OPERATIONAL AND RESERVOIR PARAMETERS INFLUENCING THE EFFICIENCY OF STEAM-ASSISTED GRAVITY DRAINAGE (SAGD) PROCESS IN FRACTURED RESERVOIRS

Transcription

1 OPERATIONAL AND RESERVOIR PARAMETERS INFLUENCING THE EFFICIENCY OF STEAM-ASSISTED GRAVITY DRAINAGE (SAGD) PROCESS IN FRACTURED RESERVOIRS a Fatemi, S. M. 1 ; b Kharrat, R. a Department of Chemical & Petroleum Engineering, Sharif University of Technology, Iran b Department of Petroleum Engineering, Petroleum University of Technology, Iran ABSTRACT The Steam-Assisted Gravity Drainage (SAGD) process is a promising enhanced oil recovery (EOR) method for certain heavy oil non-fractured sandstones, but its applicability in naturally fractured reservoirs (NFR) has not been addressed yet. In this contribution the CMG-STARS module has been applied together with a numerical model developed for SAGD, which has been qualitatively validated with previously published experiments. This non-fractured model was modified to a double porosity medium to investigate the effect of reservoir parameters such as API of crude oil, matrix permeability, initial oil saturation and fractures departure on SAGD efficiency in the presence of networked fractures. Besides, the effects of operational parameters, such as the vertical distance between the wells in a stacked configuration and the horizontal offset between wells in a staggered SAGD scheme, have also been studied. The simulation results showed higher SAGD efficiency in the case of fractured model as compared to the conventional case. Lower API of crude oil, smaller matrix permeability and lower initial oil saturation reduced the process performance in NFR. It has been observed that the vertical distance between the stacked wells should be optimized, since top injection improves the ultimate oil recovery but initial production can be delayed as compared to the bottom steam injection. The same is true in the case of staggered well configuration, and the traversal offset between wells should be optimized, since longer horizontal distance between wells causes lower initial production rates. However, higher ultimate oil recovery is achievable after steam breakthrough into the producer well, as compared to the case of stacked wells configuration. KEYWORDS steam-assisted gravity drainage; fractured reservoirs; heavy oil recovery; steam injection 1 To whom all correspondence should be addressed. Address: Department of Chemical & Petroleum Engineering, Sharif University of Technology, Tehran, Iran mobeen.fatemi@gmail.com 125

2 1. INTRODUCTION TO SAGD PROCESS The top well is the steam injector (Figure 1), and the bottom well serves as the producer. Steam rises to the top of the formation, forming a steam chamber. High reduction in viscosity (due to the steam temperature) mobilizes the bitumen, which drains down by gravity and is captured by the producer placed near the bottom of the reservoir. Continuous injection of steam causes the steam chamber to expand vertically and spread laterally in the reservoir. The process performs better with bitumen and oils with low mobility, which is believed to be essential for the formation of a steam chamber, and prevent steam channels (Butler, 1991). SAGD has been more effective in Alberta than in California and Venezuela for the same reason (Butler, 1985). There are some attempts in the literature to simulate the SAGD process and to better understand the sensitivity of the process with various reservoir and operational parameters in conventional non-fractured models. Kamath et al. (1993) developed a twodimensional numerical model of the steam-assisted gravity drainage process with a pair of horizontal wells (SAGD) for heterogeneous layered tar sand reservoirs to study the effect of heterogeneity on the growth of steam chamber and the process performance. The effect of various reservoir parameters such as porosity, permeability, initial mobile water saturation, Dykstra-Parson s permeability variation, reservoir anisotropy and shale barriers on the performance of the SAGD process was investigated. The SAGD performance improved significantly with intense steam injection procedures, low mobile water saturation near the producer, absence of continuous shale barriers, high vertical-to-horizontal permeability ratio and optimum injector-producer vertical spacing. The lateral well spacing affected the oil production in the earlier period and the project life. Kisman and Yeung (1995) developed a similar study with a two-dimensional numerical model and considered the effects of permeability, relative permeability, wettability changes, oil viscosity, thermal conductivity, flow barriers and solution gas. Elliot and Kovscek (2001) simulated a singlewell SAGD (SW-SAGD) and performed a sensitivity analysis to improve the process performance at early times. The sensitivity analysis indicated that SW-SAGD is more applicable to heavy oils with initial viscosity below 10 Pa s (10,000 cp). Additionally, the reservoir must be sufficiently thick to allow significant vertical growth of the steam chamber. The sensitivity analysis also indicated that the presence of relatively small amounts of solution gas aids the recovery process by enhancing volumetric expansion of the oil during heating. They concluded that cyclic steam injection was the most efficient pre-heating method for SW- SAGD. Akin and Bagci (2002) made an experimental investigation and optimization of startup procedure for single-well steam-assisted gravity drainage. They compared two methods of continuous and cyclic steam injection and concluded that cyclic steam injection yields better results for SW-SAGD. They also simulated the process with CMG-STARS. Using CMG-STARS, Barillas et al. (2006) determined the optimum steam injection rate for a homogenous reservoir whose sole heterogeneity was barriers. They noted that vertical permeability has a significant role in oil recovery. Parameters like horizontal permeability and viscosity had negligible effect on optimum steam injection rate. They also investigated the effect of reservoir thickness. Figure 1. Mechanism of SAGD (Thomas, 2008). The SAGD simulation appeared successful on conventional non-fractured reservoirs. However, 126

