Application of EOR techniques for oil shale fields (in-situ combustion approach).Introduction
In-situ combustion is simply combustion heating of in-place oil shale within a deposit at the fire front. The main idea in in-situ combustion is burning of a portion of oil shale to produce sufficient heat to retort the remainder. A great portion of the potentially recoverable shale oil resource is in low-grade deposits that may never be recovered by primary mining techniques. In-situ processing presents the opportunity of recovering shale oil from these low-grade deposits without the adverse environmental impacts normally related with mining and aboveground processing.
Chu  stated the important variables that characterize the performance of combustion projects: fuel content, sweep efficiency, air requirement and air/oil ratio. It was observed that oil production increases with initial oil content. An increase in oil content also increases fuel availability and air requirment. However, this increase is relatively small when compared with the increase in oil production.
Hayashitani et al.  postulated several thermal cracking reaction models that could be incorporated into numerical simulations of thermal recovery processes for the Athabasca oil sands. Several pseudo reaction mechanisms were proposed to simulate the experimental results. The reaction rate constants were represented by an Arrhenius-type expression, and the activation energies and corresponding frequency factors were determined for each reaction mechanism proposed. Diyashev et al.  focused on surface control of thermal front movement in a fire flooding process. Surface measurement methods of magnetic and electric fields, due to thermal influence on formation and saturation of fluids, were elaborated and tested. Results of the practical studies performed on a pilot field have shown application of suggested methods. Speight and Moschopedis  investigated methods of introducing oxygen functions into bitumen. Comparisons of the product properties with those of the untreated bitumen were described in order to study the effects of these functions and to evaluate their performance during in-situ recovery. Dieckmann et al.  studied primary kerogen-to-petroleum and secondary oil-to-gas conversion processes in source rocks by programmed-temperature closed-system pyrolysis of oil shale sample at different heating rates. The subsequent kinetic analysis resulted in potential versus activation energy distributions, which turned out to be comparatively broad for oil and primary gas and rather narrow for secondary gas, indicating that the former was generated from more inhomogeneous precursor materials than the latter. Mamora and Brigham  studied the effect of low-temperature oxidation (LTO) on the fuel and produced oil during in-situ combustion. Combustion tube experiments were performed with three matrix types: sand, sand and clay, and sand and sand fines. LTO was observed in the run where the matrix consisted of sand only. High temperature oxidation (HTO) was observed in runs where either clay or sand fines were a part of the matrix. Kok et al. [7-14] studied the factors influencing kinetic data, such as sample order geometry, heating rate and atmosphere, under non-isothermal conditions. It was observed that the products obtained through pyrolysis and combustion depend on oil shale composition and conditional variables, such as temperature, time, rate of heating, pressure, and gaseous environment.
In this research, oil shale samples from Seyitomer, Himmetoglu and Hatildag fields were used owing to their high grades, reserves and exploitability. These shales are of low grade or in the case of the Seyitomer deposit, a shale slightly enriched in organic matter. The oil shale samples used in this research had a particle size of <60 mesh and were prepared according to ASTM standards. Proximate and ultimate analyses  of samples are given in Table 1.
The experimental set-up used in this research is a combustion cell assembly consisting of a steel combustion tube into which oil shale is packed, and enclosed within an insulation jacket, (Fig. 1). The combustion tube is equipped with igniter, thermowell, external band heaters, and gas sampling ports. Fluids produced from the combustion tube were passed through the pressure separator to collect and measure the oil and water production and through water condenser from which the condensable gases were collected. Before venting the produced gas, it was passed through a backpressure regulator to regulate the injection pressure. The produced gas was then passed through the rotameter at atmospheric pressure to measure produced gas rate and wet test meter to meter the total gas production. A gas chromatograph was used to analyze the composition of the gases produced from the combustion tube. Gas sampling ports installed at different points on the combustion tube were used for gas sampling and analysis, as the combustion front passes through these sections. During the combustion experiments, the amounts of C[O.sub.2], CO, and [O.sub.2] in the produced gas stream were measured by a continuous gas analyzer.
