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Research Article Volume 6 Issue No. 7 Role of Well Testing and Information in the Industry- Testing in Multilayers Reservoirs Assoc. Prof. Nevton KODHELAJ M.Sc. Shkëlqim BOZGO

Abstract: The actual demand for oil and gas so as their products shows a clear increasing trend, despite some fluctuation mostly because of the global economic negative situation as well as some other geostrategic impacts. The new drilling activities and progressing toward deeper formations require stronger and abundant financial support, and looks that the safest way remains the increase of the oil recovery factor in the existing oil and gas fields. Well testing and their numerical modelling, gives responses that support their exploitation optimization. In many cases the oil and gas fields, represents as very complex systems composed by a large number of thin layers. Generally, they represent very complex hydrodynamic systems. The classification systems of these reservoirs, the calculation of the most important parameters, the factors that impacts and determine their exploitation performance as well as some economic indicators, are subject of this paper.

Key words: Well test, reservoir, petroleum engineering, production forecats, skin effect

1 INTRODUCTION The test represents one of the basic disciplines of the Oil & Gas Wells Testing reservoir engineering. The information collected regarding the fluid flow and the drawdown testing in the formation Pressure Data’s plays a key role for the determination of the productive Average Pressure of the Formation Use for Material Balance Calculations capacities of the oil and gas fields. The drawdown test gives Evaluation of the Vertical and Horizontal Evaluation of the Vertical and Horizontal Permeability an important support regarding the determination of the Anisotropy of the Permeability average pressure of the formations. This information’s Estimation of the Formation Extension Well Testing with Different Regimes allows the study and identification of the formations and Oil Saturation behavior and particularly their yield forecast, for different

Quantitative Evaluation of the Well Status, EOR regimes. The pressure data’s are without any doubt, among Pressure Data’s Interpretation the most important, regarding the reservoir engineering. Efficiency, Wells Productivity Estimation They play a key role to all the reservoirs exploitation stages Well Testing in Standard Conditions Estimation of the Formation Permeability and Well Status [7, 10]. Oil and gas wells testing, main objectives are: Quantitative Estimation of the Drainage Numerical Simulation 1. Well status evaluation and the reservoir Area and Well Status

Evaluation of the Permeability Changes form one Well to the other characterization; Parameters Estimated through the Well Testing and Data Inputs to the Model Figure 1: Information collected by the well testing 2. Calculation of the formation parameters for the purpose of the identification of its productive Is important to highlight that along the lifetime of the oil and segment; gas wells, starting from it’s drilling an up to its “abandonment”, should be collected information’s regarding 3. The identification of the well productive segment; its technical status and daily activity. Here differs the pressure data’s, to each single well or at least for the “key wells”, whose represent not less than 25% of the total wells, 4. Skin effect and factor calculation, and based in its in a periodical basis. Is very important to make careful value could be decided regarding the programming regarding the information’s that should be implementation of the EOR methods. collected through the well testing and its frequency. A very simplified logical sketch on the well testing is presented in 2 WELL TESTING AND ITS ROLE IN THE the Figure 2 [7]. It’s obvious that, the well testing could be used for the selection of the well that should be subject of the Enhancement Oil Recovery (EOR) methods. The core of the The oil and gas wells testing represent one of the strongest process stands in the identification of all factors, which tools, mostly regarding their reservoir engineering prospect, pushed toward the well productivity decline [3]. For this, as well the reservoir exploitation optimization. In a more could be very helpful the pressure build-up or pressure descriptive way the information collected by the well testing drawdown tests, the drill stem, the samples analysis or any is presented in the Figure 1 [7].

International Journal of Engineering Science and Computing, July 2016 1647 http://ijesc.org/ additional information. After “the identification”, should be optimization, and this can be achieved only after having the selected the optimal stimulation process and its correct classification. Figure 4 shows all activities that help implementation design. Summarized, the selection of all the starting from the formation classification, and the Figure 5 calculation procedures regarding the determination of the which shows the reservoir pressure data’s analysis for the exploitation parameters of each single well and the reservoir, purpose of the accurate evaluation of the reservoir of the formation properties evaluation, regarding the properties. Reservoir Characterization Through the efficiency and results of the EOR for the purpose of the OR Integrated Approach (Oil Recovery) increase with aim to maximize the financial profit, is given in Figure 3. A well testing process can be General Description of the Formation efficient regarding collecting the information’s, and later on with their analysis, but only if is done a detailed and accurate Rock Evaluation Structural Testing Reservoir Properties Estimation Integrated Approach · Lithology; · Structure; · Profiles; · Porous Volume; · Sediments Origin; · Continuation; · Reservoir Boundaries; planning, design, evaluation and coordination of all · Transmissivity. · Rock Evaluation. · Approx. Thickness. · Real Thickness. processes [1, 3].

