.80 50 DOS-2 5.735.60 0.10 40.33 40.38 1.70 0.36 0.37 0.01 15.91 15.51 0.49 10 50 DOS-3 5.48 42.48 0.37 14.82 75 DOS-1 5.58 42.07 0.33 15.00 75 DOS-2 5.27 5.38 0.14 37.71 39.86 1.78 0.31 0.31 0.01 13.92 14.29 0.50 75 DOS-3 5.29 39.80 0.30 13.95 100 DOS-1 5.77 36.38 0.28 13.42 100 DOS-2 5.56 5.58 0.15 38.62 37.65 0.94 0.27 0.27 0.01 12.62 12.97 0.34 100 DOS-3 5.40 37.94 0.26 12.86 25 DOS-1 5.77 55.41 0.27 15.34 25 DOS-2 5.93 5.70 0.22 52.98 54.31 1.00 0.27 0.26 0.01 14.89 15.41 0.45 25 DOS-3 5.41 54.53 0.25 15.99 50 DOS-1 5.35 51.32 0.32 13.30 50 DOS-2 5.61 5.55 0.15 54.78 53.03 1.42 0.30 0.30 0.01 14.91 14.19 0.67 20 50 DOS-3 5.70 52.99 0.28 14.37 75 DOS-1 5.66 46.69 0.22 13.96 75 DOS-2 5.30 5.50 0.15 48.81 46.60 1.84 0.21 0.22 0.01 12.43 13.20 0.62 75 DOS-3 5.54 44.30 0.24 13.21 100 DOS-1 5.68 42.98 0.30 13.30 100 DOS-2 5.30 5.51 0.16 43.68 42.80 0.80 0.33 0.31 0.01 14.60 13.75 0.60 100 DOS-3 5.54 41.75 0.32 13.34 25 DOS-1 5.42 51.35 0.28 17.73 25 DOS-2 5.33 5.43 0.09 48.81 49.25 1.57 0.29 0.29 0.01 16.21 16.63 0.78 25 DOS-3 5.55 47.58 0.30 15.96 50 DOS-1 5.34 48.46 0.29 12.45 50 DOS-2 5.15 5.34 0.15 44.00 46.68 1.93 0.27 0.27 0.01 11.53 12.31 0.59 25 50 DOS-3 5.53 47.58 0.26 12.96 75 DOS-1 5.02 43.76 0.37 9.99 75 DOS-2 5.02 5.17 0.21 48.33 46.76 2.12 0.380.37 0.01 9.53 9.55 0.35 75 DOS-3 5.46 48.19 0.36 9.12 100 DOS-1 4.68 37.99 0.29 18.98 100 DOS-2 4.81 4.85 0.16 40.40 39.06 1.00 0.30 0.29 0.01 20.00 19.33 0.47 100 DOS-3 5.06 38.77 0.28 19.01 Avg—Average value; SD—Standard deviation.

Table 2. Strength classification of intact and jointed rocks (Ramamurthy & Arora, 1993).

Class Description UCS (MPa) A Very high strength >250 B High strength 100–250 C Moderate strength 50–100 D Medium strength 25–50 E Low strength 5–25 F Very low strength <5 Energies 2019, 12, 14 9 of 21

(a) (b)

(c) (d)

Figure 5. SEM test results of siltstone specimens (a) dry; (b) 10% NaCl; (c) 20% NaCl; (d) 25% NaCl. Figure 5. SEM test results of siltstone specimens (a) dry; (b) 10% NaCl; (c) 20% NaCl; (d) 25% NaCl. In relation to the influence of NaCl saturation on strength reduction in various reservoir rocks, the total correspondingTable UCS 1. reduction Mechanical observed properties in of this dry study and brine-saturated upon full saturation siltstone. of siltstone (100% DOS) with varying concentrations of brine is around 36% (10%, 20%, and 25% NaCl concentrations NaCl cause the siltstone strengthSTS to be (MPa) reduced by around UCS (MPa) 38.9%, 30.5%, Poisson's and 36.7%, Ratio respectively). E (GPa) This is similar to theDOS strength (%) reduction observed for fine grained sandstone (which has similar properties % Valu Avg Valu Valu Avg Valu as siltstone) for similar NaCl saturation conditions by Rathnaweera, Ranjith [29], who also observed e . SD e Avg. SD e . SD e Avg. SD around 36.03% UCS reduction in sandstone with full saturation from 10% to 30% NaCl. This is an 0 DOS - 1 7.35 60.78 0.31 16.44 interesting0 finding, which shows that0.0 the influence61.5 of NaCl0.7 saturation is similar0.0 for sandstone16.0 and0.3 0 DOS - 2 7.35 7.33 61.32 0.29 0.30 15.68 siltstone(Dry) and possible for any reservoir rock.3 However,9 this cannot9 be confirmed without1 checking0 the 2 effect for other0 DOS rock - 3 types.7.30 Sandstone and siltstone62.66 are quite common0.30 as shales in unconventional15.87 gas reservoirs and25 DOS therefore, - 1 6.00 it can be concluded41.76 that NaCl saturation0.33 commonly creates12.50 a considerable 0.2 40.7 0.8 0.0 12.6 0.2 and similar25 influence DOS - 2 on reservoir6.00 6.20 rocks’ UCS40.67 strength. 0.35 0.34 12.50 9 5 0 1 5 2 The ARAMIS photogrammetry non-contact and material-independent measuring system was 25 DOS - 3 6.61 39.82 0.33 12.96 used in this study to investigate the effect of NaCl concentration and degree of saturation on the 50 DOS - 1 5.60 38.32 0.39 15.80 deformation characteristics of the siltstone during failure. The ARAMIS photogrammetry images of 0.1 40.3 1.7 0.0 15.5 0.4 strain development in the5.73 siltstone 5.60 specimens during failure with their0.37 failure patterns are shown 50 DOS - 2 0 40.33 8 0 0.36 1 15.91 1 9 in Figure10 6. These images have strain contours ranging from blue to red, where blue corresponds to 50 DOS - 3 5.48 42.48 0.37 14.82 the lowest strain and the red corresponds to the highest strain. As Figure6 shows, all the images 75 DOS - 1 5.58 42.07 0.33 15.00 show various colors in the specimens,0.1 confirming39.8 that the1.7 specimens underwent0.0 highly varying14.2 0.5 75 DOS - 2 5.27 5.38 37.71 0.31 0.31 13.92 strain development during failure. The4 maximum and6 minimum8 strain values for1 10%, 20%, and9 25%0 brine concentrations75 DOS - 3 are about5.29 0.65 and −0.25,39.80 respectively compared0.30 to the dry specimens.13.95 However, 100 DOS - 0.1 37.6 0.9 0.0 12.9 0.3 when both strain and failure5.77 patterns5.58 are considered, Figure5 indicates that0.27 two main failure types can 1 5 36.38 5 4 0.28 1 13.42 7 4

