EFFECT OF WELLS ARRANGEMENT ON THE PERFORMANCE OF TOE-TO-HEEL AIR INJECTION

Transcription

1 EFFECT OF WELLS ARRANGEMENT ON THE PERFORMANCE OF TOE-TO-HEEL AIR INJECTION a Fatemi, S. M. 1 ; a Ghotbi, C.; b Kharrat, R. a Department of Chemical & Petroleum Engineering, Sharif University of Technology b Department of Petroleum Engineering, Petroleum University of Technology ABSTRACT Toe-to-Heel Air Injection (THAI) is a novel EOR process, which integrates high temperature oxidation reactions and horizontal well concepts to achieve potentially high recovery rates in heavy oils. The aim of the present work is to investigate the effects of operational parameters such as injector-producer wells configuration, injectors or producers transverse distance, injector s depth of perforation and horizontal producer length on the performance of THAI in heavy oil reservoirs. Using the CMG-STARS validated combustion tube experiment model of a crushed rock and oil KEM reservoir (Fatemi and Kharrat, 2008a) and the THAI model modified with a 3D combustion cell (Fatemi and Kharrat, 2008b), a series of combustion cell assays on KEM heavy crude oil and its carbonate formation have been simulated. Different wells configurations, including vertical injector/horizontal producer (VIHP), vertical injector/2 horizontal producers (VI2HP), horizontal injector/horizontal producer (HIHP), horizontal injector/2 horizontal producers (HI2HP) and finally 2 vertical injectors/horizontal producer (2VIHP), have been evaluated to obtain the optimum scheme of the THAI technology field development scenario. Optimal wells configuration were established according to the trade-off between ultimate oil recoveries, green house gases produced, areal/vertical sweep efficiencies, possibility of carbonate rock decomposition, OPEX/CAPEX and so on. The results showed that the best strategies of field development are the 2VIHP and VIHP configurations. Sensitivity analysis confirmed that, in order to achieve the highest performance in the process, in the case of a specific reservoir, parameters like injection depth, length of horizontal producer and transverse distance between two vertical injectors (or two horizontal producers) should be optimized, since there are trade-offs between the quality of the produced oil, OPEX/CAPEX, stability of the combustion front and ultimate oil recovery achievable. KEYWORDS THAI; in situ combustion; heavy oil recovery; EOR/IOR; simulation analysis 1 To whom all correspondence should be addressed. Address: Department of Chemical & Petroleum Engineering, Sharif University of Technology, Tehran, Iran mobeen.fatemi@gmail.com 11

