Wettability Determination of the Crude Oil−Reservoir Brine−Reservoir

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Wettability Determination of the Crude Oil-Reservoir Brine-Reservoir Rock System with Dissolution of CO2 at High Pressures and Elevated Temperatures Daoyong Yang,* Yongan Gu, and Paitoon Tontiwachwuthikul Petroleum Technology Research Centre (PTRC), Petroleum Systems Engineering, Faculty of Engineering, UniVersity of Regina, Regina, Saskatchewan S4S 0A2 Canada ReceiVed January 6, 2008. ReVised Manuscript ReceiVed April 6, 2008

An experimental method has been developed to determine the wettability, i.e., the contact angle, of the crude oil-reservoir brine-reservoir rock system with dissolution of CO2 at high pressures and elevated temperatures, using the axisymmetric drop shape analysis (ADSA) technique for the sessile drop case. In the experiment, a see-through windowed high-pressure cell is prefilled with reservoir brine to submerge the reservoir rock. Subsequently, CO2 is slowly injected through the brine phase to pressurize the system to a prespecified pressure at a constant temperature. After the CO2-reservoir brine system reaches the equilibrium state, a crude oil sample is introduced by using a specially designed syringe delivery system to form a sessile oil drop on the reservoir rock inside the pressure cell. The sequential images of the dynamic sessile oil drop are acquired and analyzed by applying computer-aided image acquisition and processing techniques to measure the dynamic contact angles at different times. It is found that the dynamic contact angle between the crude oil and the reservoir rock in the presence of CO2-saturated reservoir brine remains almost constant at a given pressure and a constant temperature, though CO2 is gradually dissolved into the sessile oil drop until the latter is completely saturated with the former. It is also found that the equilibrium contact angle increases as the pressure increases, whereas it decreases as the temperature increases. In comparison with the equilibrium contact angle data for the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2, the equilibrium contact angles of the crude oil-reservoir brine-reservoir rock system with dissolution of CO2 are smaller at T ) 27 °C but larger at T ) 58 °C. Such wettability alteration will significantly affect oil recovery and subsequent storage when CO2 is injected into an oil reservoir at high pressures.

1. Introduction CO2 injection is considered to be one of the most promising enhanced oil recovery (EOR) techniques for light and medium oils because it not only effectively enhances oil recovery due to dissolution of CO2 into the crude oil but also considerably reduces greenhouse gas emissions by sequestrating CO2 in a depleted oil reservoir. Recently, CO2 EOR has gained momentum in the oil and gas industry due to its potential for mitigating greenhouse gas emissions.1 It is estimated that about 80% of oil reservoirs worldwide might be suitable for CO2 injection based on the oil recovery criteria alone.2,3 Successful CO2 EOR and CO2 storage processes are strongly affected by the interfacial interactions among CO2, crude oil, reservoir brine, and reservoir rock.4–8 The major interfacial interactions include interfacial tension, wettability, capillarity, and interface mass transfer, * Corresponding author. Tel.: 1-306-337-2660. Fax: 1-306-585-4855. E-mail: [email protected]. (1) Moritis, G. CO2 Injection Gains Momentum. Oil Gas J. 2006, 104, 37–41. (2) Taber, J. J.; Martin, F. D.; Seright, R. S. EOR Screening Criteria Revisited-Part 1: Introduction to Screening Criteria and Enhanced Recovery Field Projects. SPE ReserVoir Eng. 1997, 12, 189–198. (3) Taber, J. J.; Martin, F. D.; Seright, R. S. EOR Screening Criteria Revisited-Part 2: Applications and Impact of Oil Prices. SPE ReserVoir Eng. 1997, 12, 199–205. (4) Anderson, W. G. Wettability Literature Survey-Part 1: Rock/Oil/ Brine Interactions and the Effects of Core Handling on Wettability. J. Pet. Technol. 1986, 37, 1125–1144. (5) Buckley, J. S.; Liu, Y. Some Mechanisms of Crude Oil/Brine/Solid Interactions. J. Pet. Sci. Eng. 1998, 20, 155–160.