Information's on the Yield and Samples Analysis Oil Wells Testing Pressure Programming, Reasoning, Time, Priorities

Fluid Flow Analysis Methods, Pressure Measurement and their Utilization

Design, Direction, Analysis See Figure 5 Figure 4: Reservoir classification through the integrated Before the Exploitation Along the Exploitation testing of the system

Information's on the Fluids Well Testing Fluid Flow Analysis Methods, Pressure Measurement and their Utilization Information's on the Well Exploitation

Information's on the Injections, EOR’s etc. Well Testing Methods Most Important Interpretation Collector Parameters Others Figure 2: Well testing programming Solution for Different Initial and A n o i t a u l a v E Fluid Flow Equations Boundary conditions and Geometry

d n a s u t a t S n o i t a m r o F / l l e W n o i t a t i o l p x E s r e t e m a r a P n o i t a t i o l p x E Horizontal Wells s n o i t a l u c l a C r, r’, s, k ,k e tc. Testing v h s ’ R O E

) t c e f f e n i k S ( e g a m a D e r o b l l e W

e n i l c e D e r u s s e r P y t i l i b a e m r e P w o L y t i l i b a e m r e P e s U Drawdown & Buildup kh, s’1, s’2, s, D, pr, Δps, Testing, Initial Pressure pores volume, pi, pa

s l l e W , s i s y l a n A e l p m a S , s e u q i n h c e T g n i t s e T l l e W n g i s e D R O E s ' n o i t a m r o f n I l a n o i t i d d A , n o i t a t i o l p x E

n o i t a z i l i t U n o i t a m r o f n I n o i t a m i t s E l a n i F Fractured Collectors Cs, CsD, k, Φct, s’, sf, xf, ω, λ

s r o t c a F e n i l c e D n o i t c u d o r P t n e m e v o r p m I g n i t s e T l l e W Fluid Flow Regime k, s, kf, C, CD, CDf, m, Δ, ω A

Figure 3: Well selection for the purpose of the EOR

optimization Fracking kfu, sf, r’u, xf, (kfbf)D, r’w/xf, kfbf Drilling samples analysis ensures valuable information’s regarding the preliminary identification of the rocks. The well testing process somehow support toward its Wells Interference kh/μ, Φhc, Φμct, k, s, ν, Φt, khm improvement. Also, the well testing support, regarding the identification of all areas with low permeability whose can create difficulties during the normal process of the reservoir Heterogeneous kx, ky, kxy, kmin, kmax, Θ, Φt, Φμct, kht exploitation, or even for the identification of all areas with Reservoirs Analysis well-developed fractures, and by summarizing it helps for Figure 5: Different analysis system for the collector the preparation of the permeability map of the formation. properties calculation The main goal of the reservoir engineering is the OR

International Journal of Engineering Science and Computing, July 2016 1648 http://ijesc.org/ The multilayers reservoirs well testing, including the natural Geological Modeling Petroleum Engineering Modeling · Geophysical logging · Petroleum Engineering · Mapping and profile creation based in the · Data Entering in the Reservoir Model fracturing reservoirs, the solution of the flow differential surface geology equations with constant flow or pressure, and the economic Geophysical Modeling Geostatistical Modeling · Space Distribution of the Collector Properties aspects of the exploitation of these kinds of reservoirs, have · Seismic and their Correlations been always challenging for their industrial exploitation. · Interpretation The Information Allows a Detailed Description of Figure 6 represents the multilayers reservoirs classifications Geological Modeling the Reservoir · Geophysical Logging · Mapping and profile creation based in the systems [7]. surface geology Integrated Reservoir Modeling Multilayer's reservoirs Multilayer's reservoirs with without communication Composed reservoirs Interlayer's flow Figure 9: Reservoir modelling development communication layers layers 3.1.2 Numerical modelling simulators selection and their The layers communicate The layers communicate Differently known as Noticed the flow in between through the contact surfaces only through the wells composed reservoirs. The the very thin layers well are not yet opened and use represent the only communication way Most of the numerical simulators can be very efficiently Figure 6: Multilayer reservoirs classification systems used for the determination of the fluids properties, water cones creation process, the geological profiles creation etc., 3 NUMERICAL MODELLING AND ITS USE and also can be used as modelling methods for the reservoir engineering. To the reservoir engineers remains the difficult The numerical modelling includes the large scale use of the task for the selection of the optimal method for the purpose computers for the solution of the flow differential equations, of the modelling of the formation. Its selection could be done and in this way, to make possible the modelling of the fluids only after analysing all the parameters and gathered flow. These methods gives very satisfactory results, and information’s. This is a very complex and high costly consist in the creation of a model (spatial net) whose process. Summarized this is presented in the Figure 10 [7]. approach the porous medium (by combining of all its properties). The reservoir is divided in a number of Selection of the Reservoir elementary blocks, and to each one of them is than applied Simulation Models simultaneously the mass and energetic material balance. The Reservoir Classification technics used for this purpose are different, and they are presented in a summarized way in the Figure 7 [3, 7, and 13]. Gas Fields Oil Fields Oil & Gas Fields Condensate

