Experimental and Numerical Analysis of Dry Forward Combustion with

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Experimental and Numerical Analysis of Dry Forward Combustion with Diverse Well Configuration Serhat Akin, Suat Bagci, and Mustafa Versan Kok* Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received July 16, 2001. Revised Manuscript Received February 12, 2002

In situ combustion is a thermal recovery technique where energy is generated by a combustion front that is propagated along the reservoir by air injection. Most of the previously conducted studies report thermal and fluid dynamics aspects of the process. Modeling in situ combustion process requires extensive knowledge of reservoir data as well as reaction kinetics data. Unfortunately, limited kinetic data are available on the rates and the nature of partial oxidation reactions and the high-temperature combustion reactions of crude oils and their saturate, aromatic, resin, and asphaltene (SARA) fractions. Moreover, the impact of such data on the modeling of the in situ combustion process has not been investigated thoroughly. Thus, we modeled in situ combustion experiments conducted on a three-dimensional semiscaled physical model that represents one-fourth of a repeated five spot pattern. In all experiments a vertical injector is employed whereas, both vertical and horizontal producers have been installed to recover two different crude oils (heavy and medium). Several locations for the producers have been tried while keeping the length of the wells constant: vertical injector-vertical producer, vertical injectorhorizontal side producer, and vertical injector-horizontal diagonal producer. In these experiments horizontal side producers performed better than the others. We first simulated the experiments by incorporating a kinetic model that is based on grouping the products of cracking into six pseudo components as heavy oil, medium oil, light oil, two noncondensable gases, and coke using a commercial thermal simulator (CMG’s STARS). Five chemical reactions were considered: cracking of heavy oil to light oil and coke, heavy oil burning, light oil burning, and coke burning. Most of the experiments were history matched successfully with the exception of ones where a diagonal horizontal producer was used. We then repeated the simulations using SARA kinetic parameters and observed that all matches were somewhat improved.

Introduction In situ combustion is an important enhanced oil recovery process that has been studied extensively the past 45 years. This process has been considered particularly applicable for in situ recovery of medium and heavy oil reservoirs. In in situ combustion, heat is generated within the reservoir by igniting the formation oil and then propagating a combustion front through the oil reservoir. The fuel necessary to sustain the combustion front is supplied by the heavy residual material or “coke” that deposits on the sand grains during distillation, thermal cracking, pyrolysis etc. of the crude oil ahead of the combustion front. Horizontal wells offer the prospect of better performance over conventional vertical wells. This improved performance is due primarily to the larger contact area between the formation and the horizontal well. This is especially true for thin oil formations where the reservoir contact area for a horizontal well can be hundreds of times that for a fully penetrating vertical well. Furthermore, horizontal wells can be placed strategically away from the fluid contact to reduce water/gas coning, thereby causing smaller pressure drops with consequent improvement of cumulative oil recovery. Additionally, horizontal wells are also believed to have a much better chance than

vertical wells of intersecting systems of vertical and horizontal fractures in an oil bearing formation, and thus achieve higher production rates. Sweep efficiency during in situ combustion is one of the most important process parameters, but which has not been extensively evaluated and is least understood. Most laboratory investigations are conducted in combustion tubes that essentially use a vertical well arrangement and which, of course, because of their basically one-dimensional geometry, cannot provide information on either areal or vertical sweep. Information on the combustion sweep efficiency is very important for comparing process variations and also for predicting performance. Threedimensional scaled physical models can provide much valuable insight into the areal and vertical sweep processes and stability of the combustion front over a range of operating conditions. Results from such experiments may be used in conjunction with those from combustion tube tests to predict performance in the field, and also to validate numerical simulator models. Binder et al.1 conducted combustion experiments in several models of massive, unconsolidated reservoirs containing 2100 cp. oil and with properties generally (1) Binder, G. G.; Elzinga, E. R.; Tarmy, B. L.; Wilmann, B. T. Proceedings of the Seventh World Petroleum Congress, Mexico City, 1967; pp 477-485.