among which the wettability of the CO2-crude oil-reservoir brine-reservoir rock system has not yet been quantified at high pressures and elevated temperatures. Wettability is defined as the tendency of one liquid to spread on or to adhere to a solid surface in the presence of an immiscible fluid.4 The wettability of the reservoir rock is related to its affinity for the reservoir brine and/or the crude oil. Accordingly, wettability is a major factor controlling the flow behavior and spatial distribution of fluids in a reservoir. In a hydrocarbon reservoir, there exists a fieldwide variation of wettability.4,9 It is also found that the wettability depends on the compositions of fluids and that miscible CO2 flooding alters in situ wettability in a reservoir and thus affects oil recovery.10 Change in the wettability has been shown to affect electrical (6) Drummond, C.; Israelachvili, J. Fundamental Studies of Crude OilSurface Water Interactions and Its Relationship to Reservoir Wettability. J. Pet. Sci. Eng. 2004, 45, 61–81. (7) Emberley, S.; Hutcheon, I.; Shevalier, M.; Durocher, K.; Gunter, W. D.; Perkins, E. H. Geochemical Monitoring of Fluid-Rock Interaction and CO2 Storage at the Weyburn CO2-Injection Enhanced Oil Recovery Site, Saskatchewan, Canada. Energy 2004, 29, 1393–1401. (8) Arendt, B.; Dittmar, D.; Eggers, R. Interaction of Interfacial Convection and Mass Transfer Effects in the System CO2-Water. Int. J. Heat Mass Transfer 2004, 47, 3649–3657. (9) Aspenes, E.; Graue, A.; Ramsdal, J. In Situ Wettability Distribution and Wetting Stability in Outcrop Chalk Aged in Crude Oil. J. Pet. Sci. Eng. 2003, 39, 337–350. (10) Yeh, S. W.; Ehrlich, R.; Emanuel, A. S. Miscible-Gasflood-Induced Wettability Alteration: Experimental Observations and Oil Recovery Implications. SPE Formation EVal. 1992, 7, 167–172.