3 few studies have been done on simulation of the process in fractured formation despite the fact that there are 20 known large fractured carbonate reservoirs containing heavy oil and bitumen throughout the world spread over a dozen countries and at various depths, temperatures and API s (Chen et al., 2007). Bagci (2006) made experimental and simulation studies on SAGD process in both homogeneous and fractured reservoirs. He concluded that the shape and growth of the steam chamber in a fractured pack were different from those observed in the uniform permeability pack without fracture. An elongated steam chamber is observed in the fractured case while the homogeneous model had almost round steam chamber. The author also investigated the effects of fracture orientation. According to his studies, fractures were successful in shortening the time to generate near breakthrough conditions between the two wells. They also enhanced the expansion rate of the steam chamber. However, higher steam-oil ratios were observed in both vertical and horizontal fractures than in the conventional homogeneous model. Chen et al. (2007) investigated the effects of heterogeneity on SAGD performance. The heterogeneity included the effect of an either vertical or horizontal hydraulic fracture. For the case with a vertical fracture, the main oil production period starts shortly after steam injection and exhibits a much greater average oil rate, more than twice the oil rates observed in the cases of horizontal fracture and base without fracture. Das (2007) investigated the feasibility of thermal processes such as SAGD and CSS (Cyclic Steam Stimulation) in carbonate fractured formation. Sensitivity of the process to fracture frequency, oil viscosity, oil saturation, wettability, imbibition (i.e., capillary pressure), matrix permeability and relative permeability relationships have also been investigated. Among these parameters the oil viscosity and wettability played the most significant roles. In this work, the CMG-STARS module was applied and a numerical model has been developed for SAGD and validated qualitatively with previously published experiments. This conventional non-fractured model was modified to double porosity media to dissect the effect of reservoir parameters such as API of crude oil, matrix permeability, initial oil saturation and fractures departure on SAGD efficiency in the presence of networked fractures. Furthermore, effects of operational parameters such as vertical distance between the wells and staggered SAGD schemes have been studied. 2. METHODOLOGY 2.1 Simulation of SAGD in Conventional Model Description of the Model In order to simulate the SAGD process in laboratory scale, a rectangular model was considered. The simulator used in this study was the Computer Modeling Group s (CMG) Steam and Thermal Advance Reservoir Simulator (STARS). Phase behavior data as well as equation of state (EOS) phase envelope were processed by the Winprop module of CMG. This simulator has been successfully applied to the SAGD process by other investigators such as Akin and Bagci (2002) and Barillas et al. (2006). The model dimensions (Figure 2A) in this study were set to 3.28 ft (100 cm) by 3.28 ft (100 cm) by 0.1 ft (3.048 cm). The average porosity and permeability were set to 30% and 10 md, respectively (typical fractured reservoirs average porosity and permeability). The irreducible water saturation was set to 0.2; also, 0.15 residual oil saturation is assigned to the model. The system pressure and temperature were set at 75 psia and 51 C, respectively. The crude oil used in simulations is synthetic oil composed of three pseudocomponents known as Light Oil (LO), Medium Oil (MO) and Heavy Oil (HO), so the simulation process involves the compositional type treatment of oil and gas phases. Among the pseudocomponents, only LO and MO have been considered as volatile oil since their equilibrium constants have been identified, and HO was taken as a dead oil. The Peng-Robinson Equation of State was used in the simulations to verify the synthetic 127

