Energy & Fuels 2004, 18, 1183-1186
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Wax Precipitation in Three Chinese Reservoir Oils under Carbon Dioxide (CO2) Injection Yu-Feng Hu,*,† Shi Li,‡ Yan-Ping Chu, and Tian-Min Guo† High-Pressure Fluid Phase Behavior and Property Research Laboratory, University of Petroleum, Beijing 102200, People’s Republic of China, and Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, People’s Republic of China Received September 30, 2003. Revised Manuscript Received April 13, 2004
The wax precipitation behavior in three Chinese reservoir oil samplessXinli, MuH, and Xinmin reservoir oilsshave been studied under CO2-injection conditions. The effects of the injected CO2 concentration on the amount of wax precipitation from the three oils have been studied at the reservoir temperatures and pressures. Considerable wax precipitations have been detected as the injected CO2 concentration exceeds 50 mol %, and the amount of wax precipitation increases with the injected CO2 concentration. The hydrocarbons extracted into the vapor phase during the multiple-contact extraction process using CO2 have been observed to be mainly limited to C1-C5. The onset temperatures of wax precipitations from the CO2-saturated Xinli reservoir oil and the Xinli reservoir oil have been detected using a solid detection system (SDS) logging system at the reservoir pressure. The results show that the onset temperature of the Xinli reservoir oil is decreased as the saturation of CO2 at the reservoir pressure increases. These results, together with those of multiple-contact extraction experiments, suggest that the wax precipitation under CO2 injection may be caused by the extraction of light hydrocarbons from the reservoir oils.
Introduction Miscible/partially miscible flooding using carbon dioxide (CO2) injection has been shown to be a promising enhanced oil recovery technique for many reservoirs.1-5 Recently, it has been applied to several Chinese oil fields, including the Xinli, MuH, Xinmin, Shandong, Daqing, and Jiangsu oil fields. The Xinli, MuH, and Xinmin reservoir oils are high-wax-content oils; therefore, it would be expected that the injection of CO2 could result in wax precipitation, because of the change of the solubility of wax components in reservoir oils. Therefore, this project was designed to study the wax precipitation behavior in the three reservoir oil samples. In our previous work, we have investigated asphaltene precipitation under different conditions in many Chinese oils such as Caoqiao crude, Gudao, and Jiangsu oils.6-10 In this work, the effect of injected CO2 concentration on the amount of wax precipitation from Xinli, * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † University of Petroleum. ‡ Research Institute of Petroleum Exploration and Development. (1) Monger, T. G. SPE 1984, 12708, 71. (2) Thomas, F. B.; Bennion, D. B.; Bennion, D. W.; Hunter, B. E. J. Can. Pet. Technol. 1992, 31, 22. (3) Monger, T. G.; Trujillo, D. E. SPE 1998, 18063, 63. (4) Srivastava, R. K.; Huang, S. S.; Dyer, S. B.; Mourits, F. M. J. Can. Pet. Technol. 1995, 34, 31. (5) Huang, S. S.; Dyer, S. J. Can. Pet. Technol. 1993, 32, 42. (6) Hu, Y. F.; Li, S.; Liu, N.; Chu, Y. P.; Park, S. J.; Mansoori, G. A.; Guo, T. M. J. Pet. Sci. Eng. 2004, 41, 169. (7) Hu, Y. F.; Chen, G. J.; Yang, J. T.; Guo, T. M. Fluid Phase Equilib. 2000, 171, 181. (8) Hu, Y. F.; Guo, T. M. Fluid Phase Equilib. 2001, 192, 13. (9) Hu, Y. F.; Yang, L. Y.; Lin, X. S.; Guo, T. M. Pet. Explor. Dev. 2000, 27, 109 (in Chin.). (10) Yang, Z.; Ma, C. F.; Lin, X. S.; Yang, J. T.; Guo, T. M. Fluid Phase Equilib. 1999, 157, 143.
MuH, and Xinmin reservoir oils were investigated under the reservoir temperatures and pressures. The compositions of hydrocarbons extracted into the vapor phase during the multiple-contact extraction using CO2 were determined, and the onset temperatures of wax precipitation from the CO2-saturated Xinli reservoir oil and the Xinli reservoir oil were also detected, so that we can examine whether the wax precipitation is caused by the extraction of light hydrocarbons from the reservoir oils. Experimental Section Materials. The compositions and basic properties of the recombined Xinli, MuH, and Xinmin reservoir oils studied in this work are given in Table 1. The purity of CO2 used is 99.95 mol %. Apparatus for Measuring the Onset Temperature and Amount of Wax Precipitation. The schematic diagram of the experimental system is shown in Figure 1. The major component of the system was a mercury-free, volume-variable, visual JEFRI equilibrium cell that had been retrofitted with optical fiber light transmission probes. The working temperature range of the cell was 243-473 K, and the maximum working pressure was 69 MPa. A modern laser solid detection system (SDS) was adopted to measure the onset of wax precipitation. The incident laser was mounted in front of the equilibrium cell, ensuring that the laser beam could be transmitted through the sample chamber before reaching the light detection probe. A magnetic stirrer was used to agitate the sample to accelerate the equilibrium process. The storage cells for injection gas and oil samples were 1000-mL piston cells. The temperature in the air bath (maximum of 473 K) could be controlled within (0.3 K. The high-temperature, highpressure wax filter was filled with 0.5-µm stainless-steel fibers. The sampling system of the high-pressure experimental unit is shown in Figure 2.
