DSpace at VNU: Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System

This article was downloaded by: [Selcuk Universitesi] On: 26 December 2014, At: 10:17 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Petroleum Science and Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lpet20 Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System a a b b T Q C Dang , Z Chen , T B N Nguyen , W Bae & C L Mai a University of Calgary , Calgary , Alberta , Canada b Sejong University , Gwangjin-ku , Seoul , Korea c c Ho Chi Minh City University of Technology , Ho Chi Minh , Viet Nam Published online: 19 Jun 2013 To cite this article: T Q C Dang , Z Chen , T B N Nguyen , W Bae & C L Mai (2013) Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System, Petroleum Science and Technology, 31:14, 1454-1470, DOI: 10.1080/10916466.2010.545792 To link to this article: http://dx.doi.org/10.1080/10916466.2010.545792 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in 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a numerical investigation for evaluating the potential applicability of the steamassisted gravity drainage (SAGD) recovery process under complex reservoir conditions such as shale barriers, thief zones with bottom and/or top water layers, overlying gas cap, and fracture systems in the McMurray and Clearwater formation The simulation results indicated that the near-well regions were very sensitive to shale layers, and only long, continuous shale barriers (larger than 50 m or 25%) affect the SAGD performance in these well regions In addition, the thief zones had a strongly detrimental effect on SAGD The results also showed that the SAGD recovery process was enhanced in the presence of vertical fractures but horizontal fractures were harmful to recovery Fracture spacing is not an important parameter in the performance of a steam process in fractured reservoirs and extending horizontal fractures will reduce ultimate oil recovery in the SAGD process This article provides a guideline for SAGD operations in complex geological reservoirs Keywords: fracture system, numerical simulation, SAGD, shale barriers, thief zones INTRODUCTION Alberta’s oil sands deposits, with estimated 1.7 trillion barrels of bitumen in place, account for approximately 40% of the world’s bitumen resources (Figure 1) However, an extremely high viscosity of bitumen at reservoir temperature is one of the greatest challenges in using a recovery process At a company with recent advances in horizontal well technology, steam-based in situ recovery methods, aiming at a thermal viscosity reduction, have emerged for exploration of these vast resources (Butler, 2001) The steam-assisted gravity drainage (SAGD) recovery process has opened the door to producing a large number of bitumen reservoirs in Canada SAGD was first developed by Roger Butler and his colleagues in Imperial Oil in the late 1970s It is a thermal oil recovery process that consists of a pair of two parallel horizontal wells drilled near the bottom of the pay The top horizontal well is used to inject steam, and the bottom horizontal well is used to produce reservoir fluids (Figure 2) The heat from steam is transferred by thermal conduction into the surrounding reservoir The steam condenses and the heated oil flows to the production well located below by gravity Two types of flows exist during this process Address correspondence to Wisup Bae, Sejong University, 98 Gunja-dong, Gwangjin-ku, Seoul 143-747, Korea E-mail: wsbae@sejong.ac.kr 1454 Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS FIGURE 1455 Oil sands in Alberta, Canada (color figure available online) One is at the ceiling of a steam chamber and the other is along the slopes of the steam chamber The success of an SAGD project depends on some key factors such as an accurate reservoir description, efficient utilization of heat injected into the reservoir, understanding of displacement mechanisms, understanding of geomechanics, and overcoming various constrains (Doan et al., 1999) Successful field tests have proven that SAGD is a viable technology for in situ recovery of heavy oil and bitumen (Singhal et al., 1998; Butler, 2001; Boyle et al., 2003) The SAGD technique has many advantages over other thermal methods such as conventional steam flooding methods SAGD overcomes the shortcomings of steam override by using only FIGURE Steam-assisted gravity drainage technology (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1456 T Q C DANG ET AL gravity as the driving mechanism, which leads to stable displacement of oil and potentially high oil recovery In addition, the heated oil remains hot and movable as it flows toward the production well, whereas in conventional steam flooding, the oil displaced from the steam chamber cools down and consequently the oil-phase viscosity increases as the oil flows to the production well (Chen et al., 2008) The SAGD process is made more thermally efficient by maintaining a liquid pool that surrounds the bottom production well and preventing the escape of steam from the steam chamber However, Farouq Ali (1997) and Singhal et al (1998) pointed out some limitations of SAGD as follows: (1) the theory pertains to the flow of a single fluid; (2) only steam flows in the steam chamber, oil saturation being residual; (3) the heat transfer ahead of the steam chamber