Article pubs.acs.org/EF
Supercritical CO2 and Ionic Strength Effects on Wettability of Silica Surfaces: Equilibrium Contact Angle Measurements Jong-Won Jung† and Jiamin Wan* Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ABSTRACT: Wettability of reservoir mineral surfaces is a critical factor controlling CO2 mobility, trapping, and safe-storage in geological carbon sequestration. Although recent studies have begun to show that wettability of some minerals can change in the presence of supercritical CO2 (scCO2), different laboratories have reported significantly different wetting behavior. We studied wettability alteration of silica in CO2−brine systems through measuring equilibrium water contact angles under wide ranges of pressures (0.1 to 25 MPa) and ionic strengths (0 to 5.0 M NaCl), at 45 °C. Using two independent approaches for each of the experiments, we found the following: (1) Equilibrium water contact angles on silica increased up to 17.6° ± 2.0° as a result of reactions with scCO2. This increase occurred primarily within the pressure range 7−10 MPa, and the contact angles remain nearly constant at pressure greater than 10 MPa. (2) The contact angle increased with ionic strength nearly linearly, with a net increase of 19.6° ± 2.1° at 5.0 M NaCl. These changes in contact angle induced by changes in scCO2 pressure and aqueous solution ionic strength are approximately additive over the range of tested conditions. These findings can be used to estimate the wetting behavior of silica surfaces in reservoirs containing supercritical CO2.
1. INTRODUCTION Geological CO2 sequestration (GCS) is a promising technology for controlling global atmospheric CO2 increases. Proposed CO2 storage sites include the deep saline aquifers, depleted oil and gas reservoirs, unmineable coal seams, and the deep ocean sediments.1−3 The wettability of reservoir rocks and minerals is a key factor controlling the mobility, residual trapping, and caprock breakthrough pressures (leakage) of the injected CO2. Because of differences in mineral surface wettability by CO2 and water (brine), a capillary pressure difference Pc, between these two phases must be exceeded in order for CO2 to enter pores. As described by the Young−Laplace equation (eq 1), capillary entry of CO2 into originally water-filled pores is determined by the pore radius R, the CO2−water interfacial tension γwc, and the contact angle θ. Pc = PCO2 − Pw =
Several research groups have measured water contact angles on solids in the presence of gaseous CO2 (gCO2), liquid CO2 (lCO2), and scCO2. For example, in the gCO2−brine and lCO2−brine systems, it has been reported that contact angles on silica surface remain nearly constant with CO2 pressure increases up to ∼10 MPa (θ = 21.4° in pure water) and increase with ionic strength under the same CO2 pressure conditions (θ = 21.4 to 38.1° at 0 to 2.58 M NaCl).15 In the scCO2−brine system, however, recently reported results showed no trends for the effects of variations in either CO2 pressure or ionic strength.16,17 Bikkina (2011) reported wettability changes following repeated measurements with scCO2.18−20 Silica−CO2 interactions have been studied experimentally18,21−24 and through molecular simulations.25 Some of the previous experimental results suggested that contact angles increase when CO2 reacts with silanol groups on the modified silica surfaces.26 Through molecular simulations of interactions between scCO2 and silica nanoparticles, Vishnyakov et al. (2008) suggested that scCO2 reacts with hydroxyl groups of the silica surface via hydrogen bonds.27 Tripp and Combes showed that CO2 removes water films from the silica surface more effectively than organic solvent,23 and reacted scCO2 with silica hydroxyls could not be removed from the silica surface by simple evacuation.24 Kim et al. visualized the dewetting process (water film thinning, water droplet forming, and contact angle increase) upon silica−scCO2−brine interactions.13 These results suggest that scCO2 may alter the wettability of silica surfaces and that scCO2 may not behave as a simple nonwetting fluid as previously assumed. Because of large discrepancies among published contact angle results, the role of surface reactions with scCO2 and the
2γwc cos θ (1)
R
Values of R are fixed, and they are characteristically large for reservoir rocks and much finer for caprocks. The interfacial tension γwc only varies within a small range of about 20−35 mN/m under the GCS relevant conditions.4−11 In contrast, much less is known about mineral surface wettability, hence θ imparts the largest uncentainty to the predictions of CO2 capillary entry and its equilibrium distribution. Therefore, our ability to predict CO2 flow paths, residual trapping, and caprock integrity all critically depend on understanding wettability under geological CO2 sequestration conditions. CO2 is commonly assumed to be the nonwetting phase in predictive models for CO2 storage capacity. However, recent studies have started to show that the wettability of some minerals and rocks can be altered in the presence of supercritical CO2 (scCO2) under pressures, temperatures, and ionic strengths characteristics of geological CO2 storage conditions.9,12−14 © 2012 American Chemical Society