10.1021/ef010172x CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002

Dry Forward Combustion

lending themselves to favorable burning behavior. These experiments indicate that recovery levels of 60-70% pore volume might be obtained economically with a well spacing-sand thickness ratio of 3 and thick, shale-free sand. Experiments with a higher spacing-thickness ratio of 7, both with and without restrictions to vertical permeability, indicated that performance is adversely affected by thinner sands and by the presence of discontinuous permeability barriers simulating interbedded shale. Coates et al.2 studied a new in situ combustion strategy, top down process under detailed laboratory study. The time to reach ignition after commencing air injection was highly dependent on the degree of preheating. After a steam process in an Athabasca formation, there was enough fuel remaining to initiate and sustain an in situ combustion process. Wet combustion showed the potential to increase production rate if it was commenced before the pack was depleted. The top down in situ combustion process produced bitumen with a lower viscosity than the original native bitumen. The process resulted in the stable propagation of a combustion front from the top to the bottom of a reservoir, exploiting gravity drainage of the mobilized oil to a lower horizontal well. Garon et al.3 developed a facility for simulating fireflooding processes in a three-dimensional model. Results from three-dimensional model tests provided a basic understanding of the mechanisms of in situ combustion in nonhomogeneous reservoirs. In a reservoir heated from below by steam injection into a noncommunicating bottom water zone, a fireflood moves rapidly through the heated layer with poor vertical displacement if the air injection rates are too high. Fire-flooding at relatively low air injection rates was very effective in a reservoir with a simulated heated fracture at the midplane. Much more oil was swept from above the fracture than from below, and little or no oil was recovered from beyond the areal extent of the fracture. In another study conducted by using a three-dimensional scaled model, Garon et al.4 investigated the volumetric sweep during fire-flooding. The effect of oxygen vs air injection, water/ oxygen ratios, injection rates, and crude oil parameters on sweep efficiency and performance of the fireflood were evaluated. Results indicated that the sweep of a fireflood was similar for both oxygen and air combustion, water injection resulted in a small decrease in the sweep of the fireflood, wet combustion required less oxygen or air and increased the oil recovery and recovery rate, fireflooding a medium-gravity crude oil reservoir resulted in a larger sweep than a heavy oil reservoir, and higher injection rates improved the sweep efficiency. The application of horizontal wells was studied by Greaves and co-workers in a series of studies.5-8 A semiscaled three-dimensional physical model was used (2) Coates, R.; Lorimer, S.; Ivory, J. SPE Paper 30295. Presented at the International Heavy Oil Symposium, Calgary, Alberta, 1995; In Proceedings of the International Heavy Oil Symposium; pp 487498. (3) Garon, A. M.; Geisbrecht, R. A.; Lowry, W. E., Jr. J. Pet. Technol. 1982, 2158-2166. (4) Garon, A. M.; Kumar, M.; Lau, K. K.; Sherman, M. SPE Reservoir Engineering 1986, 565-574. (5) Greaves, M.; Tuwil, A. A.; Bagci, A. S. J. Can. Pet. Technol. 1993, 32, 58-67. (6) Greaves, M.; Al-Shamali, O. J. Can. Pet. Technol. 1996, 35, 4955.

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to investigate the process of dry forward in situ combustion using different configurations of horizontal producer wells.5 A higher overall rate of oil production and higher recovery of OOIP was achieved with both single and double horizontal well configurations compared to that achieved with a single vertical producer well. Cumulative oil recovery increased with the number of horizontal producer wells, but the net recovery per well decreased compared to the single horizontal well case. The tendency for gas override was greatly reduced by the action of the horizontal producer well, so that the vertical sweep efficiency of the combustion front was considerably improved compared to that for the single vertical producer well. For dry in situ combustion experiments and a single wet combustion experiment conducted on heavy Wolf Lake crude oil using rectangular three-dimensional combustion cell high oil recoveries were achieved during dry combustion, ranging from 64.3 to 72.3% OOIP, and 78.8% OOIP during wet combustion.6 Gas override condition was not a major problem using the horizontal producer well in direct line drive. The volumetric sweep efficiencies calculated from the vertical and horizontal temperature profiles were generally in good agreement with the measured oil recovery values, indicating almost complete recovery of oil from the swept regions of the sand pack. Low-pressure air injection/in situ combustion in waterflooded light oil reservoirs was also investigated in a three-dimensional combustion cell using a horizontal producer well in line drive.7 Oil recovery varied from 63.9 to 85% OOIP with fuel consumption varying from 3.5 to 4.5% OOIP. These values indicate that improved oil recovery from water-flooded light oil reservoirs using in situ combustion is not limited by any apparent low fuel availability, but rather the intrinsic ability to sustain a stable combustion front at a sufficiently high temperature. Greaves et al.8 furthered low-pressure air injection/in situ combustion by conducting experiments using one-quarter of a 5-spot physical model with a controlled permeability variation. For light and medium heavy crude oils, it was found that a horizontal producer well does not act as a high permeability streak, but actually increases and brings forward production. Bagci and Shamsul9 performed similar combustion experiments with the application of different well configurations in a three-dimensional semiscaled model using medium- and low-gravity crude oils. The highest oil recoveries for both crude oils were obtained with horizontal producers positioned at the boundary of the model alone as a single producer. With the same burned volume horizontal producers than by vertical producers recovered more oil. Air requirement and fuel consumption were reduced greatly by applying horizontal producer. The basic mechanism for the conventional in situ combustion process is burning of a heavy residue or coke fraction that produces heat required to increase the (7) Greaves, M.; Mahgoub, O. SPE Paper 37154, presented at SPE International Conference on Horizontal Well Technology, Calgary, Alberta, 1996, 951-962. (8) Greaves, M.; Wilson, A.; Al-honi, M.; Lockett, A. D. SPE Paper 35693. Presented at Western Regional Meeting, Anchorage, Alaska, 1996; In Proceedings of the Western Regional Meeting; pp 435-443. (9) Bagci, S.; Shamsul, A. J. Can. Pet. Technol. 1999, 38, 1-10.

