FEASIBILITY STUDY OF TOP-DOWN IN-SITU COMBUSTION IN FRACTURED CARBONATE SYSTEMS

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1 ISSN FEASIBILITY STUDY OF TOP-DOWN IN-SITU COMBUSTION IN FRACTURED CARBONATE SYSTEMS 1 S. M. Fatemi *, 2 R. Kharrat 1 Sharif University of Technology, Department of Chemical & Petroleum Engineering 2 Petroleum University of Technology, Petroleum Research Center * To whom all correspondence should be addressed. Address: Sharif University of Technology, Department of Chemical & Petroleum Engineering, Tehran, Iran Telephone / fax numbers: mobeen.fatemi@gmail.com Abstract. The In-Situ Combustion (ISC) process has been studied deeply in heavy oils and is found as a promising EOR method for certain conventional sandstone reservoirs, but its application feasibility in carbonate fractured systems remains questionable. In this contribution, firstly a model which is developed for simulation study of one of fractured carbonate, low-permeable reservoirs in Iran called Kuh-E-Mond (KEM) has been presented. The aim of this work was to dissect the effect of geometrical properties of the fractures, such as orientation, density, spacing, location and networking, on the performance of combustion tube experiments. Results indicate that the simulator can match the laboratory data. Lower vertical fracture spacing in conjunction with their higher fracture density enhanced the recovery performance. Because of the top-down mechanism of the process in the case of combustion tube, horizontal fractures had disastrous effects on the final achievement. Vertical fractures in networked model improved the performance of horizontal fractures alone. Simulation analysis confirmed that ISC will be more applicable in the case of highly networked fractured reservoirs such as those in Middle East. Keywords: in-situ combustion; combustion tube; carbonate fractured reservoir; IOR/EOR 1. INTRODUCTION For the production of oil from heavy oil reservoirs, thermal methods are widely applied. One of these is the in-situ combustion (ISC) process. In this process air is injected into the reservoir and the oxygen in the air burns part of the oil, thereby generating heat, which reduces the oil viscosity and enhances oil recovery. According to Akkutlu and Yortsus (2005), combustion front propagation is enhanced in the case of heterogeneity when the more permeable layer is of smaller thickness. However, its temperature drops significantly with the increasing heterogeneity ratio, smaller thickness ratio, and increasing heat loss rates. Below a certain limit, it is questionable that a proper combustion reaction can be sustained in the high-permeable layer (for example, in the case of fractures). In some ISC field trials, the combustion process could not be sustained if there were fractures in the reservoir (Schulte and Vries, 1982). Since fractures are much more permeable than the surrounding reservoir rocks the injected air will flow almost exclusively through the fractures and will contact only oil present in these fractures or in their immediate vicinity. In this case, not only the reaction rate is too low because of the very small contact area between air flow and fracture walls, but also the total amount of fuel available for combustion might be insufficient to sustain the combustion process. According to Schulte and Vries (1982) if only the low reaction rate is responsible for the dying out of the combustion Downloaded from World Wide Web 96

2 process, it is easily estimated that, in densely fractured reservoirs such as those occurring in Iran and Middle East, the contact area between air flow and fracture walls might be sufficiently large to sustain combustion, assuming that sufficient fuel is available. The main purpose of this paper is therefore to simulate a combustion tube experiment in the presence of fissures to study the feasibility of ISC for fractured reservoirs. 2. METHODOLOGY 2.1. Conventional Model Representation In the numerical simulation of combustion tubes based on experimental data presented by Seraji (2006) and Seraji et al. (2007), with a vertical matrix block consisting of 20 grid blocks (center of grids are located on the thermocouple locations in the experiment) along the z direction, one grid block in the x and y directions is considered. The total matrix block length (combustion tube in the experiment) is 1 m (3.28 ft) and its sizes in the x and y directions are ft (Figure 1). One additional run is performed with refined grids to study the front shape. Initial conditions and the KEM carbonate rock properties are represented in Table 1. In this model the following six components and pseudocomponents were introduced in the Computer Modeling Group (CMG), Builder module: water, heavy oil, light oil, inert gas, oxygen and coke. All non-condensable gases such as CO2, CO and N2 were lumped to a single inert gas to minimize the number of equations to be solved. To save CPU run time, flashed composition of the KEM crude oil used Figure 1. Schematic representation of conventional combustion tube model in simulation analysis. in the experiment (Table 2) has been lumped into two groups known as light oil (C1-C11) and heavy oil (C12+). Critical properties of each pseudocomponent have been obtained according to Hong mixing rules (Ahmed, 1989). The viscosity data of the oil sample as a function of temperature used to fit the correlation for Light Oil (LO) and Heavy Oil (HO) fraction. In the simulation this viscosity will be applied for the components of both light and heavy oils. Gas phase viscosity correlation parameters of oil pseudocomponents are shown in Table 3. The equilibrium ratios (K values) have been obtained from Wilson correlations (Danesh, Table 1. Initial conditions of matrix block in simulation. Parameter Value Parameter Value Pressure / psi 320 Water Saturation nil Temperature / ºF 160 Injection Rate 4.8 L/min Porosity Rock Thermal Conductivity 2.5 Btu/(ft h ºF) Matrix Permeability / md Rock Compressibility 10-6 sip Oil Saturation 0.4 Rock Thermal Expansion Coefficient ºF -1 Gas Saturation 0.6 Rock Heat Capacity 35.4 Btu/(ft 3 ºF) Downloaded from World Wide Web 97