2 1. INTRODUCTION For the production of oil from heavy oil reservoirs, thermal methods are applied widely, one of which is the In-Situ Combustion (ISC) process (Prats, 1986). ISC has some advantages over other thermal Enhanced Oil Recovery (EOR) processes for heavy oil reservoirs since the required thermal energy is generated in-situ (Green and Willhite, 1998). High efficiency in terms of heat utilization (heat loss in the injection wells is eliminated), more effective hot gas displacement (due to the high temperature of the combustion front), in-situ upgrading of heavy oil (via thermal cracking of heavy residues ahead of the combustion front) are the most important advantages of ISC processes with respect to other EOR methods. Furthermore, there is the potential of a totally self-sufficient ISC process by virtue of energy that could be recovered from the subsurface (Xia et al., 2002). However, in field trials, conventional fire flooding (CFF) using Vertical Injector/Vertical Producer (VIVP) well patterns suffers from some significant operational problems, which affect recovery rates from tar sands and heavy oil reservoirs. Gravity segregation or gas overriding, channeling, unfavorable gas/oil mobility ratio and lack of initial heated communication paths within the reservoir are some of the CFF deficiencies. In addition, it is also difficult to reach a hightemperature combustion mode, once it has slipped into a low-temperature oxidation mode, due to insufficient oxygen supply (Xia and Greaves, 2000). Toe-to-Heel Air Injection (THAI) is an advanced ISC technique, which integrates in-situ combustion and horizontal well technology. It uses a horizontal producer well (or wells), rather than a vertical producer well as in CFF, to achieve combustion front propagation along the horizontal producer, from the toe to the heel position (Greaves et al., 2005). The motivation behind THAI is to overcome the operational problems normally encountered when applying a CFF process, therefore representing a potential major breakthrough in heavy oil recovery technology. The first use of horizontal wells in the ISC process was reported by Greaves et al. (1991, 1993). Three injector-producer well arrangements were employed in a small, rectangular combustion cell. The recovery of oil from the Marguerite Lake, located in Canada, with a vertical injector (VI) and horizontal producer (HP) combination (direct line drive) was 55.7% of the original oil in place (OOIP), compared with only 39.1% OOIP for conventional vertical wells (VIVP), and over 70% OOIP for a dual horizontal well arrangement (VI2HP staggered line drive). It was very clear, therefore, that the new ISC-horizontal wells process was operationally a very significant improvement over the old process, at least at the experimental level. The other significant result was that more in-situ upgrading of heavy crude oil samples was obtained using the VIHP and VI2HP arrangements, compared to conventional VIVP. Xia et al. (2002) performed a series of 3D experimental assays on heavy crude oil samples from the Wolf Lake (with 10.5 API) and Athabasca Tar Sand Bitumen (with 8 API), both located in Canada, using the following well configurations: vertical (VI) or horizontal injection (HI) and horizontal producer (HP) wells in direct line drive (VIHP, HIHP); and staggered line drive (VI2HP, 2VIHP). In the experiments, the horizontal injector configuration (HIHP) was found to be the most efficient for achieving rapid start-up, i.e. the shortest time to achieve a stable combustion front propagation. However, injection of air via a horizontal well was not a very practical design for field operation. The single vertical injector configuration (VI2HP) was slow to achieve stable operation, due to the development of a much smaller ignition zone in the initial stage of the process, as compared with the HIHP configuration. According to Xia et al. (2002), VIHP achieved slower start-ups than VI2HP in the post-steam flood THAI process. All of their tests achieved very high oil recoveries, averaging higher than 80% OOIP, except in the VIHP test which was only 70%. Akin et al. (2002) studied in situ combustion experiments conducted on a three-dimensional semi-scaled physical model that represented onefourth of a repeated five-spot pattern. In all experiments a vertical injector was employed, although both vertical and horizontal producers had been installed. Several locations for the producers have been tried while keeping the length 12