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temperature of oil-bearing formation. The overall upgrading effect of the oil produced by in situ combustion process can be described in terms of the reduction in oil density and viscosity. However, comparing the crude and produced oil compositions can provide more insight of the reaction mechanisms. SARA (saturates, aromatics, resins, and asphaltenes) analysis is a grouping method of representing oil composition. The combustion properties of individual elements that constitute SARA fractions may be helpful in determining the source of the fuel for in situ combustion and also pathways for in situ oil upgrading. Ciajolo and Barbella10 used thermogravimetric techniques to investigate the pyrolysis and oxidation of some heavy fuel oils and their separate paraffinic, aromatic, polar, and asphaltene fractions. They found that the thermal behavior of fuel oil can be interpreted in terms of low-temperature phase involving the volatilization of paraffinic and aromatic fractions, and a high-temperature phase in which the polar and asphaltene fractions pyrolyze and leave a particulate carbon residue. Ranjbar and Pusch11 studied the effect of oil composition, characterized on the basis of light hydrocarbons, resin and asphaltene contents and the pyrolysis kinetics of the oil. The results indicated that the colloidal composition of oil as well as the transferability and heat transfer characteristics of the pyrolysis medium has a pronounced influence on fuel formation and composition. Ali and Saleem12 investigated the asphaltenes precipitated from crude oils by thermogravimetric analysis and pyrolysis-GC analysis. The evolution of methane and other normal alkanes from all the asphaltenes under mild pyrolysis conditions indicated that these asphaltenes contain thermally labile alkyl groups on the periphery. As it can be seen the importance and impact of SARA fractions to combustion process has gained impetus in recent years; however, little or no information exists regarding the effect of SARA fractions to combustion and its modeling. This paper presents experimental in situ combustion data conducted with various injectionproduction well couples (vertical injector-horizontal side producer, vertical injector-diagonal horizontal producer, and vertical injector-vertical producer) using two different heavy crude oils. In these experiments horizontal side producers performed better than the others. We first simulated the experiments by incorporating a kinetic model that is based on grouping the products of cracking into six pseudo components as heavy oil, medium oil, light oil, two noncondensable gases, and coke using a commercial thermal simulator (CMG’s STARS). Several different chemical reactions were considered: cracking of heavy oil to light oil and coke, heavy oil burning, light oil burning, and coke burning. Most of the experiments were history matched successfully with the exception of ones where a diagonal horizontal producer was used. We then repeated the simulations using SARA kinetic parameters that were obtained using thermogravimetric analysis equipment with the hope of improving the history matches. We (10) Ciajolo, A.; Barbella, R. Fuel 1984, 63, 657-662. (11) Ranjbar, M.; Pusch, G. J. J. Anal. Appl. Pyrolysis 1991, 185, 37-46. (12) Ali, M. F.; Saleem, M. Fuel Sci. Technol. Int. 1991, 9, 461468.

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observed that all matches were somewhat improved. We finally present a discussion of application of the models to field scale. Experimental Section Equipment. The schematic diagram of the experimental equipment shown in Figure 1, is comprised of four major components: three-dimensional scaled physical model, fluid injection system, fluid production system, and measuring and control system. The three-dimensional rectangular box type combustion model is a 40 cm square with a thickness of 15 cm. It is made up of 3 mm thick steel. One face of the model is removable so that sand mixture can be readily packed into the model. Combustion experiments were conducted by employing five different well configurations shown in Figure 2. Perforated stainless steel tubing of 8 mm diameter was used as injection and production wells. The lengths of vertical (14 cm) and horizontal (37 cm) wells were not changed during the course of study. The horizontal wells were positioned at the middle of the model. A total of 36 thermocouples placed at 10 cm. intervals provided temperature profiles inside the model during the course of experiments. Sixteen thermocouples are placed in the center plane of the model and the depths for the measurement plane are 7.5 cm from the top of the model. Nine thermocouples are placed in the top plane and bottom planes having the distance 3.5 cm from the top and bottom of the model (Figure 4). The design pressure for the combustion model was 3450 kPa. A coil type igniter (1000 W) was imbedded in the middle section of the sand pack near to air injection well to raise the inlet temperature to the required ignition temperature. The space between the combustion model and the isolation box, within which the model placed, was insulated by glass wool. To compensate heat loss and to maintain a near adiabatic environment, the combustion model was wound with two band heaters one located near the entrance and the other near the exit, which were automatically controlled by the heater controllers according to the propagation of combustion front. The fluid injection system was used to inject gas and water. The gas injection system delivers nitrogen prior to injection and air after the ignition from high-pressure cylinders. The fluid production system consists of high- and low-pressure separators and condensers, wet test meter, gas chromatography, reporting integrator, and helium cylinder. When two horizontal producers were used the fluid production was controlled by using two flow regulator valves connected to the same production line. A backpressure regulator was installed to set pressure in the model. A wet test meter measured produced gas volume. The composition of the produced gas was monitored by gas chromatography. The measuring and control system incorporates a digital temperature scanner and digital flow indicator. Temperature readings by thermocouples were registered with the help of a digital temperature scanner and linear mass flow meter was used to measure the mass flow rate. Pressure transducers measured inlet and outlet pressures. TG/DTG has the capability of measuring the weight loss either as a function of temperature or time in a varied but controlled atmosphere. Prior to the experiments, the TG/DTG system was calibrated with calcium oxalate monohydrate for temperature readings and silver was used in order to correct for buoyancy effects. Experimental Procedure. At first, the wells were installed inside the model with the desired configuration. The model was then packed by tamping the mixture of 75% oil and 25% water that was obtained by virtue of mixing crushed limestone, crude oil and tap water. The physical properties of the porous media were kept constant in each experiment. The sand pack properties and the operating conditions are shown in Table 1. After the completion of packing operation, ther-