to 30% NaCl. This is an interesting finding, which shows that the influence of NaCl saturation is similar for sandstone and siltstone and possible for any reservoir rock. However, this cannot be confirmed without checking the effect for other rock types. Sandstone and siltstone are quite common as shales in unconventional gas reservoirs and therefore, it can be concluded that NaCl saturation commonly creates a considerable and similar influence on reservoir rocks’ UCS strength. The ARAMIS photogrammetry non-contact and material-independent measuring system was used in this study to investigate the effect of NaCl concentration and degree of saturation on the deformation characteristics of the siltstone during failure. The ARAMIS photogrammetry images of strain development in the siltstone specimens during failure with their failure patterns are shown in Figure 6. These images have strain contours ranging from blue to red, where blue corresponds to the lowest strain and the red corresponds to the highest strain. As Figure 6 shows, all the images show various colors in the specimens, confirming that the specimens underwent highly varying strain development during failure. The maximum and minimum strain values for 10%, 20%, and Energies25% brine2019, 12concentrations, 14 are about 0.65 and −0.25, respectively compared to the dry specimens.10 of 21 However, when both strain and failure patterns are considered, Figure 5 indicates that two main failure types can be identified: shear failure and splitting failure. Interestingly, with high NaCl ionic beconcentrations identified: shear (20% failure and and25% splitting NaCl concentrations), failure. Interestingly, specimens with highare more NaCl ioniclikely concentrations to exhibit splitting (20% andfailure 25% and NaCl specimens concentrations), saturated specimens with low areNaCl more ioni likelyc concentrations to exhibit splitting of fluid failure (10% NaCl) and specimens are more saturatedlikely to exhibit with low shear NaCl failure. ionic concentrations of fluid (10% NaCl) are more likely to exhibit shear failure.

FigureFigure 6.6.ARAMIS ARAMISimages imagesof ofspecimens specimensof of failurefailure (major(major strain): strain): ((aa)) drydry specimen;specimen; ( (bb)) 10% 10% brine brine and and 100%100% DOS; DOS; ( c(c)) 20% 20% brine brine and and 100% 100% DOS; DOS; ( d(d)) 25% 25% brine brine and and 100% 100% DOS. DOS.