10.1021/ef800012w CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

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properties of porous media,11 capillary pressure,12,13 relative permeability,13,14 and flooding behavior.15,16 On the other hand, field practice indicates that wettability alteration is a major factor causing early breakthrough, water blocking, and injectivity loss in CO2 flooding processes.17,18 A number of technical papers have been published to address the role of rock wettability in oil recovery, most of which suggest that compositional changes in either the oil phase or the aqueous phase can alter wettability of rock surfaces.15,16,19–21 The salinity of the connate water has been found to be a primary factor controlling the oil recovery because increased salinity can cause wettability alternation from the water-wet condition to the mixed-wet condition.22 In gas miscible processes, it is found from experiments that the ultimate oil recovery is the highest for the oil-wet rock-fluids system and lowest for the water-wet rock-fluids system, respectively.21,23 In a CO2 flooding process, the reservoir rock becomes less oil-wet after CO2 injection.10,24 It has also been found that the wettability alteration occurs for a CO2-water-coal system,25 a CO2-water-glass system26 and a CO2-synthetic brine-mica/ quartz system,27 and a CO2-reservoir brine-reservoir rock system,28 respectively. Numerous techniques, either quantitative or qualitative, have been developed to evaluate the wettability of a fluid-liquid system on a solid surface. The quantitative methods available for determining the wettability of the reservoir rock in the (11) Anderson, W. G. Wettability Literature Survey-Part 3: The Effects of Wettability on the Electrical Properties of Porous Media. J. Pet. Technol. 1986, 38, 1371–1378. (12) Anderson, W. G. Wettability Literature Survey-Part 4: The Effects of Wettability on Capillary Pressure. J. Pet. Technol. 1987, 39, 1283–1300. (13) Masalmeh, S. K. The Effect of Wettability Heterogeneity on Capillary Pressure and Relative Permeability. J. Pet. Sci. Eng. 2003, 39, 399–408. (14) Anderson, W. G. Wettability Literature Survey-Part 5: The Effects of Wettability on Relative Permeability. J. Pet. Technol. 1987, 39, 1453– 1468. (15) Anderson, W. G. Wettability Literature Survey-Part 6: The Effects of Wettability on Waterflooding. J. Pet. Technol. 1987, 39, 1605–1622. (16) Morrow, N. R. Wettability and Its Effect on Oil Recovery. J. Pet. Technol. 1990, 42, 1476–1484. (17) Christensen, J. R.; Stenby, E. H.; Skauge, A. Review of WAG Field Experience. SPE ReserVoir EVal. Eng. 2001, 4, 97–106. (18) Rogers, J. D.; Grigg, R. B. A Literature Analysis of the WAG Injectivity Abnormalities in the CO2 Process. SPE ReserVoir EVal. Eng. 2001, 4, 375–386. (19) Melea´n, Y.; Bureau, N.; Broseta, D. Interfacial Effects in GasCondensate Recovery and Gas-Injection Processes. SPE ReserVoir EVal. Eng. 2003, 6, 244–254. (20) Robin, M. Interfacial Phenomena: Reservoir Wettability in Oil Recovery. Oil Gas Sci. Tech.sReV. IFP 2001, 56, 55–62. (21) Rao, D. N.; Girard, M. G.; Sayegh, S. G. The Influence of Reservoir Wettability on Waterflood and Miscible Flood Performance. J. Can. Pet. Technol. 1992, 31, 47–55. (22) Sharma, M. M.; Filoco, P. R. Effect of Brine Salinity and CrudeOil Properties on Oil Recovery and Residual Saturations. SPE J. 2000, 5, 293–300. (23) Wylie, P. L.; Mohanty, K. K. Effect of Wettability on Oil Recovery by Near-Miscible Gas Injection. SPE ReserVoir EVal. Eng. 1999, 2, 558– 564. (24) Huang, E. T. S.; Holm, L. W. Effect of WAG Injection and Rock Wettability on Oil Recovery during CO2 Flooding. SPE ReserVoir Eng. 1988, 3, 119–128. (25) Siemons, N.; Bruining, H.; Castelijns, H.; Wolf, K. H. Pressure Dependence of the Contact Angle in a CO2-H2O-Coal System. J. Colloid Interface Sci. 2006, 297, 755–761. (26) Dickson, J. L.; Gupta, G.; Horozov, T. S.; Binks, B. P.; Johnston, K. P. Wetting Phenomena at the CO2/Water/Glass Interface. Langmuir 2006, 22, 2161–2170. (27) Chiquet, P.; Broseta, D.; Thibeau, S. Wettability Alteration of Caprock Minerals by Carbon Dioxide. Geofluids 2007, 7, 112–122. (28) Yang, D.; Gu, Y.; Tontiwachwuthikul, P. Wettability Determination of the Reservoir Brine-Reservoir Rock Systems with Dissolution of CO2 at High Pressures and Elevated Temperatures. Energy Fuels 2008, 22, 504– 509.