4 Figure 2. Schematic repre sentation of conventional (A) and fractured (B). oil type according to its Pressure-Temperature phase diagram. The lumped oil system used in the simulations is shown in Table 1. Also, the model parameters are summarized in Table Simulation of SAGD Process in Fractured Models In a non-conventional fractured reservoir, two types of reservoirs may be distinguished: fractured reservoirs of single porosity; and fractured reservoirs of double porosity. Both reservoirs are made of a network of fractures surrounding blocks. In the case of dual porosity system, two overlapping continuums, one corresponding to the medium of the fractures and one corresponding to the medium of blocks, are considered. Also, two values of properties and permeabilities are attributed to each point, e.g. fracture porosity and block porosity. In addition, there is no matrixmatrix communication in a dual-porosity model (van-golf Racht, 1982). In this work, a double porosity model is constructed and used as fracture system. The black strips induced in the previous section model (Figure 2B) are simulated fractures. Producer and injector are placed exactly as in the conventional model. As can be seen in Figure 2, this model is based on developing the dual porosity model by using the single porosity pattern to view more clearly the performance of the process in the single block model. Properties of the fracture should be assigned to the fracture layer. For example, the fracture permeability should be much larger than the matrix permeability. Also, the fracture porosity should have a reasonable value, which is different from that of the matrix porosity. The porosity of those grids representing the fracture was taken as Table 1. The lumped oil system in the simulation. Composition (mole fraction) LO (MW=250) MO (MW=450) HO (MW=600) Table 2. Data for initializing the conventional model. Parameters Values Length in X, Y, Z-direction 100 cm * 100 cm * cm Porosity 0.3 Permeability 10 md Number of pseudocomponents 3 (LO, MO, HO) Temperature 51 C Initial pressure 517,107 pascal Initial water saturation 0.2 Residual oil saturation 0.15 Number of injection/producer wells 1/1 Steam quality

5 Figure 3. Relative permeability curves for fractures The other sensitive parameter was the relative permeability curve for the fractures (Figure 3). According to the Aguilera (1995), two straight lines crossing each other at the center have been assumed. The model dimensions, system pressure and temperature and oil sample were set similar to what we had in the simulation of the conventional model. The steam injection system was identical to that used in the previous section. The fracturedmodel parameters other than those indicated in Table 2 are summarized in Table RESULTS AND DISCUSSION 3.1 Simulation of the SAGD process in conventional model Figure 4 shows the oil saturation profiles in conventional model during SAGD process. It is clear from these figures that the development and progress of the steam front in conventional model follows the same pattern observed in laboratory studies as reported by previous researchers (Butler, 1991; Butler, 1998). According to Figure 4, the steam diffuses into the matrix and reduces the viscosity of the heavy oil by heat conduction, and the steam chamber develops laterally and vertically. Vertical and horizontal extension of the chamber is a direct function of the vertical permeability to horizontal permeability ratio. In the case of the model which is being investigated here, the kv/kh ratio is equal to one, and, because of this, the steam chamber expands symmetrically. 3.2 Simulation of the SAGD process in the fractured model In the case of fractured model, compared to the conventional case, the injected steam diffuses from the matrix around the injector into the fracture network (see Figure 2) that has higher permeability than the matrix block. This further extends the steam chamber in the fractured model as compared to the conventional case (see Figures 4 and 5). Due to the presence of fissures, the steam moves to the farther parts of the model at the top (Figure 5B), which are not accessible in nonfractured models (Figure 4B). Once the fractures become fully saturated with steam, the steam heats the matrix and diffuses into it (Figure 5C). Because of its lower density as compared to that of the viscous oil, the steam attempts to fill the space Table 3. Data for initializing the fractured model. Grid type Cartesian Fracture porosity 1.00 Fracture permeability 10,000 md 129

6 Figure 4. Oil saturation in conventional model during SAGD. above the oil, so the interface is gradually developed in each matrix and the heavy oil is accumulated at the centre of the blocks. The mobilized layer of oil drains by gravity to the fracture system. The major part of this oil is drained through the fracture system to the lower production well. This mechanism improves the recovery process in the fractured model when compared to the conventional case (Figure 6) since oil in the matrix is affected by steam via higher contact area formed in the presence of the fractures. It should be pointed out that in Figures 6-12 and 13, the simulations were performed in the year of 2008, so entry means July 2008, which is 6 months after initial production. Likewise, indicates the production results estimated for July Effect of Initial Oil in Place EOR processes like SAGD usually involve tertiary recovery mechanisms. This means that part of the initial oil in place has already been produced by primary or secondary recovery mechanisms. This reduces the amount of oil at the start of the SAGD process. To study the effect of initial oil in place (IOIP) on the ultimate oil recovery by SAGD in NFR, two additional models with initial oil saturation equal to 0.7 and 0.9 have been considered. Simulation analysis confirmed that the higher the amount of IOIP, the higher was the SAGD process outcome (Figure 7). This fact shows that the start time of SAGD in NFR as a secondary or tertiary recovery process should be optimized to attain higher process efficiencies. 3.4 Effect of API of Crude Oil Worldwide fractured reservoirs exist in the range of light crude oils to heavy viscous crudes. To investigate the applicability of SAGD in different crude oils three different models with low, medium and high API quality have been considered. Simulation analysis confirmed that SAGD process performance in NFR was higher in the case of lighter crude oil as compared to the heavier oils (Figure 8). SAGD seems to be more feasible in the case of light oil fractured reservoirs. 130