3.2.3.2. EffectEffect ofof FluidFluid SaturationSaturation onon TensileTensile Strength Strength of of Siltstone Siltstone InIn orderorder toto identifyidentify the effect of of fluid fluid saturation saturation on on the the tensile tensile properties properties of of siltstone, siltstone, a aseries series of ofsplitting splitting tensile tensile strength strength (STS) (STS) tests tests were were performed performed on onvaryingly varyingly saturated saturated siltstone siltstone specimens specimens (for (fordifferent different concentrations concentrations and and degrees degrees of ofsaturation saturation of ofNaCl) NaCl) and and the the results results are are shown shown in Figure7 7.. SimilarSimilar to to the the compressive compressive strength strength tests, tests, triplicate tripli specimenscate specimens were tested were for tested each saturationfor each saturation condition (seecondition Table1 (see) and Table the average 1) and the strength average value strength was used value as was the tensileused as strength the tensile under strength each saturationunder each condition.saturation Thecondition. STSs ofThe the STSs varyingly of the varyingly saturated saturated specimens specim are betweenens are between 4.85 MPa 4.85 and MPa 7.33 and MPa. 7.33 AccordingMPa. According to Figure to6 ,Figure dry siltstone 6, dry hassiltstone the maximum has the tensilemaximum strength tensile similar strength to the similar compressive to the strengthcompressive and it strength continuously and reducesit continuously with increasing reduces degree with ofincreasing saturation degree of NaCl of ionicsaturation solution of at NaCl any concentration, which is related to the water softening effect and quartz mineral dissolution. However, the initial strength reduction that occurs due to up to 25% DOS of NaCl ionic solution appears to be much greater than the tensile strength reduction which occurs with further increase of DOS, up to 100%. For example, up to 64%, 89%, and 76% total strength reductions in 10%, 20%, and 25% NaCl saturated specimens occur at 25% DOS. The total porosity of the tested siltstone should be greater than 20%, because according to the mercury intrusion porosity tests, the inter-granular porosity (effective porosity) of the tested siltstone is around 20%. However, this particular test can only provide the porosity available in material for mercury intrusion or in other words inter-granular pores. Since some inter-granular pores may be in closed loops and they may not contribute to the intrusion and the total porosity may therefore be slightly greater than 20%. However, it should be noted that not the effective porosity, the total porosity is governing fact for rock mechanical properties. The variation of the incremental intrusion volume with the pore diameter is given in Figure8, according to which, pore size diameter is mainly concentrated around 1 µm and 100 µm, and NaCl ions (molecular size <1 µm) ionic solution at any concentration, which is related to the water softening effect and quartz mineral dissolution. However, the initial strength reduction that occurs due to up to 25% DOS of NaCl ionic ionic solution at any concentration, which is related to the water softening effect and quartz mineral solution appears to be much greater than the tensile strength reduction which occurs with further dissolution. However, the initial strength reduction that occurs due to up to 25% DOS of NaCl ionic increase of DOS, up to 100%. For example, up to 64%, 89%, and 76% total strength reductions in solution appears to be much greater than the tensile strength reduction which occurs with further 10%, 20%, and 25% NaCl saturated specimens occur at 25% DOS. The total porosity of the tested increase of DOS, up to 100%. For example, up to 64%, 89%, and 76% total strength reductions in siltstone should be greater than 20%, because according to the mercury intrusion porosity tests, the 10%, 20%, and 25% NaCl saturated specimens occur at 25% DOS. The total porosity of the tested inter-granular porosity (effective porosity) of the tested siltstone is around 20%. However, this siltstone should be greater than 20%, because according to the mercury intrusion porosity tests, the particular test can only provide the porosity available in material for mercury intrusion or in other inter-granular porosity (effective porosity) of the tested siltstone is around 20%. However, this words inter-granular pores. Since some inter-granular pores may be in closed loops and they may particular test can only provide the porosity available in material for mercury intrusion or in other not contribute to the intrusion and the total porosity may therefore be slightly greater than 20%. words inter-granular pores. Since some inter-granular pores may be in closed loops and they may However, it should be noted that not the effective porosity, the total porosity is governing fact for not contribute to the intrusion and the total porosity may therefore be slightly greater than 20%. rock mechanical properties. The variation of the incremental intrusion volume with the pore However, it should be noted that not the effective porosity, the total porosity is governing fact for diameter is given in Figure 8, according to which, pore size diameter is mainly concentrated around rock mechanical properties. The variation of the incremental intrusion volume with the pore 1µmEnergies and2019 100µm,, 12, 14 and NaCl ions (molecular size < 1 µm) can easily enter the inter-granular 11pores of 21 diameter is given in Figure 8, according to which, pore size diameter is mainly concentrated around and break the bond between the rock grains. This may cause a significant strength reduction at 25% 1µm and 100µm, and NaCl ions (molecular size < 1 µm) can easily enter the inter-granular pores DOS. canand easily break enter the bond the inter-granular between the rock pores grains. and break This the may bond cause between a significant the rock st grains.rength Thisreduction maycause at 25% a significantDOS. strength reduction at 25% DOS.

Figure 7. Variation of splitting tensile strength of siltstone in 10%, 20%, and 25% NaCl. Figure 7. Variation of splitting tensile strength of siltstone in 10%, 20%, and 25% NaCl. Figure 7. Variation of splitting tensile strength of siltstone in 10%, 20%, and 25% NaCl.

Figure 8. Pore size distribution of tested siltstone. Figure 8. Pore size distribution of tested siltstone. Importantly, accordingFigure to Figure 8. Pore7, the size tensile distribution strength of tested of the siltstone. siltstone at any DOS continuously Importantly, according to Figure 7, the tensile strength of the siltstone at any DOS decreases with increasing NaCl concentration, which is different from the compressive strength continuously decreases with increasing NaCl concentration, which is different from the reductionImportantly, observed according upon NaCl tosaturation. Figure 7, Thethe reductiontensile strength of tensile of strength the siltstone with increasing at any ionicDOS compressive strength reduction observed upon NaCl saturation. The reduction of tensile strength strengthcontinuously of the NaCldecreases ionic solutionwith increasing from 10% toNaCl 25% isconcentration, possibly related which to the interactionis different of NaClfrom ionsthe with increasing ionic strength of the NaCl ionic solution from 10% to 25% is possibly related to the withcompressive the siltstone strength rock matrixreduction (a corrosive observed influence upon NaCl described saturation. in compressive The reduction strength of tensile reduction), strength and interaction of NaCl ions with the siltstone rock matrix (a corrosive influence described in suchwith interactionincreasing isionic obviously strength greater of the at greaterNaCl ionic ionic solution concentrations. from 10% According to 25% tois existingpossibly studies related [ 42to, 43the], rockinteraction minerals of such NaCl as quartzions andwith kaolinite the siltstone are highly rock reactive matrix with (a NaCl,corrosive and theinfluence siltstone described studied here in is abundant with these rock minerals. We can therefore expect a highly reactive environment inside the rock with NaCl saturation. The possible reactions that occur among quartz and kaolinite with NaCl are shown in Equations (2) and (3). According to these equations, these reactions commonly release HCl into the reservoir pore fluid, making the pore fluid more acidic. Such an acidic environment has a strong influence on breaking the grain-to-grain bonds in the rock, reducing its tensile strength and its compressive strength. However, one might be curious to know why the NaCl crystallization effect does not affect the tensile strength of siltstone. This is because of the difference between the load application in tensile and compressive loading. In compression, the load is applied on the bulk material, and under such conditions the reduced pore space filled with NaCl crystals offers more barriers to failing the specimen under compression as the NaCl crystals also bear some compressive loading. However, in tensile load application, the rock matrix is fractured by the load, and in such situations the existence of weak NaCl crystals in rock pore space does not help, as the whole rock is Energies 2019, 12, 14 12 of 21