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petroleum industry are the Amott method,29 the US Bureau of Mines (USBM) method,30 and the contact angle method.31 The Amott and USBM methods determine the average wettability of a reservoir core, whereas the contact angle method measures the wettability of a specific solid surface. In the literature, the reservoir rock is categorized to be water-wet, intermediate-wet, and oil-wet when the advancing contact angle of water on a solid surface is in the range of 0-75°, 75-105°, and 105-180°, respectively.31,32 It has been found that the sessile drop method or its modified technique is probably the most suitable for measuring the contact angle at high pressures and elevated temperatures.16,25–27,31–34 Recently, an advanced shape analysis technique, known as the axisymmetric drop shape analysis (ADSA) technique, has been developed to accurately measure the contact angle of a fluid-liquid-solid system.35,36 In comparison with the other existing methods, the ADSA technique for the sessile drop case is accurate for the contact angle measurement ((0.1°), fully automatic, and completely free of the operator’s subjectivity. Most importantly, the ADSA technique can be used to automatically and accurately measure the contact angle versus time, i.e., the dynamic contact angle. In this paper, an experimental technique is developed to determine the wettability of the CO2-crude oil-reservoir brine-reservoir rock system at high pressures and elevated temperatures. On the basis of the ADSA technique for the sessile drop case, this new technique makes it possible to measure the contact angles and visualize the interfacial interactions among CO2, crude oil, reservoir brine, and reservoir rock under practical reservoir conditions. More specifically, the dynamic and equilibrium contact angles of the CO2-crude oil-reservoir brine-reservoir rock system are measured at pressures of 0.1-33.6 MPa and two temperatures of T ) 27 and 58 °C. The measured contact angle data under reservoir conditions should facilitate design of the CO2 EOR and CO2 storage processes. 2. Experimental Details 2.1. Materials. A reservoir brine sample and a crude oil sample are collected from the Weyburn Oilfield in Saskatchewan, Canada, respectively. The density of the reservoir brine is 1.045 g/cm3 at 15 °C. Table 1 lists the detailed physical and chemical properties of the Weyburn reservoir brine. The density and viscosity of the Weyburn crude oil are 0.877 g/cm3 and 13.0 mPa · s at the atmospheric pressure and 27 °C, respectively. The compositional analysis result for the Weyburn crude oil is given in Table 2. The reservoir brine sample and the crude oil sample are filtered through filter papers (Grade 40 Ashless, Whatman, UK) to remove fine solids prior to the experiment. The purities of carbon dioxide (29) Amott, E. Observations Relating to the Wettability of Porous Rock. Trans. AIME 1959, 216, 156–162. (30) Donaldson, E. C.; Thomas, R. D.; Lorenz, P. B. Wettability Determination and Its Effect on Recovery Efficiency. SPE J. 1969, 9, 13– 20. (31) Anderson, W. G. Wettability Literature Survey-Part 2: Wettability Measurement. J. Pet. Technol. 1986, 37, 1246–1262. (32) Morrow, N. R. Interfacial Phenomena in Petroleum RecoVery; Marcel Dekker, Inc.: New York, 1991. (33) McCaffery, F. G. Measurement of Interfacial Tensions and Contact Angles at High Temperature and Pressure. J. Can. Pet. Technol. 1972, 11, 26–32. (34) Rao, D. N.; Girard, M. G. A New Technique for Reservoir Wettability Characterization. J. Can. Pet. Technol. 1996, 35, 31–39. (35) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. Determination of Surface Tension and Contact Angle from the Shapes of Axisymmetric Fluid Interfaces. J. Colloid Interface Sci. 1983, 93, 169–183. (36) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W. Automation of Axisymmetric Drop Shape Analysis for Measurement of Interfacial Tensions and Contact Angles. Colloids Surf. 1990, 43, 151– 167.

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Table 1. Physical and Chemical Properties of the Weyburn Reservoir Brine calcium, mg/L magnesium, mg/L

1600.00 348.00

sodium, mg/L potassium, mg/L iron, mg/L barium, mg/L manganese, mg/L chloride, mg/L sulfate, mg/L pH @ 23 °C conductivity @ 25 °C, S/m density, g/cm3 @ 15 °C @ 20 °C @ 60 °C refractive index @ 25 °C total dissolved solids, mg/L @ 110 °C @ 180 °C

20 350.00 532.00 0.22 2.60 150 s, the increased volume of the sessile oil drop and the increased contact angle are primarily due to the dissolution of CO2 in the crude oil. Figure 4b shows that the volume of the sessile crude oil drop and its contact radius slightly increase from t ) 0 to 11 700 s mainly due to the dissolution of CO2 in the crude oil (i.e., the swelling effect) and that the contact angle of the sessile oil drop remains almost constant. Figures 5a and 5b show the sequential images of the sessile oil drops on the rock slide surrounded by the reservoir brine with dissolution of CO2 at (a) P ) 4.19 MPa and T ) 58 °C

and (b) P ) 33.49 MPa and T ) 58 °C, respectively. It is illustrated in Figure 5a that the volume, contact radius, and contact angle of the sessile oil drop on the rock slide surrounded by CO2-saturated brine remain essentially unchanged from t ) 0 to 14 500 s. In this case, the rock slide surface is already saturated with the reservoir brine, crude oil, and CO2 over 40 h inside the pressure cell. In addition, the light-ends are extracted from the sessile oil drop to the reservoir brine due to the dissolution of CO2.39–41 Therefore, the counteracting effects of (39) Yang, D.; Gu, Y. Interfacial Interactions between Crude Oil and CO2 under Reservoir Conditions. Pet. Sci. Technol. 2005, 23, 1099–1112. (40) Yang, D. Interfacial Interactions of the Crude Oil-Reservoir BrineReservoir Rock Systems with Dissolution of CO2 under Reservoir Conditions. Ph.D. Dissertation, University of Regina: Regina, SK, Canada, 2005.