7 Figure 5. Oil saturation in fractured model during SAGD. Figure 6. Oil recovery factor in conventional and fractured models. 131

8 Figure 7. Better SAGD performance in the case of higher initial oil saturation. Figure 8. Higher SAGD performance in the case of lighter crude oils. 132

9 Figure 9. Higher ultimate oil recovery achievable in the case of more permeable matrix blocks. Figure 10. Effect of fractures departures on SAGD process performance in NFR. 133

10 Figure 11. Effect of vertical distance between stacked wells on SAGD process outcome. 3.5 Effect of Matrix Permeability According to the intrinsic heterogeneity of the reservoirs, matrix permeability in a NFR may change from place to place. To study the effect of matrix permeability on SAGD performance two additional models with matrix permeabilities equal to 20 and 5 md have been considered. Simulation analysis confirmed better SAGD performance in the case of highly permeable reservoirs (Figure 9). However, more simulation and analysis in the case of heterogeneous models in terms of permeability are required. 3.6 Effect of Fractures Departure Fracture departure stands as the distance between the two walls of the fissure at the entry. It has a direct effect on the absolute permeability of the fracture and obviously on the magnitude of the relative permeabilities. Fracture surface roughness can be different in the case of a reservoir from place to place, according to the prevailing stress/strain condition of the environment and mineralogy of the rocks. This fracture tortuosity will affect the fracture average departure. To study the effect of fracture departure on the SAGD performance in NFR, two additional models with and ft fracture departures have been considered. Simulation analysis showed that SAGD Figure 12. Stacked SAGD well configuration (A) vs. Staggered SAGD scheme (B). 134

11 Figure 13. SAGD performance comparison in the case of Stacked and Staggered wells configuration. initial oil production is delayed for wider fractures departures. This behavior demonstrates that fractures should first become fully saturated with steam and then steam diffuses from there into the matrix blocks, thereby producing oil according to the drainage mechanism. Once fractures have been saturated with steam the higher recovery will be achievable for wider fracture departures (Figure 10). 3.7 Effect of Vertical Spacing between Wells From the field development point of view, longer distances between producer(s) and/or injector(s) located in one pattern will be more economical in terms of OPEX/CAPEX, since the number of the wells to be drilled and/or maintained will be lower. To study the effect of vertical distance between two stacked wells in SAGD operation in NFR, two additional models with injector located at the middle part of the formation and top of the formation thickness have been considered. Simulation analysis showed that initial oil production was delayed in the case of longer vertical distance between stacked wells since warmed oil which is around the injector needs to pass through the colder region around the producer to be recovered. Also there will be longer distance for oil to be displaced from areas close to the injector region into the producer. As time passes and steam chamber breakthroughs into the producer, higher recovery is achieved in the case of longer vertical distance between stacked wells (Figure 11). This shows that the vertical distance between injector and producer should be optimized in order to achieve higher initial oil rate in conjunction with higher ultimate oil recovery at the end of process, and also reducing OPEX/CAPEX in the case of field development. 3.8 Staggered SAGD vs. Stacked SAGD In the case of drilling two horizontal wells in a real reservoir, there is always the probability that the wells are not exactly disposed on top of each other all the way along their horizontal section. This means that slight horizontal offset between the wells is not avoidable. From another point of view, staggered wells (with large horizontal offset) will cause lower OPEX/CAPEX (more economical) in the case of field development and infill drillings since the number of wells to be drilled and/or maintained per bulk volume of the reservoir will be lower. From the theoretical point of view, in the case of staggered wells configuration, gravity 135