[FIGURE 1 OMITTED]
The initial temperature was 18-21[degrees]C prior to all runs. The oil shale sample was heated to approximately 65-85[degrees]C, while nitrogen was continuously injected. When preheating was completed, the igniter was turned on in accordance with a selected temperature profile by means of the temperature programmer. While the nitrogen injection was continued, the inlet of the combustion tube was heated to 250-300[degrees]C, which was sufficient to initialize the combustion operation. At this time, nitrogen flow was terminated and air injection was started. The air injection rate and pressure were stabilized throughout the combustion tube by the backpressure regulator. During the combustion runs, temperatures, air injection pressure values, air injection rate values, produced gas rates, and oil and water productions were recorded. Produced gas samples were analyzed every 20 minutes by gas chromatography. Produced gases were also analyzed by the continuous gas analyzer. To verify the consistency and repeatability, the experiments were performed twice.
Results and discussion
In-situ processing presents the opportunity of recovering shale oil from these low-grade deposits, and the main idea in in-situ combustion is burning a portion of the oil shale to produce sufficient heat to retort the remainder. In this study in-situ combustion experiments were performed using 1-D physical laboratory models. Dry forward combustion tube runs were carried out using Seyitomer, Himmetoglu, and Hatildag oil shale samples. Parameters measured during each combustion run were: temperature along the combustion tube, inlet and outlet pressure, air injection rate, produced gas rate, and produced gas rations. Also produced oil and water amounts were recorded intensively.
The temperature profiles along the combustion tube for Himmetoglu oil shale are given in Fig. 2. Peak temperatures and thermocouple locations for Himmetoglu oil shale as a function of time are given in Table 2. Following ignition in all runs, temperatures reached their maximum values, then decreased gradually towards the end of the tube. However, for the Seyitomer oil shale sample, some fluctuations in temperature curves were observed. These fluctuations might be the results of heterogeneous permeability due to improper packing. Also heterogeneous mesh size could play an important role in temperature fluctuation.
[FIGURE 2 OMITTED]
The frontal velocity curve for Himmetoglu oil shale is given in Fig. 3. Peak temperatures are assumed to be the maximum values of the temperature curves. The maximum temperature values for each thermocouple location were read from recorded temperature data. The data were graphed with the corresponding time values. The first derivative of the time versus location graph gave the velocity of frontal movement. The combustion front velocities (cm/hr) of the oil shale samples were as follows: 0.2965 (Seyitomer), 0.2333 (Himmetoglu), 0.2948 (Hatildag). The results show that, with a constant air injection rate and operating pressure, the rate of combustion was consistent throughout the runs.
[FIGURE 3 OMITTED]
Gas produced during combustion operations was composed of mostly [N.sub.2], C[O.sub.2], CO, and [O.sub.2]. Gas analysis was performed by both continuous gas analyzer and gas chromatograph. Two different data were correlated and a result was obtained for each run. The results are given in Table 3. Gas compositions as a function of time for Himmetoglu oil shale are illustrated in Fig. 4. In Seyitomer and Himmetoglu oil shale samples, low oxygen concentrations in the exhaust gases were determined. This phenomenon is a sign of effective utilization of oxygen. Also since organic matter content of Himmetoglu oil shale is very high compared to other samples, it is hard to control the experimental parameters. Table 4 represents the calculated mean values for each combustion tube experiment. The results obtained from each experiment show no difference from the others. To calculate oil production performance of these samples, a volumetric material balance was constructed. The amounts of oil and water produced are given in Table 5. The results show that Seyitomer, Himmetoglu and Hatildag oil shales are feasible for oil production.
[FIGURE 4 OMITTED]
* Oil distillation from oil shales with in-situ combustion technique was accomplished. It was proved that Himmetoglu and Hatildag oil shale fields are suitable for retorting operations.
* Himmetoglu oil shale seemed to be highly reactive when compared to other oil shale samples. High reactivity of Himmetoglu oil shale is believed to be resulted from high organic content of this type of oil shale.