3 of the wells constant: vertical injector-vertical producer, vertical injector-horizontal side producer, and vertical injector-horizontal diagonal producer. In their experiments, horizontal side producers performed better than the others. These authors also presented new combustion kinetic model based on SARA (Saturated, Aromatic, Resin, Asphaltene) fractions in the crude oil, which successfully modeled the experiments, and, especially in the diagonal-horizontal producer case, the results were improved compared with conventional simulation. However, they mentioned that due to experimental complexity, the use of the kinetic model is not justified. Bagci (2005) presented seven dry forward combustion experiments using Raman crude oil (18 API) carried out with a 3D physical model, similar to that reported by Akin et al. (2002), with the application of five different well configurations. The highest oil recovery rates were obtained with horizontal producers positioned at the boundary of the model alone, as a single producer. With the same burned volume, more oil was recovered by horizontal producers than by vertical producers. In conjunction with all these previously published experimental works, the aim of the present contribution is to perform simulation analysis to investigate the effect of different operational parameters such as different wells configuration, injectors or producers transverse distance, injector shoe depth of completion and producer horizontal section length on the outcome of the THAI process for a low permeable heavy oil reservoir in the Persian Gulf coast called Kuh-E- Mond (KEM), in laboratory scale. Unlike previous works which have only used the oil recovery factor to choose the best scheme of field development, in this work other parameters have been also considered on the decision, including the API quality of the oil produced, rock decomposition temperature, areal/vertical/volumetric sweep efficiencies, produced green house gases (GHG), oxygen/water breakthroughs, and OPEX/CAPEX. An Operating Expenditure, or OPEX, is an on-going cost assessment for running a product, business or system. A Capital Expenditure (CAPEX) is the cost of developing or providing non-consumable parts for the product or system. KEM combustion tube experiments validated with a numerical model using CMG-Winprop, CMG- Builder and CMG-STARS and qualitatively validated THAI process numerical models for KEM have been presented elsewhere (Fatemi and Kharrat, 2008a; Fatemi and Kharrat, 2008b). 2. METHODOLOGY 2.1 THAI 3D combustion cell simulation model for KEM ISC is a complex process, which involves multiphase flow (gas, oil, and water) in porous medium and chemical reactions (fuel formation, fuel combustion). The ultimate oil recovery depends very much upon the volumetric sweep of the combustion front propagation. In a combustion tube test, because of the high temperature combustion front sweep, oil recovery reaches about 90% OOIP. However, in field conventional ISC operations, gas override and channeling can dramatically reduce the volumetric sweep efficiency, so that ultimate oil recovery is much lower. As a result, the best way to analyze feasibility of ISC process for a specific reservoir is via 3D experiments or simulation analysis. Using CMG-Builder (Computer Modeling Group) a rectangular prism combustion cell, measuring 0.8 m long 0.3 m wide m deep, was developed to carry out the THAI simulation for KEM using various injector(s)-producer(s) schemes. Permeability and porosity of the cell were equal to 127 md and 0.414, respectively, the same as the average permeability and porosity of the KEM carbonate rock. Also, in order to approach a realistic model, the actual sets of relative permeabilities data of KEM carbonate reservoir were used. External heaters were used to ignite the combustion in the cell. Dry combustion tests were completed using the different well arrangements as will be discussed later. More details on the simulation model conditions, which are also used in this work, were presented elsewhere (Fatemi and Kharat, 2008b). 2.2 THAI Different Wells Configurations A series of 3D fire flooding simulation runs (using CMG-STRAS) on KEM heavy oil were carried 13

4 out by using the following well configurations (Figure 1): vertical injector (VI) or horizontal injector (HI) and horizontal producer (HP) wells in direct line drive (VIHP, HIHP), staggered line drive (VI2HP, 2VIHP), and line drive (HI2HP). The results of the simulation task are explained in the following sections. It should be mentioned that, in previous experimental works by Xia et al. (2002), single-lateral horizontal injectors were used, and the authors simply injected air from its heel into the toe position. Since there is a pressure drop due to friction along the horizontal section which affects the sweep efficiency of oxygen in the model due to higher injection pressure at the heel compare to the toe, in the present work we have modified all HI into two-lateral wells (injection from vertical string in the mid-length of the horizontal section), so that we have the symmetry of oxygen sweep efficiency in our models. VIHP well combination The Vertical Injector-Horizontal Producer (VIHP) well configuration (Figure 1A) contains a vertical injector on the left hand side of the prism (black dot at the left side of the prism) and the horizontal producer (series of connected black dots), located in such a way that there is a distance between the shoe of the vertical injector and the toe of the producer. 2VIHP well combination The Two Vertical Injectors-Horizontal Producer (2VIHP) scheme (Figure 1B) contains two vertical injectors on the left hand side of the model (two separated black dots at the left side of the prism) and a horizontal producer located at a distance from the line which connects these two injectors (black dot at the left side of the prism). In order to ensure the consistency of operational parameters for all models, the total air injection rate for one injector (in the case of the VIHP well configuration) is equally divided between the two injectors in this Figure 1. Different production well-injection well configurations used in this study: A: VIHP, B: 2VIHP, C: VI2HP, D: HIHP and E: HI2HP. 14