Field Engineer Computer Engineer Dimensions

Data Entering Reservoir Simulation Results 0-D 1-D 2-D 3-D Modifications Results Analyses Figure 7: Reservoir modelling Geometry

3.1 Scope and aim of the reservoir modelling X X-Y Y-Z R-Z X-Y-Z The simulation programmers can be used for the purpose of the reservoirs testing, even if they are exploited by a single well, by some wells or even by systems of wells, whose Phases interact with each-other. Summarized this is presented in the

Figure 8 [7]. 1-Phase 2-Phases 3-Phases N-Components Computer Simulation Aims Figure 10: Numerical modelling optimization process Economic Original Oil in Place Calculation Reservoir Exploitation Scenarios Formation Fluid Flow Parameters The most important parameters for this selective process are Original Gas in Place Calculation Single Well Testing Oil and Gas Fields Exploitation Systems Optimization [1]: Figure 8: Computer simulation aims · The reservoir type; 3.1.1 Reservoir modelling process development · The geometry and formation extension; · Database validity; After that all information are collected they are at first · Secondary and tertiary EOR methods applied in the individually processed by creating some models. Via the reservoir; geostatistical methods is possible to intertwine all in one · Expert’s needs; single model. Summarized this process in presented in the · Logistic; Figure 9 [7]. · The financial efficiency of the numerical modelling.

International Journal of Engineering Science and Computing, July 2016 1649 http://ijesc.org/ 4 PRESSURE CHANGE ANALYSIS IN THE

MULTILAYERS RESERVOIRS k1, Φ1, h1

Clay Generally the multilayers reservoirs can be classified in tow Sandstone no k2, Φ2, h2 communication layers groups: Multilayer reservoir with interlayer’s Clay communication, whereas the thin layers do have k3, Φ3, h3 hydrodynamic communication, mainly in the contact areas, Clay k4, Φ4, h4 and the reservoir without such a communication [4, 8].

Figure 12: Four layers reservoir with non-communication 4.1 Multilayers reservoirs with communication layers layers 20 Figure 11 represents a simplified sketch of a reservoir composed by four thin layers, whose communicate in between. The transitory well testing shows that the pressure in these systems changes in the same way as in the 15

homogenous reservoirs. In this type of reservoirs can be  g fp  p g k  implemented the following equations - For the calculation of q 1 2 f .

p k 2

 10

141 100 the product permeability-layers thickness [4, 6 and 11]: kh  D

p 102 1 (1) 5 For the calculation of the product porosity-compression [4, Approximate end of the transitory flow 6, and 11]:

0 102 103 104 105 106 107 (2) 0.000264 kt t  DF  c F g t The total number of the layers is n. The permeability of each Figure 13: Muskat curves for a two layers reservoir with one of the layers can be calculated through the following non-communication layers equation [4, 6 and 11]: The pseudo-steady regime begins approximately at the time:

(3)

(4)

Impermeable sealing

Layer 1 k1, Φ1, h1 k1>k2 The beginning time of this regime depend also on the:

Layer 2 k , Φ , h 2 2 2 Communication layers · The fluctuation of the values of the porosity, width Layer 3 k , Φ , h k >k 3 3 3 3 2 and compression of the layers, by each-other; Layer 4 k4, Φ4, h4 Impermeable sealing · The geometric shape of the reservoir; · Layers number; Figure 11: Four layers reservoir with communication layers · Well location. 4.2 Multilayers reservoirs without communication Figure 13 represents the curve of the dimensionless pressure layers by the dimensionless time, for a reservoir composed by two Figure 12 represents a simplified sketch of a reservoir layers with permeability ratio k1/k2=100, 10, 2, 1. For all composed by four layers that doesn’t have inter-layers four curves the ratio rb/rw=2000. The dimensionless communication. The only communication way in this case parameters are calculated as follows: remains the well. In the initial phase of the well exploitation the pressure drawdown curve is linear, as it is showed in the (5)

Figure 12. Once the reservoir boundaries effects appear the (6) curve is bending [2, 5 and 8].