fracturing through its strongest section or rock matrix. Overall, only the grain corrosion effect created by the ions of the saturation fluid affects the tensile strength of siltstone. This is quite important for the hydro-fracturing process, as in this process fracture propagation mainly occurs through tensile failure. Therefore, tensile strength is the governing factor for the fracking process. According to the findings of this study, the hydro-fracturing process is only influenced by this grain corrosion effect created 1 by the ions of the saturation fluid. Therefore, a higher ionic concentration creates a more favorable environment for the fracturing0.014 process. 2 0.012 SiO (quartz) + NaCl + H O = H NaSiO + HCl (2) 0.01 2 2 3 4 3 Al2Si0.0082O5(OH)4(kaolinite) + 2NaCl = 2NaAlSiO4 + 2HCl + H2O (3) 4 The failure patterns0.006 of the varyingly saturated siltstone specimens under indirect tensile loading were obtained by ARAMIS0.004 photogrammetry and the results are shown in Figure9. Interestingly, all the 5 siltstone specimens under0.002 all saturation conditions appear to have undergone splitting failure with greater strain concentrations at the contact points, in accordance with the ASTM standard. According 6 to the strain contours,Incremental Intrusion (mL/g) maximum0 and minimum strain values at failure are around 0.20 and −0.075, respectively, which appear to1 only slightly 10 vary 100 with 1000 NaCl concentration 10000 100000 or DOS 1000000 levels. Moreover, in 7 dry specimens and 10% NaCl-saturated specimens,Pore size failure Diameter occurred (nm) suddenly and the entire specimen split into two parts (the width of the failure fracture is large) during the load application. In contrast, 8 in 20% and 25% NaCl-saturatedFig. 8. specimens, Pore size failuredistribu occurredtion of tested with much siltstone slower fracture propagation.

9 Dry sample

10

11

12 10% NaCl 20% NaCl 25% NaCl 13 25% 14 DOS

15

16 50% DOS 17

18 75% DOS 19

20

21 100% DOS 22

23 Fig.Figure 9. ARAMIS 9. ARAMIS strain strain distributiondistribution (− (-0.075%0.075%–0.2%) – 0.2%) and failure and failure pattern pattern in STS specimens in STS specimens for 10%, 20% and 25% NaCl concentrations for 25%, 50%, 75%, and 100% degree of saturation (DOS). 24 for 10%, 20% and 25% NaCl concentrations for 25%, 50%, 75% and 100% DOS.

35

Energies 2019, 12, 14 13 of 21

3.3. Effect of Fluid Saturation on Brittle Characteristics of Siltstone The stress-strain behavior of varyingly saturated siltstone specimens obtained from the UCS tests was then used to identify the influence of various saturations on their brittle characteristics. This is because, as explained in the introduction, brittle rocks more likely to have more favorable responses to the fracturing process than ductile rocks. In order to identify the saturation-induced brittle characteristic alterations in siltstone, the elastic modulus (E) and Poisson’s ratio (ν) of each specimen were calculated using the stress-strain curves given in Figure 10. The average values for three replicates are shown in Table1. As the table shows, the standard variation among the E and ν values among the tested triplicated specimens are at about the 5% range and therefore, the average values give a reasonable representation of the siltstone tested. Thee specimens were collected from same block and were therefore expected to have the same micro-structure and mineral composition and therefore the same mechanical properties, as shown in Table1.

Figure 10. Stress–strain curves for (a) 10% NaCl concentration; (b) 20% NaCl concentration; (c) 25% NaCl concentration.Figure 10. Stress–strain curves for (a) 10% NaCl concentration; (b) 20% NaCl concentration; (c) 25% NaCl concentration.