5 case. VI2HP well combination The Vertical Injector-Two Horizontal Producers (VI2HP) well configuration (Figure 1C) contains a vertical injector on the left hand side of the model and two horizontal producers located in such a way that there is a transverse distance between the shoe of the injector and the toe of each producer (Figure 1C). To simulate the same operational parameters for all models, the total pressure drop of one producer (in the case of the VIHP configuration) is equally divided between the two producers in this case. HIHP well combination The Horizontal Injector-Horizontal Producer (HIHP) well configuration (Figure 1D) contains a lateral horizontal injector (series of connected black dots on the left) and the horizontal producer well at a distance located in a direction perpendicular to the injector (series of connected black dots on the right). HI2HP well combination The Horizontal Injector-Two Horizontal Producers (HI2HP) well configuration (Figure 1E) contains a lateral horizontal injector on the left hand side of the model and two horizontal producer wells at a distance located in a perpendicular direction to the injector. Similarly, in order to ensure the operational parameters consistency for all models, the total pressure drop of one producer is equally divided between the two producers in this case. 3. RESULTS AND DISCUSSION 3.1 Effect of wells configuration Oil recovery factors In terms of ultimate oil recovery factors, the best methods are 2VIHP, HI2HP, HIHP, VI2HP and VIHP, in descending order, as shown in Figure 2. It should be mentioned that only 2VIHP has considerably higher ultimate recovery factors than the others. Although the differences in recovery performances in this paper are small, we believe that these differences are important because, as Figure 2. Oil recovery factor versus time for different THAI wells configurations. 15

6 Figure 3. Oxygen saturation profile for different THAI wells configuration (time: 6hr). the dimensions of the model increase, differences in terms of oil recovery between various wells configurations will be enhanced Combustion peak temperature One risk of running combustion processes in carbonate formations (like KEM) is the probability of decomposition due to very high temperatures. Whilst decomposition occurs in dolomite or in limestone formations, the rock will change into a powder-like material that will definitely cause pores clogging (Fatemi, 2008). Tests carried out with the KEM carbonate rock (Seraji et al., 2007) showed that decomposition temperatures are around 600 C (1112 F). The peak temperatures reached in different well configuration patterns indicate that the HIHP (1157 F) and HI2HP (1151 F) present some risks in terms of carbonate rock decomposition Areal sweep efficiency Areal sweep efficiencies of different well configurations in terms of oxygen saturation profiles for different models at the same time of simulation are shown in Figure 3. As observed, the areal sweep efficiencies follow the order: 2VIHP > HI2HP > HIHP > VI2HP > VIHP Volumetric sweep efficiency To specify the volumetric sweep efficiency of each wells combination at the same time, postmortem analysis of the coke deposition for each of them have been investigated at the same crosssection (perpendicular to the cell length at 40 cm from left) and compared to one another (Figure 4). According to this comparison the highest volumetric sweep efficiencies were in the order: 2VIHP > HIHP > VIHP > VI2HP > HI2HP. Figure 4. Post-mortem solid residue concentration (lb-mole/ft 3 ) for different THAI wells schemes at time = 8 hr. Zero-concentration (blue) regions depict areas which were completely burned due to oxygen. 16

7 Figure 5. Comparison of the cumulative oxygen produced in different THAI well schemes Cumulative oxygen produced The amounts of cumulative oxygen produced in terms of SCF (standard cubic feet) for all different well combination methods were assessed and compared, as depicted in Figure 5. The best oxygen breakthrough time intervals (postponed oxygen production) correspond to: 2VIHP > HIHP > HI2HP > VI2HP > VIHP. The lower amounts of oxygen produced in the 2VIHP case revealed that oxygen is better utilized in this well configuration. 3.2 Effect of horizontal producer length Additional models with the length of horizontal producer ranging from very short to very long have been developed (Figure 6). The shorter horizontal producer had the highest average reservoir temperature. The peak combustion temperature in the case of short horizontal producer was 1150 F, which may cause KEM carbonate rock decomposition in real experiments. This rock decomposition will cause oil recovery reduction in real cases, as compared to the simulation results here, since state-of-the-art simulators (and also the one we have used in this paper) are not capable of detecting this effect. Small reductions were observed in the ultimate oil recovery factor in the case of very long or short horizontal producers and, interestingly, a lower rate of oil production was provided by the longest horizontal producer. Oxygen production (Figure 7) shows accelerated breakthrough in the case of long horizontal producer. The two other cases had approximately the same time of breakthrough but shorter horizontal producers, thereby yielding lower amounts of oxygen as compared to the mediumlength case. Cumulative water produced is inversely related to the length of the horizontal production well, which means that shorter horizontal producers generate higher amounts of water (Figure 8). This is important in terms of the Figure 6. Different producer lengths in the 2VIHP configuration. 17