12 drainage will be enhanced by pressure drive until steam breaks through. The horizontal injector well in staggered wells configuration has been placed at the farthest corner (Figure 12). Simulation analysis showed that at the initial times oil recovery is higher in the case of stacked wells, but after a while the oil recovery is increased in the case of staggered wells (Figure 13). This shows that traversal distance between injector and producer should be optimized in order to achieve higher initial oil rate in conjunction with higher ultimate oil recovery at the end of the process. As a matter of fact, it takes more time for steam to breakthrough into the producer since there is a longer distance between injector and producer. This renders oil production in staggered scheme lower at the initial times. As the injection process continues, pressure drive of the injector will be augmented to the gravity drainage and, as a result, the ultimate oil recovery in this case will be higher as compared to the case of stacked wells configuration. 4. CONCLUSIONS The performance of the SAGD process in terms of ultimate oil recovery was higher in the case of fractured model as compared to the non-fractured conventional case. Investigation on reservoir parameters has shown that SAGD is more feasible in the case of wider fractures departures in terms of ultimate oil recovery that can be achievable, although the initial production rate will be somewhat lower. The amount of the produced oil was higher in mass in the case of higher initial oil saturation. This means that the start time of SAGD application in NFR as a secondary or tertiary recovery method should be accurately decided to obtain better performance of the process. SAGD is more applicable in light oil fractured reservoirs but the ultimate oil recovery in heavy oil resources was also high. As the vertical distance between stacked wells increases, the breakthrough time will be delayed. This means lower rate of oil production at the initial times. As the process continues, higher oil recovery will be achievable in the case of longer distance between wells due to the forced gravity drainage of oil in the presence of injector back pressure. This means that distance between stacked wells should be optimized. In the case of lighter oil, NFR staggered SAGD configuration can be applied to achieve higher process outcome. In the case of heavy oil reservoirs, traversal distance between injector and producer should be optimized since initial production will be delayed and the oil rate will be lower at the beginning. However, as the SAGD process moves forward, higher ultimate oil recovery will be achievable in the staggered well configuration. 5. REFERENCES AGUILERA, R. Naturally Fractured Reservoirs. Penn Well Books, ISBN: , AKIN S.; BAGCI S.A. A Laboratory Study of Single- Well Steam-Assisted Gravity Drainage Process. Journal of Petroleum Science & Engineering, v. 32 (1), p , BAGCI A.S. Experimental and Simulation Studies of SAGD Process in Fractured Reservoirs, presented at SPE/DOE Symposium on Improved Oil Recovery, Tulsa, Oklahoma, SPE paper 99920, BARILLAS, J.L.M.; DUTRA Jr., T.V.; Mata, W. Reservoir and Operational Parameters Influence in SAGD Process. Journal of Petroleum Science and Engineering, v.54 (1-2), p.34-42, BUTLER, R. M. A New Approach to the Modeling of Steam-Assisted Gravity Drainage. Journal of Canadian Petroleum Technology, v.24 (3), p.42 51, BUTLER, R. M. Thermal Recovery of Oil and Bitumen, Prentice-Hall, New Jersey, ISBN: , BUTLER, R.M. SAGD Comes of AGE. Journal of Canadian Petroleum Technology, v.37 (7), p. 9-12, BUTLER, R.M. AND STEPHENS, D.J. The Gravity Drainage of Steam Heated Heavy Oil to Parallel Horizontal Wells. Journal of Canadian Petroleum Technology, v.20 (2), p.90-96, CHEN, Q.; GERRITSEN, M.G.; KOVSCEK, A.R. Effects of Reservoir Heterogeneities on the Steam- Assisted Gravity Drainage Process, SPE paper , DAS SWAPAN. Application of Thermal Recovery Processes in Heavy Oil Carbonate Reservoirs, SPE paper , ELLIOT, K.T. AND KOVSCEK, A.R. A Numerical Analysis of Single-Well Steam Assisted Gravity 136

13 Drainage Process (SW-SAGD). Petroleum Science and Technology, v.19 (7-8), p , KAMATH, V.A.; SANDEEP, S.; HATZIGNATIOU, D.G. Simulation Study of Steam-Assisted Gravity Drainage Process in Ugnu Tar Sand Reservoir. SPE paper 26075, KISMAN, K.E. AND YEUNG, K.C. Numerical Study of SAGD Process in the Burnt Lake Oil Sands Lease. SPE paper 30276, THOMAS, S. Enhanced Oil Recovery An Overview, Oil & Gas Science and Technology Journal Rev. IFP, v.63 (1), p.9-19, VAN GOLF-RACHT, T. D. Fundamentals of Fractured Reservoir Engineering, 1st edition, Elsevier, ISBN: ,