Received: May 25, 2012 Revised: July 18, 2012 Published: July 27, 2012 6053
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using ethanol (Mallickrodt Baker, ACS reagent grade) and handled with clean gloves. Each silica plate was used only once in order to avoid uncertainty introduced from reaction history with brines and CO2. The chamber was carefully cleaned with deionized water before each use. The brine and CO2 phases were always presaturated with each other. Only the equilibrium water contact angles are reported. To determine whether results were method-dependent, contact angles were measured using two different procedures, as described next. Stepwise Pressurization on Single CO2 Droplet Method. A clean silica plate was clipped onto the bottom of the welded stainless steel plate in the chamber. The clean chamber was flushed with gaseous CO2 to remove air and then filled with CO2-saturated aqueous phase with desired ionic strength (only NaCl was used to adjust ionic strength) at 7.0 MPa CO2 pressure and 45 °C from the Parr reactor. After stabilizing the pressure for ∼2 h, a CO2 droplet was generated by injecting water-saturated CO2 (7.0 MPa, 45 °C) into the chamber through the upward-directed needle using a precision high-pressure ISCO pump. The formed CO2 droplet detached from the needle, to attach onto the silica plate. The measured average size of the CO2 droplets was 1.9 ± 0.8 mm in diameter at 7.0 MPa. The evolution of the droplet over time was recorded using high-resolution time-lapse photography. The chamber was kept at 7.0 MPa for at least 3 h, and then, the pressure was increased in 0.5 to 2 MPa steps by compressing the CO2-saturated water using the ISCO pump. The minimum time interval between each stepwise pressure increase was determined to be suitable based on previous trial tests. We observed no further measurable size and shape changes for a CO2 droplet after 3 h. Up to 15 pressure steps (from 7.0 to 25 MPa) were taken under each ionic strength conditions. The silica plate was replaced with a new one after performing each complete pressure increase series. Images of the droplets captured at the end of 3 h equilibrations were used for contact angle measurements and analyzed with ImageJ.28 The roughness of a used silica plate was measured for comparison with fresh silica surfaces. One Pressure, One CO2 Droplet Per Silica Plate Method. The previously described stepwise pressurization method has the advantage of conveniently maintaining the same experimental parameters while only increasing the pressure. However, the CO2 droplet size decreases with increased pressure, and the duration of the substrate exposure to CO2 increases. To determine if the pressurization sequence and cumulative exposure time influenced the contact angle values, measurements were obtained using an alternate procedure. In the same experimental setup, freshly prepared CO2-saturated brine was used to fill the chamber at a specific test pressure. A new (brinesaturated) CO2 droplet was formed at end of the needle and allowed to equilibrate on a new silica plate. In this method, only one contact angle measurement is obtained at the single test pressure, and the silica surface is used for only the single contact angle determination. The procedure was repeated on each new silica surface, with a CO2 droplet generated at a selected pressure (0.1 to ∼25 MPa) and equilibrated for 20−24 h. Water Droplet in CO2 versus CO2 Droplet in Water. Contact angles obtained with water droplets in CO2 were measured to determine whether any systematic differences exist when compared with CO2 droplets in water, under otherwise identical experimental conditions. To generate water droplets, the pressure chamber was inverted and filled with water-saturated CO2 at the desired pressure. Then, CO2-saturated water was introduced into the chamber (flow rate = 0.01 mL/min) through a needle to form a water droplet (2.1 ± 0.4 mm diameter) on the silica surface (detached from the needle). The image monitoring and analyses have the experimental uncertainty of ±0.3°. Contact angles were also measured with water droplets under atmosphere conditions and at different ionic strengths (0 to 5 M NaCl) using the same experimental procedure. Nitrogen versus CO2. Nitrogen is a relatively inert gas compared to CO2. Therefore comparisons of wettability between these two nonaqueous phases could help identify causes of contact angle alteration. We measured water contact angles in silica−N2−brine systems under at 0.1, 7, and 12 MPa, by generating N2 bubbles in brine. The experimental procedures were the same as the previously described single droplet method for the CO2 measurements.
extent of their impact on silica wettability are still inconclusive, despite important implications for GCS. The objective of this study is to systematically and carefully measure contact angles of silica surfaces upon reactions with scCO2 and brine under a range of conditions relevant to GCS.