8 Figure 7. Oxygen breakthrough for horizontal producers with different lengths as a function of time. probability of forming more water-oil emulsions (which would be disastrous) and also in terms of requirements for water-treatment facilities (higher OPEX/CAPEX). The most important factor in the case of field development, which also refers to sweep efficiency, is related to the length of the horizontal producer. According to Figure 9 the best areal sweep efficiency corresponds to the case of a medium-length horizontal producer. The same is true in the case of volumetric sweep efficiency (post-mortem analysis of the coke deposition at the same cross-section perpendicular to the cell length at 40 cm from left, as shown in Figure 10). Figure 8. Water generated in producers with different lengths as a function of time. 18

9 Figure 9. Areal sweep efficiency (oxygen saturation profile) for horizontal producers with different lengths (time = 6hr). Figure 10. Comparison of the volumetric sweep efficiency for horizontal producers with different lengths. Solid concentration in lb-mole/ft 3 at post-mortem, (time = 8hr). 19

10 Figure 11. Schematics of producers with different transverse distances for the VI2HP configuration. 3.3 Effect of vertical injector depth To study the effect of vertical injector depth on the simulation results of THAI in the case of VIHP and 2VIHP configurations, additional runs have been done with the well completed at the top, in the middle section and at the bottom of the layer. According to the results and the negligible difference in the ultimate oil recovery factor detected between them, the best location of vertical injector completion for THAI in the case of VIHP and 2VIHP wells configurations were at the top layer. This was also proposed considering the lower amounts of OPEX in the case of shorter vertical injectors. We believe that this idea can be confirmed by carrying out further simulation analyses with larger-dimension configurations, especially in terms of thickness. 3.4 Effect of horizontal producer wells spacing To study the effect of horizontal producers transverse distance in the case of VI2HP, three additional short-, medium- and long-distance runs have been performed, with the configurations depicted in Figure 11. The highest ultimate oil recovery factor was obtained with the longer horizontal producer (Figure 12). The oxygen breakthrough time is the same for all cases (Figure 13) but lower amounts of oxygen have been produced in the case of longer traversal distances. Superficial, areal and volumetric sweep efficiencies are illustrated in Figures 14 through 16, which depict better sweep efficiencies in the case of longer distances. From another point of view, water breakthrough (Figure 17) occurred faster in the case of shorter distances, but after breakthrough the cumulative volume produced was lower in the case of shorter distances. Lower Figure 12. Oil recovery factor for producers with different transverse distances (2VIHP) as a function of time. 20