Figure 1. Schematic drawing of the experimental setup. mocouples were inserted inside the model. Prior to ignition, the model was heated with the aid of the band heaters until the temperature inside the model reached to 50 °C that was presumed to be the reservoir temperature. During this period nitrogen was continuously injected to prevent low-temperature oxidation. After preheating, the igniter was switched on to heat the combustion cell to 500 °C with the aid of a temperature programmer. At this instant, nitrogen flow was terminated and air injection was started. Temperature distribution inside the model was registered every 10 min during each experiment. The other parameters that were recorded during the experiment were air injection pressure and rate, volume of produced gas, production pressures and, oil and water productions. Produced gas samples were fed to gas chromatography every 20 min for compositional analysis of gas. The combustion runs were terminated as soon as the front arrived at the thermocouple nearest to the producer. The precipitation and column chromatographic methods achieved separation of the crude oil into SARA fractions.

Diluting the crude oil with forty volumes of n-hexane separated the asphaltene fractions of oils. The mixture was shaken for about 1 h and stored overnight in the dark. The filtration of this mixture was the next step to obtain asphaltene. The asphaltene fraction was washed with n-hexane until no yellow color due to resins or oil was visible in the wash. The asphaltene fraction was dried in an oven at 70 °C for 6 h under helium atmosphere. To separate saturates, aromatics, and resins a combined alumna/silica column was prepared. The column (60 × 1 cm2 ID) was slurry packed, the alumina was preheated for 6 h at 300 °C and silica gel was preheated for 6 h at 130 °C. The height ratio of alumina and silica was 1:1 in the column. The solvent (n-hexane) soluble compounds were separated into alkanes, aromatics, and resins by chromatography using the prepared column. The carrier solvent for separation of alkanes was n-hexane. A 1:1 mixture of n-hexane/ benzene was used to elute aromatics from the column. Finally, 1:1 benzene/methanol was used to obtain resins from the column. The separation efficiency of saturates from aromatics

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Figure 2. Schematic drawing of experimental well configurations. Table 1. Experimental Packing Data and Operating Conditions Raman

°API k, Darcy φ, % So, % Sw, % Sg, % Voil, cc Vwater, cc PInj, kPa Qair, m3/m2 hr QInj.m3/hr

RM-1 configuration: vi-vp

RM-2 configuration: vi-hp-d

RM-3 configuration: vi-hp-r

RM-4 configuration: vi-hp-l

RM-5 configuration: vi-2hp

RM-6 configuration: vi-vp

RM-7 configuration: vi-hp-d

18.3 9.8 38 73.5 25 1.5 6705 2280 300 8.35 0.626

18.3 9.8 38 75 25 0.0 6850 2280 294 8.45 0.634

18.3 9.8 38 75 25 0.0 6850 2280 290 4.61 0.348

18.3 9.8 38 75 25 3.99 6477 2280 250 2.40 0.180

18.3 9.8 38 71.0 25 2.82 6583 2280 210 2.88 0.216

18.3 9.8 38 72.2 25 2.st0 6663 2280 205 1.68 0.126

18.3 9.8 38 73 25 5.64 6326 2280 230 1.84 0.138

B. Kozluca

°API K, D φ, % So,% Sw,% Sg, % Voil, cc Vwater, cc Pinj, Kpa Qair, m3/m2 h

BK-1 configuration: vi-vp

BK-2 configuration: vi-hp-d



12.4 9.8 38 69.36 25 0.0 6850 2280 200 14.44

12.4 9.8 38 75 25 0.0 6850 2280 120 9.97

12.4 9.8 38 75 25 0.0 6850 2280 178 7.57

12.4 9.8 38 75 25 0.0 6850 2280 248 2.76

was checked by means of UV detection. Thermal experiments (TG/DTG) were performed with a sample size of ∼10 mg, at heating rate of 10 °C/min. Air flow rate through the sample pan was kept constant at 50 mL/min. in the temperature range of 20-600 °C. Experiments were performed twice for repeatability.