(7)

where:

(8)

(9)

(10)

International Journal of Engineering Science and Computing, July 2016 1650 http://ijesc.org/ 4.3 The composed reservoirs The equivalent length of the fractures and their equivalent conductivity are calculated as follows: The kinds of reservoirs, as mentioned above are named composed reservoirs. The thin layers do communicate with (12) each-other, only by the well, as it showed in the Figure 14.

Practically the fluid flows by the thin layer and raise to the (13) surface only via the well [4, 6, 11]. The dimensionless conductivity is calculated [4, 6, and 11]:

Zone 1 k1, Φ1, h1

n (14)

e o s i ' n t k’ , Φ’ , h’ r 1 1 1 a o

e c z i y

a n l d

Zone 2 k’ , Φ’ , h’ u

2 2 2 r e e m t x As the solution of the diffusion problem for the multi-layers n i m k’3, Φ’3, h’3 I o M c n

o s i ' t

k” , Φ” , h” r reservoirs as very complicated, by Camacho is showed how 1 1 1 a e c i y a n l

Zone 3 k” , Φ” , h” u

2 2 2 r e m t can be handled this problem by considering the reservoir as n m k”3, Φ”3, h”3 I o c it represent a single layer system. The basic assumption is that the fractures don’t communicate in between. In the Figure 14: Composed reservoir with communication layers fracture would communicate than the value of the CfD would be somehow bigger and the value of the ratio hf/xfj plays an 4.4 The reservoirs with inter-layers communication important role regarding the well performance. In the layers Figure 15 represents a composed reservoir with inter-layer have been subject of hydraulic fracking, than the maximal communication. The flow and pressure changes profile, for a yield could be taken only in case that the flow front reach gas well, can be based in the homogenous reservoir theory. any fracture simultaneously. Regarding the gas reservoirs, Whilst, with regard to the oil wells, that behaves as the maximal yield of a two layers reservoir could be gotten homogenous, with product permeability-thickness, kh, equal when is fulfilled the following criteria [4, 6, and 11]: with the value of the entire system. The presence of the inter- layers communication can be verified only by analysing the (15) wells flow and pressure profiles [4, 6, and 11].

where: (16)

Sealing The dash line in the Figure 16 is drawn by the solution of the equation 15. Its solution is done based in the assumption Low permeability layer that the boundaries effect is negligible. Is not possible to use the reservoir conductivity it the boundaries of the reservoirs a e

r effects plays any role.

a F lu 2 e d i 10

g d i a re fl n c o i ti w a o r n D High permeability layer

10

2 B 1 h h 1 Sealing 2 çD 1 çD C C Figure 15: Two communication layers reservoir A

4.5 Fractures conductivity in the multi-layers 10-1 reservoirs This concept is introduced by Raghavan et al, and is 10-2 calculated with the following equation: 10-2 10-1 100 10 102 103 CçRShD 2 xç2 C x çRShD1 ç1 Figure

16: ProductivityFigura 15. 7: Kr icriteria’steret për prod hforues hthemëri maximalmaksimale s ipyieldas Cam acho (11)

where: ηj diffusion coefficient of the layer j.

5 FACTORS THAT IMPACT THE MULTI- LAYERS RESERVOIRS PERFORMANCE

Generally, the factors that impact the multi-layers reservoirs exploitation are [12]:

International Journal of Engineering Science and Computing, July 2016 1651 http://ijesc.org/ · Relative permeability: If all thin layers have the the decision making regarding the EOR same value of the relative permeability, the average implementation; water saturation will be larger in the less permeable 5. The reservoir numerical modelling, as the most layers than in the more permeable ones. This is important scope toward the reservoir exploitation because their average pressure is higher in the optimization is based in the identification of the layers will lower permeability; type, spatial extension and its geometry, human and · Pores size: If the pores size in the less permeable financial resources; layers are smaller than those of the more permeable 6. The multilayers reservoirs exploitation process is layers, than to the fluid flow in the reservoir would very complex, mostly due to their heterogeneity and appear additional resistance. This effect can be anisotropy; calculated by the utilization of the capillary 7. The well testing allows the evaluation of the pressure curves; conditions and characteristics of these kinds of · The formation geometry: The geometric shape and reservoirs; the inter-layer communication spatial distribution, 8. Though the well testing is possible the have strong impacts with regard to the multilayers determination of the parameters for each layer for reservoir exploitation; the purpose of the accurate description of the · The permeability anisotropy: In most of cases of the multilayers reservoir; oil reservoirs, the vertical permeability is much 9. This process allows the prediction of the reservoirs smaller than the horizontal one; performance along their exploitation period; · N-Layers systems: The exploitation performance 10. The economic indicators of these reservoirs analysis is strongly depending by the accuracy of exploitation tend to be more positive than of the the flow equations, especially in the case of the sum simple ones. formulas. 8 REFERENCES 6 THE ECONOMIC INDICATORS OF THE [1]. Amanat U. C., Pressure transient test analysis MULTILAYERS RESERVOIRS user’s handbook, Advanced TWPSOM Systems The multilayers reservoirs type has direct impact with regard Inc., Houston, Texas, Vol. 8, Oct. 1995; to the economic efficiency (feasibility) of their exploitation. Some of the inter-layer communication advantages are [7, [2]. Camacho, V., Raghavan, R., and Reynolds, A. C. 12]: “Response for Wells Producing Layered Reservoirs, Unequal Fracture Length”, SPE Formation Eval · Shorter exploitation time; (Feb. 1987), 9-28; · Higher oil recovery coefficient; · Lower cost for the well perforation and completion; [3]. Cinco, H., Miller, F. G., and Ramey, H. J. Jr., · Shorter time for the well test information “Unsteady state pressure distribution created by a interpretation. directionally drilled well”, SPE-AIME, 1975;

In many cases the reservoirs with lack of inter-layers [4]. Cobb, W. M., “A Study of Transient Flow in communication, could be converted in reservoirs with Stratified Reservoirs with Commingled Fluid communication via the application of the hydraulic fracking. Production”, PhD, Dissertation, Stanford The artificially vertical fractures created, allows the fluid University, Stanford, CA, 1970; flow by all layers, whose are already communication in between and not only by the well, whose was representing [5]. Cobb, W. M., Ramey, H. J. Jr., and Miller, F. G., the only communication way. “Well Test Analysis for Wells Producing Commingled Zones”, J. Petroleum Tech.(Jan. 1972), 27-37; 7 CONCLUSIONS 1. The well testing process allows the wellbore status [6]. Earlougher, R. C. Jr., Kersch, K. M., and Kunzman, evaluation and the reservoir characterization; W. J., “Some Characteristics of Pressure Build up 2. It allows the reservoir properties determination for Behaviour in Bounded Multiple Layers Reservoirs the purpose of the accurate reservoir description; Without Cross flow”, J. Petroleum Tech.(Oct. 3. It helps for the identification of the productive 1974), 1178-1186; section of the well; 4. The wellbore status, reservoir permeability [7]. Kodhelaj, N. “Studimi hidrodinamik i puseve të calculation combined with the statistical and naftës, Pjesa I+II”, Sh. B. L. U., Tiranë, 2009; graphical processing of the information’s allows the skin effect calculation. These processes facilitate

International Journal of Engineering Science and Computing, July 2016 1652 http://ijesc.org/ [8]. Lefkovits, H. C., Hazebroek, P., Allen, E., and Matthews, C. S., “A study of the Behaviour of Bounded Reservoirs Composed of Stratified Layers”, Soc. Petroleum Eng. J. (March 1961), 43- 58;

[9]. Raghavan, R., Topaloglu, H. N., Cobb, W. M., and Ramey, H. J. Jr., “Well Test Analysis for Wells Producing from two Commingled Zones of Unequal Thickness”, J. Pet. Tech. (Sept. 1974), 1035-1043;

[10]. Ramey, H. J. Jr., and Cobb, W. M., “A general build-up theory for a well in closed drainage area”, J. Pet. Tech. (Dec 1971) 1493-1505;

[11]. Russell, D. G., and Prats, M., “The practical Aspects of Inter Layer Cross flow”, J. Petroleum Tech. (June 1962), 589-594;

[12]. Russell, D. G., Goodrich, J. H., Perry, G. E., and Brushkotter, J. F., “Methods for Predicting Gas Well Performance”, J. Pet. Tech. (Jan. 1966);

[13]. Saidikowski, R. M., “Numerical simulations of the combined effects of wellbore damage and partial penetration”, paper SPE 8204 (September 1979).

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