10.1021/ef0340662 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
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Table 1. Compositions and Properties of the Recombined Crude Oils value composition (mol %)
MuH
Xinli
Xinmin
N2 CO2 CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 C11+ sum reservoir temperature (K) reservoir pressure (MPa) viscosity of reservoir oil at 339 K and 15 MPa (MPa s) specific gravity of C11+ at 293 K (g/cm3) molecular weight of C11+ (g/mol) wax content (wt%)
1.30 0.07 22.12 0.26 0.06 0.05 1.04 0.03 0.09 0.10 1.00 1.98 0.68 2.1 69.12 100.0 318 5.07 5.6 0.9129 426.9 26.13
0.42 0.24 6.00 0.25 0.22 0.02 0.02 1.01 0.06 1.02 0.2 1.42 2.25 1.05 85.82 100.0 339 11.6 5.9 0.9268 460.2 28.80
1.64 0.05 2.26 0.70 1.76 0.09 0.25 0.08 1.08 0.06 1.2 1.00 2.15 3.62 84.06 100.0 336 6.0 5.8 0.9206 437.6 26.44
Figure 1. Schematic of the high-pressure, high-temperature wax precipitation system. Legend is as follows: (a) air bath, (b) floating piston oil sample cylinder, (c) floating piston injection gas storage cylinder, (d) windowed equilibrium cell, (e) magnetic stirrer, (f) pressure transducer, (g) solid detection system (SDS), (h) laser beam receiver, (i) circulation liquid, (j) piston, (k) reservoir fluids, and (l) laser emission. Panel (m) is a side view.
Figure 2. Schematic diagram of the sampling system of the high-pressure experimental unit. Experiments with CO2 Injection. The effect of injected CO2 concentration on the amount of wax precipitation from Xinli, MuH, and Xinmin reservoir oils was measured indi-
rectly, through the change of wax content in the oil phase. This experimental procedure has been used to determine the amount of asphaltene precipitation from petroleum fluids under gas injection,10,11 and now is briefly summarized as follows: (1) Charge a given amount of oil sample into the equilibrium cell and maintain the system at the reservoir temperature and pressure. (2) Introduce a preset amount of CO2 into the cell. (3) Agitate the (CO2 + oil) mixture in the cell for 2 h and then allow to settle in a vertical position for 72 h, to ensure full wax precipitation. (4) Sample the oil and vapor phases under constant pressure (by adjusting the floating piston). Open the top valve of the sampling cell slightly, to allow the flashed gas to flow into the bubbling flask. The displaced oil was collected in the oil trap. (5) Determine the wax content in the flashed oil, using the acetone method. (6) Perform the composition analysis of the vapor phase, using a Hewlett-Packard model 6890 gas chromatograph. (7) Determine if the wax components are extracted into the vapor phase during the equilibrium periods. The results show that no wax components exist in vapor phase, and, therefore, the amount of wax precipitated was determined by the difference between the wax contents in the feed oil and flashed oil. The experimental data obtained for the MuH, Xinli, and Xinmin reservoir oils are listed in Table 2. Effect of Multiple-Contact Extraction. The experimental procedure of multiple-contact extraction is described as follows:7 (1) Introduce a given amount of CO2 into the reservoir oil contained in the equilibrium cell at reservoir temperature and a selected pressure to obtain a CO2-saturated oil mixture. The selected pressure was 22 MPa. (2) Charge a given amount of CO2 (20 mol % of the CO2saturated oil mixture) into the equilibrium cell. Agitate the mixture for 1 h. (3) Sample the equilibrium vapor phase at constant pressure and perform the composition analysis using a HewlettPackard model 6890 gas chromatograph. (4) Remove the vapor phase at constant pressure and recharge the same amount of CO2 into the equilibrium cell to achieve multiple-contact extractions. (5) Repeat steps (2)-(4) for another extraction. (11) Novosad, Z.; Costain, T. G. SPE 1990, 20530, 599.