to cold oil is by conduction only; (4) sand control may be necessary; (5) there is a hot effluent/high water cut production; (6) frequent changes in operating regimes and high operating costs occur; and (7) deterioration of production at late stages occurs This article presents a comprehensive evaluation of SAGD’s performance in presence of continuous shale barriers, discontinuous shale barriers, bottom and top aquifers, and gas cap layers In particular, the effect of a fracture system on SAGD operation is also described STATEMENT OF THE PROBLEM The success of SAGD has been mostly demonstrated by numerical simulation with homogeneous reservoir models However, this process is very sensitive to reservoir heterogeneity; therefore, it is necessary to have a comprehensive understanding of the effects of reservoir heterogeneity on SAGD performance for wider and more successful implementation The efficiency of the SAGD process is significantly decreased in the presence of shale barriers or thief zones such as bottom and top aquifers, overlying gas caps, or fracture systems Thus, the first attempt of this research is motivated by the need for an improved SAGD process in heterogeneous reservoirs Such an improvement is crucial to broaden the applications of SAGD and unlock vast discovered heavy oil/bitumen resources worldwide In addition, the performance of SAGD is compared in different geological areas including the McMurray formation and the Clearwater formation This comprehensive comparison will allow us to fully evaluate the effect of reservoir properties on the SAGD process in hostile conditions DESCRIPTION OF A SYNTHETIC RESERVOIR MODEL The advanced thermal reservoir simulator, STARS, developed by the Computer Modeling Group Ltd (Calgary, Alberta, Canada), was used to construct a reservoir model and evaluate the performance of the SAGD process A synthetic reservoir model that represents two generic formations in the Alberta oil sands was selected for this research The main reservoir properties of two formations are shown in Table In order to reflect the reservoir heterogeneity, the formation consisted of clean sands and shaly sands that contained some thin shale lenses The bitumen viscosity of the McMurray formation was much higher than that of the Clearwater formation The producers, with a length of 900 m, were located at the bottom of the reservoir and the injectors were m above the producers The horizontal spacing between well pairs was 50 m The steam injection pressure was set at 2,500 kPa for the McMurray formation and 3,600 kPa for the Clearwater formation In the first months, we specified a line heater in the grid cells that contained the wellbores instead of using steam circulation through both the left and right injection and production wells The heat flux was determined by the amount of latent heat in which 400 m3 /day of 0.95 quality steam was delivered to the reservoir A steam trap control is important in SAGD as well as SAGD RECOVERY PROCESS 1457 TABLE Typical Reservoir Properties of McMurray and Clearwater Formations Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 Reservoir Parameter Reservoir depth, m Porosity Vertical permeability (Kv ), D Permeability ration (Kv =Kh ) Oil saturation Reservoir pressure, kPa Reservoir temperature, ı C Bitumen viscosity at reservoir temperature, cP Rock compressibility, 1/kPa Formation heat capacity, kJ/m3 K Rock thermal conductivity, J/m.d.C Oil thermal conductivity, J/m.d.C Water thermal conductivity, J/m.d.C Gas thermal conductivity, J/m.d.C McMurray Formation Clearwater Formation 210 0.35 0.8 1,800 11 2,000,000 7E 06 2.39EC06 6.6EC05 1.15EC04 5.3496EC04 139.97 450 0.31 1.5 0.7 2,900 12 60,590 9.6E 06 2.35EC06 6.6EC05 1.15EC04 5.3496EC04 139.97 fast-SAGD to prevent or reduce steam production from the reservoir This steam trap control should result in keeping all of the latent heat generated by the steam inside the reservoir and producing only bitumen and condensed hot water In this study, the operating constraint at the production wells imposed a maximum temperature difference between the saturation temperature corresponding to the pressure of the fluids and the temperature in the wellbore equal to 5ıC RESULTS AND DISCUSSION Shale Barriers Heterogeneity plays a critical role in understanding steam chamber growth at the actual field scale and within simulations It is important and necessary to understand the factors determining growth rates and areal propagation Unfortunately, most numerical simulation investigations have been conducted with homogeneous systems, so these studies cannot be applied directly to provide accurate, reliable predictions for a field-type system During the last two decades, several researchers have attempted to evaluate the effect of reservoir heterogeneity on steam chamber development for the SAGD process One of the first to present their research on this topic was Joshi and Threlkeld (1985) Through experiments at the laboratory scale, Yang and Butler (1992) studied the effect of a shale barrier length (short and long horizontal barriers) for both top steam injection and bottom steam injection cases With a top steam injection, the presence of a short horizontal barrier has no effect on the general performance and a long horizontal barrier decreases the production rate, though not as much as expected