* According to the combustion front velocities calculated, it can be said that the combustion rates were consistent throughout the runs. All runs were carried out at a constant air injection rate.
* Since the oxygen percentage in output gases was relatively less, oxygen consumption during combustion operations was concluded to be effective.
The authors would like to express their appreciation for the financial support of TUBITAK (The Scientific and Technological Research Council of Turkey), MISAG-141.
Received May 14, 2007
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M. V. KOK *, G. GUNER, S. BAGCI
Dept. of Petroleum and Natural Gas Engineering Middle East Technical University 06531 Ankara, Turkey
* Corresponding author: e-mail firstname.lastname@example.org
Table 1. Proximate and ultimate analysis of studied oil shales Oil shale Calorific value, Components, % cal/g [H.sub.2]O Ash Seyitomer 1006 2.8 70.9 Himmeto lu 1086 12.9 60.5 Hatilda 744 1.6 66.2 Oil shale Components, % C H O, N S Seyitomer 8.58 1.4 4.39 0.19 Himmeto lu 13.6 1.5 10.48 0.99 Hatilda 5.63 1.3 3.89 1.25 Table 2. Peak temperatures of combustion of Himmetoglu oil shale, [degrees]C Distance, Time, min cm 105 165 205 235 5 166 861 900 828 12 112 637 900 900 19 93 415 695 890 26 87 229 444 632 33 84 144 296 423 40 82 112 197 346 47 76 91 145 150 54 72 90 115 157 61 67 89 94 114 68 66 90 92 98 75 63 90 92 94 82 60 89 91 92 89 52 85 87 88 96 44 81 83 84 Distance, Time, min cm 265 295 325 5 663 513 389 12 839 683 566 19 900 827 673 26 900 900 785 33 651 900 884 40 450 705 900 47 328 412 736 54 233 358 456 61 161 234 394 68 128 179 342 75 106 136 277 82 93 104 181 89 89 94 144 96 85 85 107 Table 3. Stabilized composition of produced gas, vol. % Composition Seyitomer Himmetoglu Hatildag Oxygen 1.85 2.58 3.00 Carbon dioxide 16.59 14.32 16.09 Carbon monoxide 2.60 4.14 1.91 Nitrogen 78.96 78.96 79 Table 4. Calculated results of combustion experiments of oil shale samples Results Seyitomer Himmetoglu Carbon dioxide, vol. % 16.59 14.32 Carbon monoxide, vol. % 2.60 4.14 Nitrogen, vol. % 78.96 78.96 Atomic H/C ratio 0.26 0.44 Av. peak temperature, [degrees]C 730 900 Fuel cons. Rate, g fuel/min 0.32 0.31 Fuel/sand ratio, g/[cm.sup.3] 0.017 0.021 Air/fuel ratio, L/g 9.52 9.75 Air/sand ratio, L/[cm.sup.3] 0.16 0.21 Carbon burned, g/min 53.97 49.62 Air/oil ratio, L/[cm.sup.3] 0.16 0.21 Results Hatildag Carbon dioxide, vol. % 16.09 Carbon monoxide, vol. % 1.91 Nitrogen, vol. % 79.00 Atomic H/C ratio 0.21 Av. peak temperature, [degrees]C 577 Fuel cons. Rate, g fuel/min 0.29 Fuel/sand ratio, g/[cm.sup.3] 0.016 Air/fuel ratio, L/g 10.19 Air/sand ratio, L/[cm.sup.3] 0.16 Carbon burned, g/min 51.33 Air/oil ratio, L/[cm.sup.3] 0.16 Table 5. Oil and water production of tube runs Oil Shale. Sample amount, Oil production, g cc Seyitomer 4260 19 Himmetoglu 3600 116 Hatilda 6950 127 Oil Shale. Oil production, Water production, L/ton cc Seyitomer 4.46 357 Himmetoglu 32.22 298 Hatilda 18.27 67