3 Table 2. Sarvak oil sample flash composition. Component Percentage Component Percentage C C C C C C ic C nc C ic C nc * C *C12+ Molecular Weight: 485; C12+ Specific Gravity: Table 3. Gas phase viscosity of oil pseudocomponents in simulation. Gas-Phase Viscosity = A T B Light Oil Heavy Oil A B ) for each light and heavy oil components. According to Danesh (1998), the K values are determined as the ratio between the mole fraction of component i in the vapour phase and the mole fraction of the same component in liquid phase Simulation Model Validation by Experiment The system average temperature, the front propagation rate and the total experiment time had been chosen to match the experimental data published by Seraji (2006) and Seraji et al. (2007). Figure 2 shows average system Figure 2. Comparison between experimental and simulation reservoir average temperature. 98 Downloaded from World Wide Web

4 Table 4. Comparison between experimental and simulation results. Matching Parameter Experimental Results Simulation Results Front Propagation Rate 0.42 ft/hr 0.42 ft/hr Total Process Time 7.88 hr 7.92 hr Front Average Temp. 587 C 1090 F ( C) Front Location at end 2.25 ft 2.3 ft Production Well Temperature at end 285 C 550 F ( C) temperatures, for both experimental and simulation up to 4 hours. Other parameters have been compared in Table 4. The simulation results are in fair match with experimental data. Figure 3. Schematic representation of fractured combustion tube (base model) Fractured Model Representation The fractured combustion tube simulation model used in this work consists of two vertical fractures induced at the lateral sides of the previous conventional non-fractured combustion tube model section, and is depicted in Figure 3. There are three grid blocks in the x direction, one grid block in y and 20 grids in the z direction. The first and third grid columns in the x direction are simulated fractures and the second ones are matrices. Each fracture has a ft width. Fissures have been illustrated by vertical red stripes at either side of the model in Figure 3. Relative permeabilities of the fractures are assumed to be linear, as compared with saturations reported in the literature (Van Golf-Rakht, 1982). To study the effect of geometrical properties of fractures on the ISC process performance, other fissured models have also been developed and are depicted in Figure 4. Patterns A and B are conventional and fractured base models respectively. During the simulation the first grid block was continuously preheated up to ignition temperature for 0.5 hr, after which the heater was switched off. During the process, air is Figure 4. Conventional model and different fractured models. Downloaded from World Wide Web 99

5 elevated temperatures from the start. Ignition immediately occurs in fractures and the front propagates towards the production well. Heat generated by the fracture s oil ignition is conducted to the matrix and combustion takes place in the matrix blocks. The first trials were to ignite oil by the same air injection rate, as reported by Seraji (2006) and Seraji et al. (2007) in their conventional combustion tube experiments (4.8 L/min). At this rate of injection, the combustion front could not be sustainable neither in the fracture nor in the matrix and the combustion was extinguished after a short period of time. Therefore, the injection rate was reduced to 2.4 L/min. Combustion started and propagated steadily towards the producing well. Figure 5. Oxygen saturation profiles in conventional (left) and fractured (right) models. injected into the fractures (no air is injected into the matrix; there are two injectors, one in each fracture) to simulate the flow of air in the fracture system. The fractured model contains the same amount of fluids as the conventional one. Besides, it is assumed that, for initial conditions, the fractures are fully saturated with oil. It was found that the combustion process would not start without the inflow face being at 2.4. Comparison of Fractured and Conventional Models Oxygen diffusion causes some kind of coneshaped combustion front. This is represented by the concave-shaped profile in the fractured model, as compared with the convex-shaped profile in the conventional model (Figure 5). The higher rate of the front propagation in the fracture, as opposed to that in the matrix, causes the matrix to be preheated forward of its own combustion front. This preheating reduces oil viscosity and oil is transferred to the lower parts because of gravity drainage. As a result, low amount of oil will be present in the face contacting the matrix combustion front and thereby less coke will be generated. Figure 6. Effect of vertical fractures on ISC process performance. 100 Downloaded from World Wide Web