in some configurations They also concluded that the heated bitumen above the barrier may not be produced even though it is hot because of the steam pressure holding up the oil at the bottlenecks to the flow Additionally, Yang and Butler (1992) showed that long shale barriers can cause a difference in the advancement velocity of the interface above and below the barrier This difference is reduced by the drainage of heated bitumen through conduction above the barrier Pooladi-Darvish et al (2002) proposed a better way to investigate the effect of shale barriers in complex geological characterizations by using a stochastic model based on geostatistical methods to represent the shale distribution Chen et al (2008) conducted a numerical simulation study on Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1458 T Q C DANG ET AL the stochastic of shale distribution near the well region and above the well region They stated in their conclusion that the SAGD performance was affected adversely only when the above-well region contained long, continuous shales or a high fraction of shales Recently, Ipek et al (2008) conducted numerical studies of interbedded shales in SAGD The purpose of this research was to determine the potential of pressure cycling as a method of enhancing the reservoir permeability Le Ravalec et al (2009) conducted a numerical investigation and showed that the influence of shale baffles depended upon their locations relative to the well pairs Shin and Choe (2009) constructed a two-dimensional homogeneous model and tested the effect of shale barriers that were located in the above- and between-well pairs The effect of reservoir heterogeneity on SAGD performance was studied by including randomly distributed, discontinuous or continuous, thin shale lenses Shale is characterized by low permeability, typically in the range of 10 to 10 mD The effects of shale barriers have been investigated in many case studies (Yang and Butler, 1992; Pooladi-Darvish et al., 2002; Chen et al., 2008) and different depending on the location, size, and volume of the shale layers Shale Barriers between Injector and Producer First, the effect of discontinuous shale barriers in the horizontal direction was evaluated; the size of shale barriers varied from to 30 m Steam cannot perfectly propagate in a reservoir when a shale barriers exist; thus, the cumulative oil recovery continuously decreases as the size of the shale barriers increases (Figures and 4) The shale barriers had a great effect on the amount of oil recovery in the Clearwater formation Figure shows the effect of shale barrier orientation on cumulative oil recovery in the McMurray formation The numerical simulation indicated that the thermal efficiency would be significantly decreased in the presence of vertical shale barriers due to the fact that a steam chamber cannot perfectly develop in the sideway as shown in Figure As a result, the SAGD performance is higher in the case of horizontal shales as compared to vertical shale barriers In FIGURE Effect of discontinuous shale barriers in the horizontal direction on cumulative oil recovery in the McMurray formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS 1459 FIGURE Effect of discontinuous shale barriers in the horizontal direction on cumulative oil recovery in the Clearwater formation (color figure available online) FIGURE online) Effect of discontinuous shale barrier orientation on cumulative oil recovery (color figure available Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1460 T Q C DANG ET AL FIGURE Effect of discontinuous shale barrier orientation on the steam chamber (color figure available online) addition, an increase in shale barriers in both cases led to an increase in cumulative steam-oil ratios (CSOR; Figures and 8) The effect of continuous shale barriers in the horizontal and vertical directions was compared and is shown in Figure The existence of continuous shale barriers in the vertical direction is the worst-case scenario for SAGD operation; it prevents the steam chamber from forming in the sideway and, as a result, the CSOR is the highest and the cumulative oil recovery is the lowest among three cases Figures 10 and 11 indicate the dominant effect of continuous shale barriers on the cumulative oil recovery in the McMurray and Clearwater formations Oil recovery is much lower in the presence of lengthy continuous shale barriers FIGURE online) Effect of discontinuous shale barriers in the horizontal direction on CSOR (color figure available Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS FIGURE online) 1461 Effect of discontinuous shale barriers in the vertical direction on CSOR (color figure available FIGURE Effect of continuous shale barriers in the horizontal and vertical directions on CSOR and cumulative oil recovery in the McMurray formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1462 T Q C DANG ET AL FIGURE 10 Effect of continuous shale barrier size on cumulative oil recovery in the McMurray formation (color figure available online) FIGURE 11 Effect of continuous shale barrier size on cumulative oil recovery in the Clearwater formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS 1463 FIGURE 12 Effect of shale volume (horizontal direction) on cumulative oil recovery in the McMurray formation (color figure available online) The volume