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Figure 7. Measured dynamic contact angles of the crude oil-reservoir brine-reservoir rock system without dissolution of CO2 as a function of time under different pressures at T ) 27 °C.

Figure 8. Measured dynamic contact angles of the crude oil-reservoir brine-reservoir rock system with dissolution of CO2 as a function of time under different pressures at T ) 58 °C.

the dissolution of CO2 and the light-ends extraction lead to essentially unchanged volume, contact radius and the contact angle of the sessile drop. Nevertheless, Figure 5b shows that, at P ) 33.49 MPa, T ) 58 °C, and t e 150 s, the volume of the sessile oil drop slightly increases due to the dissolution of CO2, whereas, at t > 150 s, it slightly decreases because of the light-ends extraction from the crude oil to the CO2-saturated brine phase. For comparison purposes, the equilibrium contact angles of the crude oil surrounded by the reservoir brine without any (41) Yang, D.; Tontiwachwuthikul, P.; Gu, Y. Interfacial Tensions of the Crude Oil + Reservoir Brine + CO2 Systems at Pressures up to 31 MPa and Temperatures of 27 and 58 °C. J. Chem. Eng. Data 2005, 50, 1242–1249.

dissolution of CO2 are measured to be in the range of 104.8-135.3° at P ) 0.20-30.30 MPa at T ) 27 °C and 66.6-104.3° at P ) 0.42-30.69 MPa at T ) 58 °C, respectively. At the same temperature, the contact angle increases as pressure increases. At the same pressure, the temperature increase has an effect of contact angle reduction.16,31–33,43 As shown in Figures 4, panels a and b, and 5, panels a and b, once CO2 is introduced into the crude oil-reservoir brine-reservoir rock system, the crude oil has a tendency to wet the rock slide (42) Buckley, J. S. Effective Wettability of Minerals Exposed to Crude Oil. Curr. Opin. Colloid Interface Sci. 2001, 6, 191–196. (43) McCaffery, F. G.; Mungan, N. Contact Angle and Interfacial Tension Studies of Some Hydrocarbon-Water-Solid Systems. J. Can. Pet. Technol. 1970, 9, 185–196.

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Figure 9. Measured dynamic contact angles of the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2 as a function of time under different pressures at T ) 58 °C.

Figure 10. Measured equilibrium contact angles of the crude oil-reservoir brine-reservoir rock system with and without dissolution of CO2 versus pressure at two temperatures.

at T ) 27 °C, whereas the crude oil becomes less wetting to the rock slide surface at T ) 58 °C. It should be noted that the visibility of the contact line cannot be discerned clearly in the experiments mainly due to the fact that the reservoir rock is porous and that its polished surface is still not perfectly smooth. 3.2. Dynamic Contact Angle. The measured dynamic contact angles of the crude oil-reservoir brine-reservoir rock system with dissolution of CO2 as a function of time under different pressures at T ) 27 °C are shown in Figure 6. It should be pointed out that the contact angle measurements generally lasts 6 h and some even last about 24 h. At P e 2.13 MPa, the dynamic contact angle fluctuates with time and reaches its equilibrium value after t ) 450-500 s. This is mainly attributed

to the penetration of the crude oil into the rock slide, lightends extraction, and possible dissolution of microwater drops trapped in the crude oil to the reservoir brine. On the other hand, this contact angle fluctuation also results from the strong electrostatic interactions between the crude oil and the reservoir brine.5,42 It should be noted that, in general, the dynamic contact angle reaches a constant value, i.e., the equilibrium contact angle, after 20 s at P g 5.76 MPa. This is ascribed to the fact that only about 5 mm3 of fresh oil is added to the existing CO2saturated oil drop at a higher pressure. It can be seen from Figure 6 that the contact angle of the CO2-crude oil-reservoir brine-reservoir rock system generally increases with pressure.