6 Figure 7. Effect of vertical fractures spacing on oil recovery. Lower coke deposition means lower front temperature. This reduces the average temperature and causes lower coke deposition. As a result, lower ultimate oil recovery would be achievable in the fractured model (Figure 6). In the KEM fractured model, the rate of front advance in the matrix is not so delayed with respect to the conventional model. This may have occurred due to the preheating of the matrix rock forward of its front by the fracture front, and also because of the higher thermal conductivity of carbonated rock (2.5 Btu/ft-hr- ºF), as compare to the sand case (1 Btu/ft-hr-ºF, according to Tabasinejad et al., 2006) Effect of Vertical Fractures Spacing Fracture spacing is really important in failure or success of ISC for fractured model. Additional runs with the same fracture opening size and same total amount of oil in the system has been done, with different fracture spacings, namely , (base case) and ft (Figure 4, Patterns C, B and D respectively). In the case of lower fracture spacing, average temperature and oil recovery would be higher (Figure 7). Higher fracture spacing causes more extended cone-shaped patterns and steady state conditions under which the delay of the coneshaped front diminishes. In both cases, less opportunity is given for oxygen to diffuse into the matrix for higher fracture spacing and as a result the oil recovery is reduced and lower system temperature is attained. Figure 8. Effect of horizontal on ISC process performance. Downloaded from World Wide Web 101

7 Figure 9. Effect of horizontal fracture location on ISC performance Effect of Horizontal Fractures Patterns A and E (Figure 4) have been compared to study the effect of horizontal fractures on the ISC process efficiency. Simulation results showed that the top-down combustion performance decreased in the case of one horizontal fracture (Figure 8). This can be due to the difficulty of oxygen diffusion in this case from the matrix into the fracture and vice versa, which reduces the ISC performance Effect of Horizontal Fracture Location Patterns E, F and G (Figure 4) were used to elucidate the effect of horizontal fracture location on the process performance. As is obvious from Figure 9, oil recovery increases when the horizontal fracture is located next to the producer. This can be explained by the fact that the higher volume of the combustion tube could be swept in this case since the fracture is located at the end of the combustion tube, as compared to the other two cases. This fact will impair the diffusion mechanism based in the fracture model and as a result the ISC performance increases Effect of Horizontal Fracture Density Patterns E, H and I (Figure 4) have been used in this study. Simulation results confirmed that the higher horizontal fracture density enhances the process performance. This may be partially due to higher contact area of oxygen with the fracture system. From another point of view the volume of the matrices that should be affected by oxygen have been reduced and Figure 10. Effect of horizontal fracture density on ISC performance. 102 Downloaded from World Wide Web

8 Figure 11 of horizontal fracture spacing on ISC performance. oxygen finds its path into the following highly permeable media of the next fracture due to the low pressure of the production well. These combined processes enhance the ISC process performance (Figure 10) Effect of Horizontal Fractures Spacing Patterns H and J (Figure 4) have been compared in this case. Figure 11 confirms that lower fracture spacing has the higher ultimate oil recovery. This can be displayed by the fact that lower fracture spacing means higher fracture density if the fracture system is uniform, so the ultimate oil recovery is enhanced just as much as in the previous section. In fact, some authors (Fatemi et al., 2008) believe that there is a higher horizontal fracture density limit for this behavior, which decreases after the ultimate oil recovery is achieved Effect Networked Fractured To study the effect of horizontal and vertical fractures simultaneously on ISC performance in naturally fractured reservoirs (NFR), networked fracture should be developed on the model. There are two well-known models to simulate networked fractures, the Warren and Root Model (1963) and the De-Swaan-O Model (1976). In the first one the fractures form a continuous and uniform network oriented in such a way as to be parallel to the principal Figure 12. Effect of vertical fractures on ISC process performance. Downloaded from World Wide Web 103