Results and Discussions Table 2 summarizes the experimental results obtained from the tests. During the course of study, a total of seven dry forward combustion experiments using Raman (18.3 °API) and four experiments using B. Kozluca (12.4 °API) crude oils were carried out in a three-dimensional physical model with the application of different well configurations (Figure 2). Three vertical

injection vertical production experiments were conducted. Of these three experiments one experiment (RM-6) was a rerun of experiment RM-1 with a lower air rate. The temperature distributions at two time levels in RM1 and in BK1 as shown in Figure 3 were discussed to make a comparison between the two runs. The center plane was observed to be much warmer than the top and bottom planes in both runs. This is because the igniter was located near the center plane. The occurrence of HTO at the vicinity of injection end in both runs can be verified by looking into reaction kinetics of Raman and B. Kozluca crude oils (see discussion in the following paragraphs). The heat front propagation in RM1 was faster at the beginning and slowed in later time steps (Figure 4). The peak temperatures observed


Energy & Fuels, Vol. 16, No. 4, 2002 897 Table 2. Overall Summary of the Combustion Test Results Raman

Taverage peak °C Vfront, cm/min oxygen utilization % air requirement m3(st)/m3 AOR, m3(st)/m3 AFR, m3/kg fuel consumption rate, kg/m3 oil recovery, (% OOIP)

RM-1

RM-2

RM-3

RM-4

RM-5

RM-6

RM-7

428 0.065 85.50 1395 2675 11.32 123.2 33.45

411 0.097 83.10 847 2313 11.47 73.82 36.35

390 0.064 80.10 423 1121 13.03 32.42 54.46

403 0.073 83.40 133 328 11.04 12.04 55.14

401 0.103 82.16 183 558 10.84 17.05 74.91

450 0.051 89.80 376 640 12.50 30.08 30.66

420 0.065 86.00 245 702 10.66 22.90 34.88

B. Kozluca Taverage peak °C Vfront, cm/min oxygen utilization % air requirement m3(st)/m3 AOR, m3(st)/m3 AFR, m3/kg fuel consumption rate, kg/m3 oil recovery, (% OOIP)

BK-1

BK-2

BK-3

BK-4

458 0.0650 90.65 3930 4864 12.8 305.4 25.9

460 0.0820 90.80 1329 4234 10.6 124.6 28.4

445 0.0690 92.21 562 1859 10.4 53.92 45.1

465 0.0612 95.06 229 595 10.9 20.9 43.2

Figure 3. Left: Temperature profiles at the center plane at t ) 95 and t ) 450 min for RM-1. Right: Temperature profiles at the center plane at t ) 100 and t ) 300 min for BK-1.

in RM1 were lower than those in BK1. Stabilized combustion front was observed in RM1, whereas no period of stabilization was apparent in BK1 due to severe bypassing. However in RM6, which was a rerun of RM1 with a lower air rate, the channeling occurrence was severe. The reduction of channeling results from the lower rate. The superficial burning of the sand pack was also noted in this experiment. Therefore, it can be concluded that superficial burning of sand pack is usual in all vertical-vertical well configurations, which leads to a poor vertical sweep in this configuration. Five experiments were conducted employing vertical injector-horizontal producer configuration to recover

Raman crude oil whereas; three experiments were also conducted by using the same well configuration for the production of B.Kozluca crude oil. After air injection, a uniform temperature distribution throughout the center plane in both runs was observed. The instant of active combustion was obvious in RM3 and BK3 at 247 and 279 min, respectively. The front stabilized in both runs and proceeded in the direction of the producer. The creation of isotherms were parallel to the producer in both runs which is suggestive of distributed flow field at the center plane as shown in Figure 5. Nevertheless, isotherms at the center plane in BK3 spreaded to the left boundary of the model. Early production of hot fluids

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Figure 4. Temperature (°C) isotherms for RM-1 (vertical injection-vertical production) at different stages of the experiment.

Figure 5. Left: Temperature profiles at the center plane at t ) 140 and t ) 615 min for RM-3. Right: Temperature profiles at the center plane at t ) 95 and t ) 485 min for BK-3.


Figure 6. Experimental recovery curves for different well configurations using raman crude oil.

that transport heat as a result of convection was noted. It was thought that heat was removed from the burnedout sand pack by vaporization of water behind the combustion front. The heat was deposited ahead of the burning zone by condensation in cooler regions of the reservoir. The horizontal production well conveyed heat downstream into the colder regions. However, another important factor was the conductive heat loss toward the walls of the three-dimensional model because of the controlling of band heaters. No indication of flow channel or bypass was noted. Since horizontal producers cause a larger areal contact, they can drain fluid without creating a by passing flow path. At the beginning of all runs, the rate of frontal advance was observed to be very low. This was probably due to the higher fuel deposition at the beginning of the experiments as a result of LTO and cracking. Since the frontal advance is dependent on the residual material burned per unit of sand cleaned, propagation of front is very slow at the beginning. Additionally, in a three-dimensional model the front is liable to change its direction of propagation during the courses of its travel to the producers, which is one of the reasons of stagnancy in frontal advance along the diagonal of the model at earlier time steps. The average fuel consumption rates in vertical-vertical configuration for both crude oils were the highest, which, in turn, resulted in the highest overall fuel consumption. This event indicates the involvement of severe cracking and distillation all through these experiments, which enhance fuel deposition. The fuel consumption rate was greatly reduced in all runs with both crude oils by employing horizontal producers. For both crude oils, air requirements were higher in vertical-vertical configuration, since higher fuel deposition takes place in this configuration. The AOR for both crude oils were highest in vertical-vertical configuration. Figures 6 and 7 represent the time and corresponding recovery of Raman and B.Kozluca crude oil respectively, employing different well configurations. With the same burned volume, horizontal producers than by vertical producers recovered more oil, which is very pronounced in both figures. The amount of oil recovered in RM3 and RM4 was almost the same, at around 55% of OOIP. BK3 and BK4 runs were similar such that they employ the same well configurations that were used in RM3 and RM4, respectively, except the


Figure 7. Experimental recovery curves for different well configurations using B. Kozluca crude oil.