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This means that the rock slide surface becomes more oilphobic or hydrophilic as pressure increases. For comparison purposes, the dynamic contact angle of the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2 under different pressures at T ) 27 °C are shown in Figure 7. The contact angles of the sessile oil drop on the rock slide increase as pressure increases. Also, there exists slight fluctuation for the contact angle of the crude oil-reservoir brine-reservoir rock system at P ) 0.20 MPa and T ) 27 °C. This contact angle fluctuation may be ascribed to the strong electrostatic interactions between the crude oil and the reservoir brine.5,42 The measured dynamic contact angles of the crude oil-reservoir brine-reservoir rock system with dissolution of CO2 as a function of time under different pressures at T ) 58 °C are shown in Figure 8. It can be seen from this figure that the dynamic contact angle reaches the equilibrium contact angle after 10-400 s under different pressures. In addition, the contact angle of the crude oil on the rock slide surrounded by the reservoir brine with dissolution of CO2 increases as pressure increases. This means that the rock slide surface alters from oil-wet to intermediate-wet and finally to water-wet. It is worthwhile emphasizing that the dynamic contact angles fluctuate at P e 4.19 MPa mainly because of the penetration of the crude oil into the rock slide, light-ends extraction, and possible dissolution of microwater drops trapped in the crude oil to the reservoir brine. On the other hand, this contact angle fluctuation is also attributed to the strong electrostatic interactions between the crude oil and the reservoir brine.5,42 At P ) 12.22 MPa, the dynamic contact angle increases at the beginning and reaches its equilibrium value after 150 s due to the phase change of CO2 and formation of hydrates.28,44 At P g 20.59 MPa and T ) 58 °C, the dynamic contact angle reaches its equilibrium values quickly. This is because the sessile oil drop is saturated with CO2 quickly at high pressures. The dynamic contact angles of the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2 at T ) 58 °C are shown in Figure 9. This figure shows that the contact angle of the sessile oil drop on the rock slide generally increases with pressure. As shown in Figure 8, when CO2 dissolves into the crude oil-reservoir brine-reservoir rock system, the contact angle of the crude oil surrounded by the reservoir brine is larger than that in the absence of CO2 under the same pressure. This is mainly due to dissolution of CO2 in both the crude oil and the brine phase.39,40,44–48 This also means that the reservoir wettability alters from intermediate-wet to water-wet once CO2 is injected into an intermediate-wet reservoir. 3.3. Equilibrium Contact Angle. A constant or equilibrium contact angle is always obtained at the end of each dynamic contact angle measurement. The measured equilibrium contact angles of the crude oil-reservoir brine-reservoir rock system (44) Tewes, F.; Boury, F. Thermodynamic and Dynamic Interfacial Properties of Binary Carbon Dioxide-Water Systems. J. Phys. Chem. B 2004, 108, 2405–2412. (45) Enick, R. M.; Klara, S. M. Effect of CO2 Solubility in Brine on the Compositional Simulation of CO2 Flood. SPE ReserVoir Eng. 1992, 7, 253–258. (46) Chun, B. S.; Wilkinson, G. T. Interfacial Tension in High-Pressure Carbon Dioxide Mixtures. Ind. Eng. Chem. Res. 1995, 34, 4371–4377. (47) Yang, D.; Tontiwachwuthikul, P.; Gu, Y. Interfacial Interactions between Reservoir Brine and CO2 at High Pressures and Elevated Temperatures. Energy Fuels 2005, 19, 216–223. (48) Hebach, A.; Oberhof, A.; Dahmen, N.; Ko¨gel, A.; Ederer, H.; Dinjus, E. Interfacial Tension at Elevated Pressure: Measurements and Correlations in the Water + Carbon Dioxide System. J. Chem. Eng. Data 2002, 47, 1540–1546.