of shales is also a critical issue that strongly affects the SAGD performance Figures 12 and 13 indicate that the cumulative oil recovery largely decreased as the shale volume increased in terms of shale barriers in the horizontal direction Once it reached the shale layers, steam chamber development and oil recovery did not increase much due to the restricted area available for steam flow to pass the layers However, the dependence of bitumen recovery on the shale volume in the vertical direction was slight because steam could move up to the top of the reservoir without any difficulty Shale Barriers Above-Well Pairs Discontinuous shale barriers had a minor effect on the SAGD performance; the cumulative oil recovery and CSOR were almost similar even when the length of the shale barriers increased from 10 to 40 m (Figures 14 and 15) However, there was a significant difference between continuous and discontinuous shale barriers The continuous shale layers that exceed 70 m in length were detrimental (low cumulative oil recovery and high CSOR) to the SAGD process In the Clearwater formation, the CSOR sharply increased when the shale barrier extended from 30 to 70 m Figures 16 and 17 show the effect of the location of the shale barriers on cumulative oil recovery and CSOR The shale layers were located in three important positions: near the injection well, in the middle of the reservoir, and at the top of the reservoir The simulation results indicate that the well pairs should not operate near a shale layer because those layers will prevent the propagation of steam in the vertical direction As the volume of discontinuous shale barriers increased, the cumulative oil recovery slightly decreased (Figure 18) On the contrary, the amount of bitumen recovery greatly decreased as the volume of continuous shale barriers in the reservoir increased (Figure 19) It is important to note that this phenomenon is immutable even when the shale layers are located near the injection well, in the middle of the reservoir, or at the top of the reservoir Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1464 T Q C DANG ET AL FIGURE 13 Effect of shale volume (vertical direction) on cumulative oil recovery in the McMurray formation (color figure available online) FIGURE 14 Effect of discontinuous shale barrier size (above-well pair) on cumulative oil recovery in the McMurray formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS 1465 FIGURE 15 Effect of discontinuous shale barrier size (above-well pair) on cumulative oil recovery in the Clearwater formation (color figure available online) FIGURE 16 Effect of shale barrier location (above-well pair) on cumulative oil recovery in the McMurray formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1466 T Q C DANG ET AL FIGURE 17 Effect of shale barrier location (above-well pair) on CSOR in the McMurray formation (color figure available online) FIGURE 18 Effect of shale volume (discontinuous shale barriers, above-well pair) on cumulative oil recovery in the McMurray formation (color figure available online) Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS 1467 FIGURE 19 Effect of shale volume (continuous shale barriers, above-well pair) on cumulative oil recovery in the McMurray formation (color figure available online) Thief Zone Layers In order to investigate the effect of thief zones including a bottom water zone (BWZ), overlaying water zone (OLWZ), and gas cap, another numerical simulation study was conducted An active aquifer, 10 m thick (either above or below bitumen zones), was added to the reservoir model The cumulative oil recovery in the case of OLWZ was much lower than in the BWZ because the injected steam was diverted into the water zone (Figure 20) Another disadvantage of the OLWZ is the moving of oil in the pay zone into the top water zone when a very small pressure gradient exists between the steam chamber and the top of the water zone The OLWZ acts as an absolute thief zone in the SAGD process because it delays sweeping of the pay zone by the steam chamber These important features may significantly reduce the efficiency of the SAGD process However, Doan et al (2003) found that as steam was continuously injected into the confined overlying water sand, enough heat was available to flash water into steam and allow gravity to drive oil from the top of the reservoir In addition, simulation results indicated that the effect of a gas cap on cumulative oil recovery and CSOR was slightly higher than BWZ and OLWZ As the steam chamber approached the gas cap, and if the steam pressure was kept higher than the gas cap pressure, steam and possibly some oil were pushed into the gas cap (Pooladi-Darvish and Matter, 2002) Fractured Reservoir The numerical simulation of the SAGD process was investigated in a fractured reservoir A dual-porosity model was applied to represent the fractured system The fractured porosity and permeability were 0.65 and 10 Darcy, respectively From the simulation results, the formation of a steam chamber in a fractured reservoir is faster than in a conventional reservoir due to the large Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 1468 T Q C DANG ET AL FIGURE 20 Effect of BWZ and OLWZ on cumulative oil recovery and CSOR in the McMurray formation (color figure available online) difference in permeability between the