Figure 8. Thermograms of Raman crude oil and its SARA fractions.

crude was different. The recoveries in BK3 and BK4 were around 45 % of OOIP. Although, the highest amount of crude was recovered by dual horizontal producers (72% of OOIP), the net recovery per producer was lower than that of a single producer placed alone at the boundary. The main reason for this is presumably the flow interference between the twin producers due to the restricted drainage area. The vertical-vertical configurations for both crude oils exhibit poor performances. Early production noted in horizontal wells remove oil without the creation of any extensive mobile path in the colder region whilst, because of their flow geometry more volume should be burned to create a flow path between a vertical injector and a vertical producer within which the fluid can flow easily. TG analysis (Figure 8) showed that LTO is very weak in asphaltenes and occurs with very little weight loss (2.1%). This weight loss is probably due to the visbreaking and pyrolysis of the asphaltenes. After this slight reaction, medium-temperature oxidation (MTO) starts near 380 °C that creates a 32% weight loss for asphaltenes. Since these are the heaviest fractions in crude oil, they lose almost rest of their weight in hightemperature oxidation (HTO) region. This means that the overall oxidation of asphaltenes is slow with little production of higher molecular weight compounds to be burned. Saturates are at the other extreme as far as oxidation of crude oil fractions are concerned. Saturates show a huge weight loss (88.6%) until the end of LTO reaction that start at 310 °C and their weight losses in this region are around 12.6%. Saturates show very slow MTO reaction and a weak HTO of the fuel formed from the oxidation of paraffin in LTO. So, saturates do not

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Table 3. Chemical Reaction Schemes and Kinetic Parameters Used in the Simulations

description

chemical reaction C7+ f C2-C6 + coke

cracking of heavy oil to light oil and coke cracking of heavy oil to gas and coke coke burning heavy oil burning

coke + O2 f H2O + CO + energy C7+ + O2 f H2O + CO + energy

light oil burning

C2-C6 + O2 f H2O + CO + energy

gas burning carbon monoxide burning

CH4 + 3/2O2 f 2H2O + CO + energy CO + 0.5 O2 f CO2 + energy

C7+ f CH4 + coke

stoichiometry C14.39H23.75 f 0.20C3.12H8.51 + 13.75C2H6 C14.39H23.75 f 0.20C3.12H8.51 + 13.75CH1.6 C1.0H1.6 + 0.9O2 f 0.8H2O + CO C14.39H23.75 + 13.13O2 f 11.87H2O + 14.3949CO C3.11H8.51 + 3.68 O2 f 4.25H2O + 3.11CO CH4 + 1.5O2 f 2H2O + CO CO + 0.5 O2 f CO2

contribute too much to HTO reaction, in oil phase. Between these two extremes, asphaltenes and saturates, there are aromatics and resins or polar materials. TG behavior of the aromatic fraction is very similar to that of resins and this observation supports the hypothesis of the formation of resins is due to the reaction between aromatics and oxygen. For the resin fraction of both crude oils, LTO region is between 320 and 370 °C, with a weight loss of 11%. It is also important to note that the behavior of resins in distillation and LTO region is dependent on the chemical structure of resins or the amount of waxy compounds, which is distilled at low temperatures as inferred from such weight losses. The very same behavior is also observed for aromatics. They have almost the same LTO region (330-390 °C), and also the LTO starting temperature is the same with resins. Numerical simulations of the experimental tests were performed using the CMG model STARS13 by incorporating a reaction model for in situ combustion of the crude oils used in this research. Cartesian gridding with 12 × 12 × 5 (3.33 × 3.33 × 3) grid blocks were used. A constant pressure producer and a constant rate injector located at the appropriate plane and location were used. The igniter was modeled by constant addition of heat to the injection block. We first modeled experiments using a kinetic model that is a consolidation of individual kinetic studies on thermal cracking, low-temperature oxidation, and hightemperature oxidation given in detail by Greaves et al.15 In this model, describing the oil component in terms of heavy, medium, and light fractions with two noncondensable gases as well as a solid coke component followed a pseudo component approach. Five chemical reactions were used. Two reactions dealt with cracking of heavy oil. Other reactions described burning of the heavy, medium, light oil, hydrocarbon gases, and the coke components respectively (Table 3). However, as reported in Kumar,16 it was observed that only heavy oil burning reaction was enough to describe the reaction kinetics for the heavy oils used throughout the experiments. Equilibrium K-values for the model were estimated from the Table 2 in STARS manual. For the (13) Computer Modelling Group. STARS Version 99 User’s Guide; Computer Modelling Group: Calgary, Alberta, Canada, 1999. (14) Hirasaki G. J. SPE J. 1975, Feb, 39-49. (15) Greaves, M.; Ron, S. R.; Rathbone, R. R. SPE Paper 40062. Presented at SPE/DOE Improved Oil Recovery Symposium, Tulsa, Oklahoma, 1998; In Proceedings of the SPE/DOE Improved Oil Recovery Symposium; pp 479-492. (16) Kumar M. SPE Reservoir Engineering 1991, 46-54.