9 direction permeability. The fractures are assumed to be of constant width. In the case of anisotropic network or a variation in a given direction, the anisotropy must be simulated. The fracture spacing associated to fracture density is directly related to the fracture permeability and porosity. In the De Swaan-O model, instead of matrix blocks shaped as parallelepipeds, the block units are shaped as spheres. The spheres are superimposed in a regular orthogonal distribution. The fracture volume is represented by the spherical interspace which is further correlated with porosity values. In this paper, the Warren-Root model has been used. Patterns B, H and K (Figure 4) have been compared for this purpose. Vertical fractures really improved the performance of the process, and higher oil recoveries were obtained. As a matter of fact oxygen has the ability to diffuse from each matrix from all sides, which improves oxygen availability in the matrix. In addition to this, there is a higher fracturematrix contact area in the latter case which enhances the combustion process by providing more coke to be burnt at the fracture face. This kind of ignition will preheat the matrix block ahead and improve the process performance further (Figure 12). 3. CONCLUSIONS The simulator presented here has the ability to fairly match the combustion tube experiment of KEM crushed rock samples from the Middle East. In-Situ Combustion has been studied in fractured modes and the effects of geometrical properties of the fractures have been investigated. The recovery mechanism is somewhat different in fractured and conventional cases, since the recovery is based on oxygen molecular diffusion from fissures into the matrix or vice-versa in the first case. Although the horizontal fractures have disastrous effect on the performance, their higher density increases the ultimate oil recovery achievable. The same is true in the case of vertical fractures. In networked fractures, the pattern recovery was considerably higher than in the case of unaccompanied horizontal fractures, acknowledging the presence of vertical fractures. Simulation results confirm that the ISC is more applicable in densely networked fractures carbonate heavy oil reservoirs such as those occurring in Iran and Middle East. CONVERSION FACTORS 1 ft = m 1 psi = Pascal 1 (Fahrenheit) = 1.8 (Celsius) 1 Btu = joule F = 1.8 C Darcy = m2 REFERENCES AHMED, T. Hydrocarbon Phase Behavior, Houston: Gulf Publication Company, p. AKKUTLU, I.Y. and YORTSUS, Y.C. The Effect of Heterogeneity on In Situ Combustion: Propagation of combustion Fronts in Layered Porous Media, SPE Paper 75128, presented at the 2002 SPE/DOE Symposium on Improved Oil Recovery, Tulsa, USA, June DANESH, A. PVT and Phase Behavior of Petroleum Reservoir Fluids, Netherlands: Elsevier Science, 1998, 400p. FATEMI, S.M.; KHARRAT, R. and VOSSOUGHI, S. Feasibility Study of In- Situ Combustion (ISC) in a 2-D Laboratory- Scale Fractured System Using a Thermal Reservoir Simulator, presented at 2nd World Heavy Oil Congress (WHOC NO: ), Edmonton, Canada, SAIDI, A.M. Reservoir Engineering of Fractured Reservoirs (Fundamental and Practical aspects), Paris: TOTAL Edition Press, 1987, 864p. SERAJI, S. Feasibility Study of In-Situ Combustion Process for Carbonated Heavy Oil Reservoirs, MSc. Thesis, Department of Chemical and Petroleum Engineering, Sharif University of Technology, autumn (in Persian) SERAJI, S.; KHARRAT, R.; RAZZAGHI, S. and TAGHIKHANI, V. Kinetic Study of 104 Downloaded from World Wide Web

10 Crude Oil Combustion in the Presence of Carbonate Rock, SPE Paper , Presented in 15th SPE Middle East Oil & Gas Show and conference, Bahrain, March SCHULTE, W.M and VRIES, A.S. In-Situ Combustion in Naturally Fractured Heavy Oil Reservoirs, SPE Paper 10723, presented at SPE Enhanced Oil Recovery Symposium, Tulsa, USA, April TABASINEJAD, F.; KHARRAT, R. and VOSSOUGHI, S. Feasibility Study of in- Situ Combustion in Naturally Fractured Heavy Oil Reservoirs, SPE Paper , presented at the First International Oil Conference and Exhibition, Cancun, Mexico, September VAN GOLF-RAKHT, T.D. Fundamentals of Fractured Reservoir Engineering, Netherlands: Elsevier Scientific Publication Company, 1982, 710p. WARREN, J.E. and ROOT, P.J. The Behavior of Naturally Fractured Reservoirs, SPE Journal, September, p , Downloaded from World Wide Web 105