matrix and fracture systems The steam spreads through the fractures and then starts to heat up and diffuse into the matrix Figure 21 shows the effect of vertical fractures on cumulative oil recovery High conductivity of the vertical fractures helped steam to propagate deep into the reservoir, diffuse into a matrix block, affect the matrix for more contact, and lead to higher ultimate oil recovery than in a conventional reservoir The simulation results also indicated that with a higher density of vertical fractures, the final cumulative oil recovery increased An unexpected effect in the lateral expansion of the steam chamber was that only a small amount of the injected steam could diffuse upward from the fractures to the top of the reservoir Thus, the cumulative oil recovery in the presence of horizontal fractures was greatly decreased compared to a conventional reservoir (Figure 22) Moreover, increasing the horizontal fracture density was detrimental to the operation of the SAGD process due to low bitumen recovery and high CSOR CONCLUSIONS A simulation study was conducted to examine the feasibility of bitumen recovery using the SAGD process in hostile conditions such as shale barriers, thief zones, and fractured reservoirs of two main formations in the Athabasca oil sand area Following are the conclusions of this study: The near-well regions were very sensitive to shale layers and only long, continuous shale barriers (larger than 50 m or 25%) affected the SAGD performance at these well regions The location and direction of these shale barriers also played important roles in the SAGD performance The Clearwater formation was more sensitive to the degree of reservoir heterogeneity than the McMurray formation Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 SAGD RECOVERY PROCESS 1469 FIGURE 21 Effect of vertical fractures on cumulative oil recovery in the McMurray formation (color figure available online) FIGURE 22 Effect of horizontal fractures on cumulative oil recovery in the McMurray formation (color figure available online) 1470 T Q C DANG ET AL In both formations, the OLWZ acted as an absolute thief zone to the SAGD process because it delayed the sweeping of the pay zone by the steam chamber, whereas the BWZ may be useful for remaining the reservoir pressure Vertical fractures can sharply improve the performance of the SAGD process However, horizontal fractures can be detrimental to the SAGD process by significantly decreasing the amount of bitumen produced as well as thermal efficiency Downloaded by [Selcuk Universitesi] at 10:17 26 December 2014 REFERENCES Boyle, T B., Gittins, S D., and Chakrabarty, C (2003) The evolution of SAGD technology at East Senlac J Can Petrol Technol 42(1):58–61 Butler, R M (2001) Application of SAGD, related processes growing in Canada Oil Gas J 99(20):74–78 Chen, Q., Gerritsen, M Q., and Kovscek, A R (2008) Effects of reservoir heterogeneities on the steam assisted gravity drainage process SPE Reservoir Eval 11:921–932 Doan, L T., Baird, H., Doan, Q T., and Farouq Ali, S M (1999) An investigation of the steam assisted gravity drainage in the presence of a water leg Paper No SPE 56545, SPE Annual Conference and Exhibition, Houston, TX, October 3–6 Doan, L T., Baird, H., Doan, Q T., and Farouq Ali, S M (2003) Performance of the SAGD process in the presence of a water sand—A preliminary investigation J Can Petrol Technol 42(1):25–31 Farouq Ali, S M (1997) Is there life after SAGD J Can Petrol Technol 36:55–60 Ipek, G., Fraunenfeld, T., and Yuan, J Y (2008) Numerical study of shale issues in SAGD Paper No SPE 2008150,Canadian International Petroleum Conference, Calgary, Alberta, Canada, June 17–19 Joshi, S D., and Threlkeld, C B (1985) Laboratory studies of thermal aided gravity drainage using horizontal wells AOSTRA Journal of Research 2:11–19 Le Ravalec, M., Morlot, C., Marmier, R., and Foulon, D (2009) Heterogeneity impact on SAGD process performance in mobile heavy oil reservoirs Oil Gas Sci Technol 64:469–476 Pooladi-Darvish, M., and Matter, L (2002) SAGD operations in the presence of the overlying gas cap and water layer— Effect of shale layers J Can Petrol Technol 41(6):40–51 Shin, H., and Choe, J G (2009) Shale barrier effects on the SAGD performance Paper No SPE 125211, SPE Reservoir Characterization and Simulation Conference, Abu Dhabi, UAE, October 19–21 Singhal, A K., Ito, Y., and Kasraise, M (1998) Screening and design criteria for steam assisted gravity drainage projects Paper No SPE 50410, SPE International Conference on Horizontal Well Technology, Calgary, Alberta, Canada, November 1–4 Yang, G., and Butler, R M (1992) Effects of reservoir heterogeneities on heavy oil recovery by steam assisted J Can Petrol Technol 31:72–79  ... feasibility of bitumen recovery using the SAGD process in hostile conditions such as shale barriers, thief zones, and fractured reservoirs of two main formations in the Athabasca oil sand area Following... Figures 10 and 11 indicate the dominant effect of continuous shale barriers on the cumulative oil recovery in the McMurray and Clearwater formations Oil recovery is much lower in the presence of lengthy... field scale and within simulations It is important and necessary to understand the factors determining growth rates and areal propagation Unfortunately, most numerical simulation investigations have