reaction frequency

activation energy kJ/kmol

enthalpy kJ/kmol

4.167 × 105

7.75 × 106

0.00 × 100

4.167 × 105

6.28 × 106

0.00 × 100

4.192 × 102 3.0 × 1010

5.86 × 106 7.75 × 106

5.23 × 107 1.10 × 109

3.0 × 1010

7.75 × 106

2.09 × 109

3.02 × 1010 8.064 × 108

1.38 × 107 1.76 × 106

2.91 × 109 6.60 × 107

Figure 9. Water-oil (top) and gas-oil (bottom) relative permeability curves used in the simulations.

relative permeability data Corey-like relative permeabilities were used.14 Figure 9 shows the relative permeabilities used in the simulations. The numerical model incorporated external heater option to raise the temperature of the injector in the beginning. To simulate adiabatic conditions, no external heat losses or gains were allowed. The modeling efforts produced relatively satisfactory results in terms of oil production especially in vertical injector-horizontal side producer configurations using both crude oils. Figures 10 and 11 show sample history matched experiments, RM-1 and BK-1 where vertical injector-vertical producers for different crude oils were used. It can be observed that recovery predictions and oil matches are comparable for RM-1. On the other hand cumulative oil production for BK-1 is apparently reasonable, but the oil rate is not. This mismatch could be due to the plugging of the exit lines. Similarly successful cumulative oil production matches were observed with vertical injector-horizontal side producer configurations (RM-3 and BK-3) as shown in Figures 12 and 13. The same oil rate mismatch was also observed for RM-3. The pressure matches were


Figure 10. History match of experiment RM-1.

Figure 11. History match of experiment BK-1.

Figure 12. History match of RM-3.

Figure 13. History match of BK-3.

somewhat poor as the experimental pressures were almost constant. A sample pressure match for BK-3 (vertical injector-vertical producer) is given in Figure 14. Recovery and pressure matches were the poorest for the vertical injector-diagonal horizontal producer configuration (RM-2) as shown in Figures 15 and 16. For both crude oils the numerical model severely underestimated the experimental recovery especially after the


Figure 14. Differential pressure match of experiment BK-3.


Figure 16. Differential pressure match of experiment RM-2.

ignition. We believe this problem is related to the definition of horizontal wells in the numerical model and further studies are required. Figure 17 gives temperature distribution at the center plane for the experiment RM-1. When compared with the experimental temperature profiles presented in Figure 3, it can be observed that model temperature profiles underestimated experimental ones. Although the simulation displays rapid channeling down the communication path observed in the experiment it does not duplicate the progression of a high-temperature zone through the pack. This behavior was also observed in other injector-producer configurations. The purpose of history matching the experimental tests is that an acceptable reaction scheme and other operational parameters can be obtained for field scale simulations that will ultimately predict a possible process performance in the field. That’s why proper modeling of the experiments is extremely important. Thus, to improve the matches, we have incorporated a kinetic model that was obtained using the observations carried out using the thermogravimetric experiments.

902


Akin et al.


Figure 17. Numerical temperature profiles at the center plane at t ) 135 (top) and t ) 615 min (bottom) for RM-3. Table 4. Kinetic Parameters of the Raman Crude Oil

enthalpy

activation energy (kJ/mol)

Arrhenius constant (1/min)

saturates aromatics resins asphaltenes

Low-Temperature Oxidation 138.8 18.9 120.5 103.1 54.35 60.9 0.3248 83.1

1.27 × 105 3.61 × 107 9.14 × 107 5.67 × 105


Middle-Temperature Oxidation 50.75 7.3 15.6 41.9 113.1 46.3 193.5 124.5

2.15 × 104 7.50 × 105 1.76 × 106 2.43 × 107


High-Temperature Oxidation 238 136.6 837.9 152.7 803.8 49.5 1798 183.5

7.43 × 108 1.36 × 109 5.74 × 106 1.93 × 1011

data cannot be used directly for field scale simulation. The size of the grid blocks used in the experimental history matching is comparable to the actual size of the combustion front. However, field scale grid blocks are several magnitudes larger. Moreover, in the numerical model that simulates the experimental data, temperature is the average of the entire grid block that adequately represents the temperature in the reaction zone. It is inappropriate to represent the peak combustion zone temperature in the field case. A mathematical model that incorporates dynamic grid refinement around the injector and producer as suggested by Coates et al.17 may be used to solve the aforementioned problem. Scaling of Model. Scaling of a model and process defined by the governing equations, constitutive relationships and constraints, and conditions requires fulfillment of scaling criteria which are represented by similarity groups. In as much as in situ combustion is a complex process, it is very difficult to satisfy all scaling criteria and besides, some of the scaling criteria must be relaxed to verify others. Various approaches are set forth to be applied during scaling of an in situ combustion process and model. Since a different porous media than that of field was used during the course of experiments, an approach proposed by Islam and Farouq Ali18 was used to back-scale the model. As the length and the width of the prototype are not known, it was decided to ascertain the scaling factor “a”, by determining thickness ratio of the model and the prototype. The width and length of the prototype are then calculated by employing this scaling factor together with the similarity groups:

L H W , , X1 X2 X3

Table 5. Chemical Reaction Schemes for the SARA Simulations description

chemical reaction

aromatics (Ar) cracking resins (Re) cracking asphaltenes (as) cracking

Ar f As + C2-C6 Re f As + C2-C6 As f coke + C2-C6

Thus we described the Raman crude using its SARA fractions and developed a new kinetic model on the bases of the experimental observations given in Table 4. When compared with the previous model, the new model has additional oil fractions and the reactions are slightly different (Table 5). The matches were somewhat improved, as it can be observed from Figure 18. However, the usage of SARA fractions and their kinetic parameters is not justified because of the experimental complexity and time. Finally, we can conclude that the results obtained from history matching experimental

[ ] H X2

) model

[ ] H X2

(1) )a

(2)

prototype

Hprototype 50m ) ) 333 ) a Hmodel 0.15m

(3)

Since the length and width of the model are same and known to be 40 cm, the length and the width of the prototype will also be the same and are calculated as 133.2 m by applying scaling factor along with above (17) Coates R.; Lorimer, S.; Ivory J. SPE Paper SPE 30295. In Proceedings of the International Heavy Oil Symposium held in Calgary, Alberta, Canada; 1995; pp 487-498. (18) Islam, M. R.; Farouq Ali, S. M. J. Pet. Sci. Eng. 1992, 6, 367379.



similarity groups. The well spacing in the prototype was calculated as

[wellspacing]prototype [wellspacing]model

) a ) 333

[wellspacing]prototype )a 49.5m

(4)

(5)

therefore, the well spacing in prototype is 165 m. The permeability of the sand pack should be “a” times to that of prototype. The ratio can be found as

Kmodel 9800md ) ) 338 = a ) 333 Kprototype 29md

(6)

which is close to the scaling factor. This relationship is similar with the assumption used in approach (Islam and Farouq Ali18). Now, if the differential pressure in the prototype is assumed to be “a” times to that in the model, then the time in the prototype can be scaled as “a2/3” to that of model from the similarity groups. The porosity of the model assumed three times to that of prototype. Setting (aK) in the model group and (a∆P) in the prototype group

[

]

3φSoµoX21 taK∆P

)

model

[

]

φSoµoa2X21 taK∆P

[t]prototype )

a2 [t] 3 model

prototype

(7) (8)

The model was almost fully scaled commensurate with Raman field while it merely corresponds to geometrically similar prototype in B. Kozluca field. The scaling factor for the model according to Raman field was found to be 333, whereas the geometric similarity factor between the model and the prototype in B. Kozluca field was selected to be 250. The scaling parameters for both the B. Kozluca and the Raman models are shown in Table 6. Conclusions We present in situ combustion experiments conducted on a three-dimensional semiscaled physical model that represents one-fourth of a repeated five spot pattern using two crude oils: heavy and medium. In all experiments a vertical injector is employed whereas, both vertical and horizontal producers have been installed to recover two different crude oils (heavy and medium). On the bases of the experimental analyses, the following conclusions can be drawn:

Table 6. Scaling Parameters for B. Kozluca and Raman Prototype Reservoirs model scaling factor lithology porosity, % perm., md length, m thickness, m width, m well spacing, m °API gravity So, % Sw, % Ti, °C pressure drop, kPa time

crushed limestone 38 9800 0.40 0.15 0.40 0.495 18.3, 12.4 75 25 50 35-70 1 min

prototype (Raman)

prototype (B. Kozluca)

333 limestone

250 limestone

13.5 29 133.2 50 133.2 165 18.3 75 25 50 11655-23310 20 days

25 400 100 37 100 124 12.4 75 25 50

1. The superficial burning of the sand pack was usual in all vertical-vertical well configurations, which leaded to poor sweep efficiency in this configuration. 2. No indication of flow channel or bypass was noted during vertical injector-horizontal producer configurations. Since horizontal producers cause a larger area contact, they can drain fluid without creating a by passing flow path. Therefore, it was concluded that the vertical injector-horizontal side producers performed better than the other configurations. 3. There is almost no weight loss of asphaltenes due to distillation and LTO reaction. Asphaltenes are the strongest fractions toward oxidation. 4. Saturates show a huge weight loss till the end of LTO reaction. Unlike asphaltenes, saturates are the easiest oxidizable compound. 5. A kinetic model based on SARA fractions in the crude oil successfully modeled the experiments. However, due to experimental complexity, the use of the kinetic model was not justified. 6. Numerical temperature profiles underestimated experimental temperature profiles. Nomenclature H ) thickness of the model or prototype, L K ) permeability, L2 L ) length of the model or prototype, L So ) oil saturation, dimensionless X1 ) Cartesian coordinate in model or prototype, L W ) width of the model or prototype, L ∆P ) differential pressure, M L-1 T-2 t ) time, T φ ) porosity, dimensionless µo ) oil viscosity, L-1 T-1 M EF010172X

Experimental and Numerical Analysis of Dry Forward Combustion with

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