Wax Precipitation in Chinese Reservoir Oils
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Table 2. Amount of Wax Deposited in the CO2-Injected Reservoir Oils at the Reservoir Temperatures and Pressures temp, T (°C)
pressure, P (MPa)
amount of CO2 injected (mol %)
wax content in feed oil (wt %)
amount of wax deposited (wt %)
relative precipitationa,b (%)
45.0 45.0 45.0 45.0
5.0 5.0 5.0 5.0
0.0 50.0 65.0 75.0
MuH 26.13 25.28 24.33 18.88
0.0 0.85 1.8 7.25
0.0 3.3 6.9 27.7
66.0 66.0 66.0 66.0
11.6 11.6 11.6 11.6
0.0 50.0 65.0 75.0
Xinli 28.80 27.60 23.91 19.88
0 1.20 4.89 8.92
0 4.2 17.0 30.94
65.0 65.0 65.0 65.0
6.0 6.0 6.0 6.0
0.0 50.0 65.0 75.0
Xinmin 26.44 24.87 24.08 22.40
0 1.57 2.36 4.04
0 2.6 6.3 14.8
a Refers to feed oil. b The relative deposition is given as follows: relative deposition(%) ) 100 × (wax deposited (wt%))/(wax content in the feed oil (wt%)).
Table 3. Compositions of the Vapor Phase for Each Extraction from the Xinli Reservoir Oil composition of vapor phase (mol %)
a
Na
C1
CO2
C2
C3
C4
C5
C6
∆Voil b (%)
1 2 3 4
20.0113 12.9182 7.6521 3.6960
77.4062 84.8630 90.5602 94.6585
1.5152 1.2856 0.9888 0.9185
0.8176 0.6467 0.5255 0.4906
0.2048 0.1969 0.1850 0.1608
0.0396 0.0830 0.0760 0.0620
0.0052 0.0065 0.0128 0.0142
4.98 8.15 10.69 12.82
N is the number of the extractions. b ∆Voil is the total reduction in the volume of the oil phase after each extraction.
(6) Repeat steps (1)-(5) for another operating pressure. The results show that the hydrocarbons extracted into the vapor phase during the multiple-contact extraction are mainly limited to C1-C5. For illustration, the results for the Xinli reservoir oil are shown in Table 3. However, the results from Tables 2 and 3 indicate that the rapid increase in the amount of wax precipitation is not proportional to the gradual increase in the amount of light hydrocarbons removed. Experiments for the Onset Temperature of Wax Precipitation. This experiment was performed to determine if the wax precipitation from a reservoir oil is induced by the extraction of light hydrocarbons from the reservoir fluids. The Experimental Procedure. The experimental method and procedure is briefly described as follows: (1) The equilibrium cell is thoroughly cleaned, evacuated, and maintained at the reservoir temperature and pressure. (2) To remove any possible solid particles present in the oil, a given amount of reservoir oil is filtered first and then charged into the cell under single-phase conditions. (3) The system temperature is reduced at a constant rate of 5 °C/h under isobaric conditions. At each temperature step, the oil in the cell is agitated for 30 min and then the light transmittance through the oil is measured using an SDS logging system. (4) Repeat steps (1) and (2) and then introduce a given amount of CO2 into the cell to obtain CO2-saturated reservoir oil at the reservoir pressure and temperature. (5) Maintain the CO2-saturated reservoir oil under singlephase conditions, agitate the mixture for 24 h under isothermal and isobaric conditions, and measure the light transmittance data. (6) Repeat step (3). When the system temperature is reduced, the oil becomes denser, and, thus, a weaker light signal will be received. When wax precipitation occurs, the incoming light will be scattered and a significantly decreased intensity of the light signal will be received.12 (12) Bryant, D. W.; Monger, T. G. SPE 1985, 14150, 22.
Figure 3. Plots of light transmittance versus temperature for the Xinli reservoir oil and the CO2-saturated Xinli reservoir oil systems.
Experimental Results The measured data for the Xinli reservoir oil and the CO2-saturated Xinli reservoir oil are represented in Figure 3. The experimental results show that the onset temperature of wax precipitation in the Xinli reservoir oil is 53.4 °C, whereas that in CO2-saturated Xinli reservoir oil is 43.2 °C, indicating that the dissolution of CO2 in a single phase results in a decrease in the onset temperature of wax precipitation of the reservoir oil. Conclusions Based on the aforementioned experimental results, the following conclusions can be drawn: (1) Wax precipitation can be induced in the Xinli, MuH, and Xinmin reservoir oils as the injected CO2 concentration exceeds 50 mol %, and the amount of wax
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precipitation in these oils increases rapidly as the injected CO2 concentration increases. (2) The hydrocarbons extracted into the vapor phase during the multiple-contact extraction are mainly limited to C1-C5. (3) The saturation of CO2 in the Xinli reservoir oil in a single phase decreases the onset temperature of wax precipitation, in comparison with that of the Xinli reservoir oil. (4) Conclusions (1)-(3) suggest that wax precipitation is caused by the extract of light hydrocarbons of the reservoir oil, presumably because of the decreased solvent ability of the oil phase for waxes as some light hydrocarbons were extracted. It is important to note that the injection of CO2 in the Xinli, MuH, and Xinmin reservoir oils indeed has induced in situ wax precipitations and seriously hindered the recovery process. Therefore, the Natural Gas
Hu et al.
and Oil Company of Jilin has consulted the authors, in regard to reasons for the wax precipitation and possible ways to solve the problems. On the basis of the present results, it was suggested to limit the injected CO2 to