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with and without dissolution of CO2 under different pressures at two temperatures are plotted in Figure 10. It is shown from this figure that the equilibrium contact angle (solid lines) of the CO2-crude oil-reservoir brine-reservoir rock system increases as the pressure increases, whereas it decreases as the temperature increases. This is because CO2 solubility is higher in both the crude oil and reservoir brine at a higher pressure but lower at a higher temperature.39,40,44–48 In the literature, it is found that an increase in temperature results in a decrease in the contact angle of the hydrocarbon/oil-water system.16,31–33,43,49 In comparison with the equilibrium contact angle data (dashed lines) for the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2 as shown in Figure 10, the equilibrium contact angles of the CO2-crude oil-reservoir brine-reservoir rock system are smaller at T ) 27 °C but larger at T ) 58 °C under the same pressure. At T ) 27 °C, the reduction of the contact angle of the CO2-crude oil-reservoir brine-reservoir rock system is mainly due to dissolution of CO2 in both the reservoir brine and crude oil. This trend was also reported for the CO2-pure hydrocarbon-water system inside a capillary.50 According to the Young equation:51 γob cos θ ) γsb - γso, where γob, γsb, and γso are the interfacial tensions of the crude oil-CO2-saturated brine interface, the reservoir rock-CO2saturated brine interface, and the reservoir rock-crude oil interface, respectively, and θ is the contact angle of the sessile oil drop on the reservoir rock surrounded by CO2-saturated reservoir brine; γob cos θ is defined as the adhesion force.31 It has been found that, at a constant temperature, the equilibrium interfacial tension γob of the crude oil-CO2-saturated reservoir brine system decreases as the pressure increases because of a higher CO2 solubility in the both crude oil and the reservoir brine at a higher pressure.39,40,44–48 Meanwhile, the contact angle θ increases with pressure at a constant temperature as shown in Figure 10. Consequently, the adhesion force decreases with pressure at a constant temperature and thus the capillary number, defined as Nca ) µν/γob cos θ (where µ is the viscosity of the crude oil and ν is the velocity of CO2-staurated brine), increases with pressure. This means that it is easier to displace the crude oil by injecting CO2 into an intermediate-wet hydrocarbon reservoir at a higher pressure. In this way, more oil can be recovered and subsequent CO2 storage capacity will be significantly increased. 4. Conclusions The dynamic and equilibrium contact angles of the CO2-crude oil-reservoir brine-reservoir rock systems under reservoir conditions are measured by using the ADSA technique for the sessile drop case. It is found that the measured dynamic contact angles quickly reach their constant values at different pressures and two constant temperatures. It is also found that, for the CO2-crude oil-reservoir brine-reservoir rock system, the equilibrium contact angle increases as the pressure increases, whereas it decreases as the temperature increases. This is attributed to higher CO2 solubility at a higher pressure but lower CO2 solubility at a higher temperature. In comparison with the equilibrium contact angle data for the crude oil-reservoir brine-reservoir rock system without any dissolution of CO2, the equilibrium contact (49) Shedid, S. A.; Ghannam, M. T. Factors Affecting Contact-Angle Measurement of Reservoir Rocks. J. Pet. Sci. Eng. 2004, 44, 193–203. (50) Aguilera, M. E.; Lo´pez de Ramos, A. L. Effect of CO2 Diffusion on Wettability for Hydrocarbon-Water-CO2 Systems in Capillaries. Int. Commun. Heat Mass Transfer 2004, 31, 1115–1122. (51) Adamson, A. W. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons, Inc.: New York, 1996.

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angles of the CO2-crude oil-reservoir brine-reservoir rock system are smaller at T ) 27 °C but larger at T ) 58 °C. Therefore, the wettability alteration occurs when CO2 is injected into an oil reservoir and thus will significantly affect the oil recovery and subsequently CO2 storage to a large extent.

Council (NSERC) of Canada to D.Y. and Y.G. The authors also thank the Saskatchewan Subsurface Geological Laboratory for providing the Weyburn core samples and the Saskatchewan Research Council (SRC) for providing the compositional analysis of the Weyburn crude oil and reservoir brine, respectively.

Acknowledgment. The authors acknowledge the Discovery Grants from the Natural Sciences and Engineering Research

EF800012W