According to Figure 11a, the elastic moduli of dry and saturated specimens vary between 19.33 and 9.55 GPa, the dry specimens commonly have the highest elastic modulus (E) (around 16 GPa), and saturation generally causes the reduction of the E values of the siltstone. If the effect of NaCl DOS on the rock specimen E values is considered, elastic modulus seems to generally decrease with Energies 2019, 12, 14 14 of 21

According to Figure 11a, the elastic moduli of dry and saturated specimens vary between 19.33 and 9.55 GPa, the dry specimens commonly have the highest elastic modulus (E) (around 16 GPa), increasingand saturation degree generally of saturation. causes the This reduction is consistent of the Ewith values theof Ethe reduction siltstone. behavior If the effect with of brine NaCl saturationDOS on the observed rock specimen by Nasvi, E values Ranjith is considered,[30] for sandstone elastic and modulus geo-polymer. seems to The generally reduction decrease of elastic with modulusincreasing represents degree of saturation.the enhancement This is consistentof ductile withprop theerties E reduction in the siltstone behavior with with NaCl brine saturation, saturation regardlessobserved byof Nasvi,saturation Ranjith condition. [30] for This sandstone proves andthat geo-polymer.increasing NaCl The ionic reduction concentration of elastic causes modulus the siltstone’srepresents brittle the enhancement properties to of be ductile reduced. properties The interaction in the siltstone of strong with NaCl NaCl ions saturation, with the regardless siltstone rockof saturation matrix possibly condition. changes This provesits polymer that increasingstructure. NaClHowever, ionic this concentration is not favorable causes for the the siltstone’s hydro- fracturingbrittle properties process, to as be ductile reduced. rocks The need interaction more ofenergy strong for NaCl fracturing ions with compared the siltstone to brittle rock matrixrocks. Accordingpossibly changes to Dyke its and polymer Dobereiner structure. [44], However, the polyme this isr structure not favorable of the for siltstone the hydro-fracturing rock matrix process,can be changedas ductile by rocks the exchange need more of energy strong forSi–O–Si–bond fracturing compareds with weaker to brittle hydrogen rocks. bonds According which to eventually Dyke and breaksDobereiner the strong [44], the inter-granular polymer structure atomic of thebonds, siltstone resulting rock matrix in enhanced can be changed ductile byproperties. the exchange If the of hydro-fracturingstrong Si–O–Si–bonds process with is weaker considered, hydrogen additional bonds which costs eventually and a greater breaks theinjection strong inter-granularpressure are requiredatomic bonds, to create resulting fractures in enhanced in the ductilereservoir properties. rock with If the enhanced hydro-fracturing ductile properties, process is considered, which is certainlyadditional not costs a favorable and a greater condition. injection pressure are required to create fractures in the reservoir rock withInterestingly, enhanced ductile as can properties, be clearly which seen in is certainlyFigure 11a, not when a favorable the NaCl condition. concentration is 25%, there is a quiteInterestingly, high E value as (19.33 can be GPa) clearly in seenthe tested in Figure siltstone 11a, whenfor the the 100% NaCl saturated concentration condition, is 25%, which there is evenis a quite higher high than E valuethe E (19.33value GPa)for the in thedry testedcondition siltstone (16 GPa). for the This 100% high saturated E value condition,for this particular which is saturationeven higher condition than the can E valuebe observed for the in dry all conditionthe triplicated (16 GPa). specimens This high(Table E 1). value Therefore, for this this particular result issaturation reliable although condition unexpected. can be observed It is possible in all the that triplicated, at high specimens salinity concentrations, (Table1). Therefore, the occurrence this result of is NaClreliable crystals although in the unexpected. pore structure It is possible is much that, harder at high than salinity at low concentrations,saturation conditions. the occurrence Therefore, of NaCl the generallycrystals in expected the pore structure brittle ischaracteristic much harder affects than at lowthe saturationfailure behavior conditions. of Therefore,the siltstone. the generallyThis is confirmedexpected brittle by the characteristic fact that this affects is seen the only failure at 100% behavior degree of theof saturation, siltstone. This at which is confirmed all the bypores the factare fullythat thisfilled is seenwith onlyNaCl at and 100% there degree is therefore of saturation, a higher at which probability all the poresfor applied are fully load filled to be with transferred NaCl and throughthere is thereforethe NaCl acrystals. higher probability for applied load to be transferred through the NaCl crystals. InIn relationrelation toto the the effect effect of NaClof NaCl ionic ionic solution soluti saturationon saturation on siltstone’s on siltstone’s Poisson’s Poisson’s ratio, according ratio, accordingto Figure 11 tob, Figure the Poisson’s 11b, the ratios Poisson’s of varyingly ratios of saturated varyingly siltstone saturated specimens siltstone (including specimens dry) (including are in the dry)0.22 toare 0.37 in the range. 0.22 However, to 0.37 range. a closer However, look on a Figure closer 11 lookb shows on Figure that there 11b isshows no such that direct there influence is no such of directNaCl saturationinfluence onof NaCl the tested saturation siltstone, on and the thetested variation siltstone, of the and Poisson’s the variation ratio with of the NaCl Poisson’s concentration ratio withand DOSNaCl appears concentration to be random. and DOS However, appears more to be tests random. need toHowever, be performed more beforetests comingneed to tobe a performedfinal conclusion. before coming to a final conclusion.

(a)

Figure 11. Cont. Energies 2019, 12, 14 15 of 21

(b)

FigureFigure. 11. 11. Variation of ( aa)) Yong’s Yong’s modulus modulus and and (b (b) )Poisson’s Poisson’s ratio ratio of ofsiltstone siltstone in 10%, in 10%, 20%, 20%, and and25% 25%NaCl. NaCl.