11 Figure 13. Oxygen breakthrough for producers with different transverse distances as a function of time. volume of the produced water means fewer facilities are required for water treatment and disposal (lower CAPEX), thereby reducing operating expenses during water treatment processes (OPEX) as compared to the case with large amounts of produced water. Consequently, there will be fewer chances for formation of water-oil emulsions in the generated liquid. 3.5 Effect of vertical injector wells spacing To study the effect of vertical injectors distance in the case of 2VIHP, three additional short-, medium- and long-distance runs have been carried out (Figure 18). Since the general trend of the oil recovery factors were the same and the differences between them was not clear, we have just shown the oil recoveries factors for the time interval between 7.5 hr and 8.5 hr (Figure 19) to highlight their difference. Figure 19 shows that the oil recovery factor is slightly higher in the case of medium transverse distance between injectors as compared to the long and short transverse distances. This result may be interesting from the OPEX and CAPEX point of view, since there is no need to fill the model with nearly spaced injectors to get the higher oil recovery. We believe that there is an upper limit for the transverse distance to render oil recovery optimal. The areal sweep efficiency in terms of oxygen saturation was higher in the case of longer transverse distance (Figure 20). The same was true for the vertical sweep efficiency (Figure 21). The volumetric sweep efficiency according to the post-mortem analysis of deposited coke was also higher (Figure 22). Better oxygen sweep efficiency and utilization in the case of longer transverse distance caused a delay in the oxygen breakthrough in this case (Figure 23) and yielded even lower amounts of oxygen produced after breakthrough. From another point of view, the peak combustion temperature in the case of longer distances was higher (1152 F), which is above the KEM carbonate rock decomposition temperature. However, in the case of medium distances between injectors, the temperature detected (1082 F) was lower than the critical temperature for the KEM rock (1100 F). On another hand, lower amounts of GHG (Figure 24) and water (Figure 25) were produced in the case of shorter-distance configurations, thereby reducing the costs for water and GHG treatments in this case. These are important parameters that should be accounted for in the optimization of transverse distance between producers. 21

12 Figure 14. Superficial oxygen saturation profile for producers with different transverse distances. Figure 15. Vertical oxygen saturation profile for producers with different transverse distances. 22

13 Figure 16. Volumetric sweep efficiency for horizontal producers with different transverse distances. Figure 17. Produced water in the case of different producers traversal distance. Figure 18. Schematics of injectors with different transverse distances for the 2VIHP configuration. 23

14 Figure 19. Oil recovery factors for different transverse distance between injectors in the case of 2VIHP wells configuration. Figure 20. Areal sweep efficiency for different transverse distances between vertical injectors. 24

15 Figure 21. Vertical sweep efficiency for different transverse distances between V-injectors. Figure 22. Comparison of the volumetric sweep efficiency for different transverse distances between vertical injectors. Figure 23. Oxygen production for different transverse distances between vertical injectors. 25

16 Figure 24. GHG produced for different transverse distances between vertical injectors. Figure 25. Water produced for different transverse distances between vertical injectors. 26

17 4. CONCLUSIONS CMG-Builder THAI process 3D simulation models have been applied in the investigation of parameters that affect THAI operations involving KEM heavy oil reservoir. The effects of variables such as injector(s)-producer(s) wells configuration, transverse distance between injectors or producers, depth of injector shoe completion and length of horizontal producer have been investigated. When applied for KEM carbonate rock and heavy crude oils, THAI achieved excellent ignition and very stable combustion propagation. There was a probability of carbonate rock decomposition in the case of HIHP and HI2HP configurations, since the peak temperature of the combustion was higher than 1100 F. This may reduce the ultimate oil recovery in real applications, but these two schemes had high oil recovery factors according to the simulation analysis discussed here. Furthermore, in real field developments, only configurations featuring vertical injectors can be used. The configurations with the horizontal well as the injector should be put aside due to very serious safety concerns. As a result, HIHP and HI2HP are not feasible in the case of field developments; which is also the case if one considers higher OPEX and CAPEX for multi-lateral horizontal injectors, with regard to their minor effects on the recovery factor. The 2VIHP configuration was the best scheme for THAI field developments employing KEM, since there is no probability of carbonate rock decomposition from its use, yielding the highest amount of recovered oil, the best sweep efficiency and a delayed oxygen breakthrough. From another point of view, VIHP could be also considered since there is no risk for carbonate rock decomposition from its implementation, generating the lowest amounts of GHG (which is one of the top advantages of THAI over other thermal methods like SAGD, rendering it more Kyoto-friendly and releasing less pollution to the environment), the lowest amounts of water produced (lower chances of forming water-oil emulsions and therefore less OPEX) and lower amounts of CAPEX, since the number of wells drilled in the development is minimal. The most interesting aspect is that, in the VIHP configuration, the vertical section of the horizontal wells may be used as future vertical injectors, hence reducing further expenses, since there will be no need for future in-fill injectors drilling. The length of horizontal well(s) should be optimized for THAI, since a very long horizontal producer adds on CAPEX, accelerates oxygen breakthrough, and reduces sweep efficiency of the process. A very short horizontal producer may induce high combustion temperatures, therefore reducing the ultimate oil recovery and sweep efficiency obtained by the simulation, because of the disastrous effect of carbonated rock decomposition. A longer distance between the two horizontal producers in the case of VI2HP increased the ultimate oil recovery factor, delayed water and oxygen productions and enhanced the sweep efficiency. Since the depth of vertical injector had a negligible effect on the average reservoir temperature and ultimate oil recovery of the THAI process, the best location for vertical well completion seems to be the top of the formation. This reduces the costs of well drilling and, from the operational point of view, may even enhance the process by means of top burning and gravity drainage to the horizontal producer. Due to the small dimensions of the cell used here, the ultimate oil recovery factor was not affected so much by the vertical injectors transverse distance in the case of 2VIHP. From another point of view, better sweep efficiencies and delayed oxygen breakthrough are advantages provided by longer-distance configurations, as indicated by the higher amounts of water and GHG produced. Also, the probability of carbonate rock decomposition due to high peak temperatures must be considered in the case of short-distance configurations. 5. REFERENCES AKIN S., BAGCI A.S. and KOK M.V., Experimental and Numerical Analysis of Dry Forward Combustion with Diverse Well Configurations, Energy Fuels, 16, 4, , BAGCI A.S., Investigation of Combustion Process through Horizontal Wells, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 27, 6, ,