3.4.3.4. EffectEffect ofof FluidFluid SaturationSaturation onon FractureFracturePropagation Propagation in in Siltstone Siltstone TheThe fracturefracture propagation propagation in thein the reservoir reservoir rock rock is highly is highly important important in the hydro-fracturingin the hydro-fracturing process, andprocess, may and be affected may be by affected the properties by the properties of the pore of fluid the andpore the fluid saturation and the conditions.saturation conditions. Therefore, thisTherefore, was considered this was inconsidered the next stage in the of next the study, stage basedof the onstudy, comprehensive based on comprehensive acoustic emission acoustic (AE) analysis.emission AE (AE) capture analysis. and AE analysis capture is one and of analysis the most is effective one of the methods most toeffective understand methods crack to propagation understand behaviorcrack propagation in rocks, having behavior the abilityin rocks, to effectivelyhaving the exhibit ability the to major effectively fracture exhibit propagation the major steps fracture under loadpropagation application: steps crack under initiation, load application: crack propagation crack initiation, and final crack failure propagation [45–49]. Among and final the failure initial step[45– belongs49]. Among to initiation the initial of AEstep release belongs upon to initiation load application, of AE release where upon in the load beginning application, of load where application in the therebeginning is no anyof load crack application in the rock there or rock is atno crack any closurecrack in state the androck continuous or rock at loadcrack application closure state causes and tocontinuous initiate cracks load atapplication one stage andcauses this to is initiate called crackcracks initiation. at one stage Further and this load is application called crack on initiation. the rock beyondFurther this load point application causes the on created the rock cracks beyond to propagate this point through causes the rockthe withcreated continuously cracks to increasingpropagate AEthrough energy the release. rock with This iscontinuously called crack increasing propagation. AE In en thisergy region release. a gradual This is AE called release crack can propagation. be seen and thisIn this is called region stable a gradual crack propagation.AE release can This be isseen followed and this by moreis called rapid stable or exponential crack propagation. energy release, This is calledfollowed unstable by more crack rapid propagation. or exponential Further energy increasing release, of called load beyondunstable unstable crack propagation. crack propagation Further causesincreasing the rock of load to fail beyond [35,49 unstable,50]. crack propagation causes the rock to fail [35,49,50]. InIn thethe present present study, study, AE AE technology technology was was utilized utilized to identify to identify the effectthe effect of NaCl of NaCl saturation saturation on crack on formationcrack formation behavior behavior in siltstone in silt specimens.stone specimens. The results The of results measured of measured cumulative cumulative AE energy AE with energy axial stresswith foraxial varyingly stress for saturated varyingly specimens, saturated including specimens, dry including specimens, dry are specimens, shown in Figure are shown 12. The in failureFigure occurring12. The failure stress thresholdoccurring values stress for threshold each DOS values under for the consideredeach DOS threeunder brine the concentrationconsidered three levels brine are shownconcentration in Table 3levels. According are shown to Figure in Table 12 and 3. Accordin Table3, dryg to specimen Figure 12 is atand crack Table closure 3, dry stress specimen condition is at upcrack to 5.3 closure MPa axialstress stress, condition where up is crack to 5.3 closer MPa period axial willstress, be reducedwhere is up crack 4.3 MPa closer and period 3.4 MPa will at the be presencereduced ofup 10% 4.3 andMPa 20% and brine 3.4 inMPa the at pore the fluid presence and is of increased 10% and up 20% to around brine in 6.7 the MPa pore at thefluid presence and is ofincreased 25% concentration up to around brine 6.7 MPa in the at pore the presence fluid at fullyof 25% saturated concentration (100% brine DOS) in condition. the pore fluid Upon at crack fully initiation,saturated the(100% released DOS) AE condition. energy increases Upon crack almost initiation, linearly up the to released around 23.8 AE MPa,energy 17.6 increases MPa, 17.2 almost MPa, andlinearly 20.5 MPaup to in around dry, 10%, 23.8 20%, MPa, and 17.6 25% MPa, brine-saturated 17.2 MPa, and specimens, 20.5 MPa respectively in dry, 10%, under 20%, and fully 25% saturated brine- conditionssaturated specimens, (100% DOS). respectively However, thisunder energy fully release saturated trend conditions in 25 % brine-saturated(100% DOS). However, specimens this is quiteenergy different release fromtrend the in 25 other % brine-saturated specimens (dry, spec 10%,imens 20% is brine quite saturated different specimens),from the other where specimens when (dry, 10%, 20% brine saturated specimens), where when the specimen is saturated with high concentrations of NaCl (25% here), two fracture propagation periods can be seen. This is probably Energies 2019, 12, 14 16 of 21 the specimen is saturated with high concentrations of NaCl (25% here), two fracture propagation periods can be seen. This is probably due to the previously-mentioned hard NaCl crystallization effect, due to the previously-mentioned hard NaCl crystallization effect, where the fracture propagation where the fracture propagation occurred in these pores filled with this strong NaCl crystals may be occurred in these pores filled with this strong NaCl crystals may be exhibited through this exhibitedsecondary through fracture this propagation secondary fractureand specimen propagation would fail and only specimen after completion would fail onlyof the after both completion fracture of thepropagation. both fracture This propagation. shows the importance This shows of the the importance NaCl ionic of concentration the NaCl ionic for concentration fracture propagation for fracture propagationbehavior in behavior reservoir inrock. reservoir rock.

FigureFigure 12. 12.Cumulative Cumulative absolute absolute energyenergy withwith axial stress stress for for ( (aa) )dry dry specimen; specimen; (b ()b 10%) 10% NaCl; NaCl; (c) (20%c) 20% NaCl-saturatedNaCl-saturated specimen; specimen; ( d(d)) 25% 25% NaCl-saturated NaCl-saturated specimen.

AccordingAccording to to Figure Figure 12 12,, the the NaCl NaCl concentration concentration ofof thethe brinebrine solutionsolution of the rock rock specimens specimens has has a cleara clear effect effect on the on energythe energy released released from from the acoustic the acoustic system. system. Increasing Increasing the NaCl the NaCl concentration concentration causes a greatercauses a degree greater of degree AE energy of AE release,energy release, which which is to be isexpected, to be expected, as the as electron the electron conductivity conductivity of brine of increasesbrine increases with increasing with increasing numbers numbers of NaCl ionsof NaCl or NaCl ions concentration, or NaCl concentration, as this affects as thethis transferabilityaffects the oftransferability the acoustic energy of thereleased acoustic fromenergy the released rock to thefrom sensors the rock through to the thesensors pore through fluid [29 the]. However, pore fluid this is also[29]. affectedHowever, by this the NaClis also crystallization affected by the effect, NaCl where crystallization the greater effect, the NaCl where concentration, the greater the the NaCl greater theconcentration, ability to create the NaClgreater crystals the ability in the to rockcreate pore NaCl space. crystals The in application the rock pore of load space. on The the application rock not only breaksof load the on rock the matrix,rock not it only also causesbreaks the rock NaCl matrix, crystals it toalso break, causes causing the NaCl a higher crystals number to break, of AEcausing counts. a higherThe influence number of of AE brine counts. saturation on fracture formation behavior in reservoir rock is clear and therefore needs to be considered in the hydro-fracturing process. Table 3. Stress threshold value for dry and NaCl-saturated specimens.

Table10% 3. Stress NaCl threshold value for dry 20% and NaCl NaCl-saturated specimens. 25% NaCl DOS (%) σcc σ10%ci NaClσcd σucs σcc 20%σci NaClσcd σucs σcc 25%σci NaClσcd σucs 0 5.3 23.8 55.8 61.3 5.3 23.8 55.8 61.3 5.3 23.8 55.8 61.3 DOS (%) σ σ σ σ σ σ σ σ σ σ σ σ 25 1.3cc 14ci 40cd 41.5ucs 2.4cc ci20 cd61 ucs65.3 cc9.2 30.7ci cd57.9 ucs59.4 50 0 2.5 5.3 20 23.8 55.860 61.361.6 5.32.5 23.818 55.870 61.374 5.33.4 23.812.9 55.847.8 61.348.4 25 1.3 14 40 41.5 2.4 20 61 65.3 9.2 30.7 57.9 59.4 75 7 18 46.9 48.6 12.6 18.6 43.7 46.6 5.5 13.1 41.5 43.7 50 2.5 20 60 61.6 2.5 18 70 74 3.4 12.9 47.8 48.4 10075 4.3 7 17.6 18 46.939 48.640.6 12.63.4 18.617.2 43.741.4 46.642.9 5.56.7 13.120.5 41.5 40 43.7 68.9 σ100cc – Stress 4.3 at crack 17.6 closure; 39 σci – Stress 40.6 at crack 3.4 initiation; 17.2 σ 41.4cd – Stress 42.9 at crack 6.7 damage; 20.5 σucs – 40Stress at 68.9

σcccrack—Stress final at failure. crack closure; σci—Stress at crack initiation; σcd—Stress at crack damage; σucs—Stress at crack final failure. Energies 2019, 12, 14 17 of 21

3.5. Correlation of Reservoir Rock strength with NaCl Ionic Concentration and its DOS According to the all the research findings summarized above, the significant influence of NaCl ionic concentration and saturation level on reservoir rock compressive and tensile strengths is clear. An effort was therefore made to develop a relationship to exhibit the influence of these NaCl ionic solution properties on both the compressive and tensile strength of siltstone, by performing non-linear regression analysis using measured data and MATLAB software. The obtained relationships are shown in Equations (4) and (5). The first shows how the compressive strength of siltstone varies with NaCl ionic concentration and saturation and the other shows how the tensile strength of the same rock varies with these parameters, and both are in the 99% confidence band. However, it should be noted that the developed equations can only be used for Eidsvold siltstones with the brine conditions considered here.

2 3 2 σc = −80.2145D+134.2635D − 76.6561D +346.2722C−908.417C +4.5637CD + 32.0057 (4)

2 3 2 σt = −8.4069D+12.9294D − 6.2115D +9.6456C−32.5277C − 1.7638CD + 6.7607 (5) where σc is the average UCS, σt is the average STS, D is the degree of saturation and C is the NaCl concentration in percentage by weight. It is important to note that in Equations (4) and (5), the C and D values are in decimals. To date, no data are available on the effect of brine concentration and degree of saturation on the mechanical properties of siltstone reservoirs. The findings of this study are therefore vital for the hydro-fracturing process in low permeable reservoir rocks and will enable identification of the effects of NaCl concentration and degree of saturation on the strength properties and therefore the hydro-fracturing process in low permeable reservoir rocks such as shales. However, the mechanical properties of such reservoir rocks also vary with the injection fluid properties. Therefore, further studies are suggested to obtain a comprehensive understanding of the subject.