18 FATEMI, S.M.; KHARRAT, R. Feasibility Study of Top- Down In-Situ Combustion in Fractured Carbonate Systems, Brazilian Journal of Petroleum and Gas, Vol. 2, Issue 3, p , 2008a. FATEMI S.M. and KHARRAT R., 3D Simulation Study on the Performance of Toe-to-Heel Air Injection (THAI) Process in Fractured Carbonate Systems, Brazilian Journal of Petroleum and Gas, Vol. 2, Issue 4, p , 2008b. FATEMI, S.M. Feasibility Study of THAI for KEM Naturally Fractured Carbonated Heavy Oil Reservoir in Laboratory Scale, MSc Thesis, Department of Chemical and Petroleum Engineering, Sharif University of Technology, 2008 (in Persian). GREAVES M., TUWIL A.A. and BAGCI A. S., Horizontal producer well in in-situ combustion (ISC) processes, Journal of Canadian Petroleum Technology, Vol. 32, No. 4, pp 58-67, GREAVES M., TUWIL A.A. and FIELD R.W., Horizontal Wells in In Situ Combustion (ISC) Processes, CIM/AOSTRA Technical Conference, Banff, Canada, April 21-24, GREAVES M., XIA T.X. and AYASSE C., Underground Upgrading of Heavy Oil Using THAI, SPE 97728, presented at the SPE International Thermal Operations and Heavy Oil Symposium, Calgary, Canada, November GREEN D.W. and WILLHITE G.P., Enhanced Oil Recovery, Textbook Vol. 6, Society of Petroleum Engineers publication, ISBN: , PRATS M., Thermal Recovery, Monograph Vol. 7, Society of Petroleum Engineers publication, ISBN: , SERAJI S., KHARRAT R., RAZZAGHI S. and TAGHIKHANI V., Kinetic Study of Crude Oil Combustion in the Presence of Carbonate Rock, SPE , Presented in 15 th SPE Middle East Oil & Gas Show and conference, Bahrain, March XIA T.X. and GREAVES M., Upgrading Athabasca Tar Sand Using Toe to Heel Air Injection, SPE 65524, presented at SPE/Petroleum Society of CIM International Conference on Horizontal Well Technology, Calgary, Canada, November XIA T.X., GREAVES M. and TURTA A.T., Injection Well- Producer Well Combinations in THAI, Toe to Heel Air Injection, SPE 75137, presented at the SPE/DOE Improved Oil Recovery Symposium, Tulsa, USA, April