4. Conclusions The presence of brine in reservoir rock pores is believed to have an important influence on the hydro-fracturing process in unconventional gas extraction. This was considered in this study by conducting a wide range of laboratory tests (UCS and STS experiments) for varyingly saturated siltstone specimens and observing their fracture propagation behavior (AE and ARMIS technologies). According to current research findings:

­ Any type of NaCl ionic solution causes the strength to be reduced, mainly through weakening the bonding between the clay mineral and rock and breaking the solid–solid bonds and creating new weaker solid-liquid bonds. For example, dry specimens exhibited the highest UCS value in testing (61.6 MPa) and full saturation with 10%, 20%, and 25% NaCl caused the strength to be reduced by around 38.9%, 30.5%, and 36.7%, respectively. ­ Increasing the NaCl concentration in saturation fluid generally enhances the compressive strength of the rock through the NaCl crystallization effect in the pore structure, an excessive amount of NaCl molecules may cause the rock strength to be reduced through the corrosive influence which occurs at high ionic concentrations. For example, according to the test results, there is a strength increment in brine-saturated specimens up to 20% NaCl concentration of the saturation fluid and further increasing the NaCl ionic concentration to 25% causes strength reduction. ­ Similar to the compressive strength, the tensile strength of siltstone also decreases with NaCl ionic solution saturation, due to the same bond breaking and weakening effects. ­ However, the rate of strength reduction becomes much smaller after a particular degree of saturation, probably after filling the inter-granular pores. For example, in the test results, there is a high strength reduction in siltstone up to 25% DOS and further increasing the NaCl ionic solution saturation level up to 100% DOS causes the reduction rate to be significantly reduced. Energies 2019, 12, 14 18 of 21

­ Further, increasing the NaCl ionic concentration at any DOS causes siltstone tensile strength to be reduced, probably due to the interaction of NaCl ions with the siltstone rock matrix or the corrosive influence that is obviously greater at greater ionic concentrations. However, this trend is different to the compressive strength behavior, because the significant influence of NaCl crystals on compressive strength does not affect the tensile strength much due to the different load application. This is quite important for the hydro-fracturing process, as in this process fracture propagation mainly occurs through tensile failure and tensile strength is therefore the governing factor for the fracking process. ­ Regarding the effect of NaCl ionic solution saturation on the brittle characteristics of siltstone, saturation of siltstone with any NaCl ionic concentration generally causes its ductile properties to be enhanced, and the influence is greater at greater DOS, due to the polymer structure rearrangement which occurs in siltstone with fluid saturation. For example, according to the results, the elastic moduli of dry and saturated specimens vary between 19.33 and 9.55 GPa, and the dry specimens commonly have the highest elastic modulus and any saturation generally causes the E value to decrease. This is however not favorable for the hydro-fracturing process, as ductile rocks need more energy for fracturing, compared to brittle rocks. ­ However, having very highly concentrated NaCl ionic solution in siltstone causes its brittle properties to be enhanced, perhaps under high salinity concentrations, NaCl crystals occur in pore structure is much harder than low salinity concentrations and therefore the generally expecting its brittle characteristic affects the overall failure behavior of the siltstone. For example, according to the results, when the NaCl concentration is 25%, there is a quite high E value (19.33 GPa) in the tested siltstone for the 100% saturation condition, which is even higher than the E value for the dry condition (16 GPa).

To date, little information is available on the effect of brine concentration and degree of saturation on the mechanical properties of siltstone reservoirs. The findings of this study are therefore vital for the hydro-fracturing process in low permeable reservoir rocks. This study will enable the identification of the effects of NaCl concentration and degree of saturation on the strength properties, and therefore improve the understanding on hydro-fracturing process in low permeable reservoir rocks such as shales.

Author Contributions: A.W., R.P.G. and Q.L. conceived and designed the experiments; H.W. and A.W. performed the experiments; S.P. and T.R. analyzed the data; A.W. wrote the paper. Funding: This research was funded by the China Postdoctoral Science Foundation grant number 2018M630913. Acknowledgments: The authors would like to thank all the technical staffs of the Geo-lab at Monash University for their help with the experimental work and the financial support from the China Postdoctoral Science Foundation (Grant No.: 2018M630913). Conflicts of Interest: The authors declare that they have no conflict of interest. Ethical approval: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This article does not contain any studies with animals performed by any of the authors. Informed Consent: Informed consent was obtained from all individual participants included in the study.

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Author/s: Wanniarachchi, A; Gamage, RP; Lyu, Q; Perera, S; Wickramarathne, H; Rathnaweera, T

Title: Mechanical Characterization of Low Permeable Siltstone under Different Reservoir Saturation Conditions: An Experimental Study

Date: 2019-01-01

Citation: Wanniarachchi, A., Gamage, R. P., Lyu, Q., Perera, S., Wickramarathne, H. & Rathnaweera, T. (2019). Mechanical Characterization of Low Permeable Siltstone under Different Reservoir Saturation Conditions: An Experimental Study. ENERGIES, 12 (1), https://doi.org/10.3390/en12010014.

Persistent Link: http://hdl.handle.net/11343/227470

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