2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. The sessile drop technique was used to measure contact angles on silica surfaces. Figure 1 shows the
Figure 1. Schematic diagram of the setup for contact angle measurements under high pressure and temperature using the Sessile Drop technique. apparatus used. The high-pressure chamber has two transparent windows allowing illumination and imaging. A stainless steel plate was welded to the center of the chamber to hold a horizontally oriented silica surface. A CO2 droplet was introduced into the water-filled chamber from the bottom, and released onto the silica plate (hangingdrop method). Evolution of the CO2 droplet was monitored using high-resolution time-lapse photography (12.3 Megapixel, Nikon D90). To compare the results obtained using CO2 droplets with those obtained using water droplets, water droplets were introduced from the top into the CO2 filled chamber. No systematic differences were found between the two approaches, and the hanging-drop method was used throughout this study because it is operationally easier for measurements of contact angles when the substrate is primarily water wet. The chamber was instrumented with a pressure transducer (OMEGA PX309), a thermocouple (T-type, copper-constantan), and a movable needle used to inject CO2 to form droplets. The chamber had two separate ports connected to a high-pressure pump (Teledyne ISCO, 500HP) to control the chamber pressure and to deliver both CO2 and water. Heating tapes surrounded the chamber were used to maintain the chamber temperature with PID (proportional−integral− derivative) controllers (Cole Parmer, EW-89000−10). The entire system was enclosed in a temperature-controlled glovebox maintained at the constant temperature of 45 ± 1 °C during experiments. Ultrapure deionized (DI) water with a resistivity ≥18 MΩ (Barnstead, NanoPure) and three different salinity brines (1.0, 3.0, and 5.0 M NaCl) were used. CO2 (Airgas, 99.99%) and brines were presaturated with each other using a high-pressure stirred reactor (Parr instrument, Model 4560) at 0.1 up to 25 MPa, and 45 °C. Smooth fused pure silica plates (VWR VistaVision−Cover Glasses, amorphous SiO2) were used as the substrate. The surface roughness of the silica plates was measured using a three-dimensional (3D) optical profilometer (New View 6K Microscope by ZYGO) before and after tests. The average roughness of the silica plates was ∼25 nm, over 710 μm × 530 μm areas, before the contact angle measurements. 2.2. Methods and Procedures. The variables tested in this study of scCO2−brine−silica contact angles are pressure (0.1 to 25 MPa) and ionic strength (0, 1, 3, and 5 M NaCl). Temperature was a constant (45 °C). To prepare an experiment, a silica plate was cleaned 6054
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3. RESULTS The measured contact angle values obtained using the stepwise pressurization method (Step-P method) are presented in Figure 2a. The data show three clear trends: (1) the contact angles
Figure 3. One pressure-one CO2 droplet-one plate method measured contact angles in silica−water−CO2 and silica−water−N2 systems, and using both CO2 droplet in water and water droplet in CO2 methods. Results from the stepwise pressure increase method are also shown for comparison.
measured contact angles resulting from an additional different approach, by filling the chamber with water-equilibrated CO2 and generating water droplets in the bulk CO2 phase for contact with the silica surface. The results of six measurements under pressures 10, 15, and 20 MPa and two salt concentrations, 3 and 5 M NaCl, are also presented in Figure 3 (the colored solid symbols with black crosses). Again the two different approaches of CO2 droplet-in-water and water droplet-in-CO2 resulted in nearly the same contact angle values. These aforementioned experiments indicate that the equilibrium contact angles presented in Figures 2 and 3 are not method-dependent and are reproducible. Nitrogen gas (N2) was used as the nonreactive nonaqueous phase, to help identify the cause of contact angle changes with pressure. The results of three measurements conducted in the system of silica−N2−brine of 1.0 M NaCl under pressures 0.1, 7.0, and 12 MPa (One-P method) are presented in Figure 3. The measured contact angles maintained a nearly constant value of about 33° as pressure was increased up to 12 MPa. These contact angles are lower than those obtained with CO2 as the nonaqueous phase.
Figure 2. The stepwise pressurization method measured contact angles in silica−water−CO2 systems. (a) Water contact angles as the function of pressure and ionic strength. (b) Raw photographs of a CO2 droplet in brine at steady states showing changes of droplet shape and size as pressure was increased stepwise. (c) Kinetics of contact angle and drop-size changes of a CO2 droplet as pressure increase from 7 to 8 MPa in 1.0 M NaCl brine at 45 °C.
increase steeply as the pressure increases from 7 MPa to about 10 MPa; (2) the contact angles remain relatively constant for pressures above 10 MPa; and (3) the contact angles increase with the ionic strength increase. Figure 2b shows the raw images of a CO2 droplet in brine at its steady states after different stepwise increased pressure. Shown in Figure 2c is the kinetics of contact angle and drop-size changes of a CO2 droplet following pressure increase from 7 to 8 MPa in 1.0 M NaCl brine at 45 °C. The increased water contact angle with increased pressure is visually evident, and the decreased droplet size is due to the increased CO2 density and increased solubility in water at higher pressure. Because such large extent of contact angle changes and the relationship with pressure and salinity (Figure 2a) have not been seen previously in the literature, also because the Step-P method involved another two variables as pressure increase (drop size and contact time), we decided to use an independent method to check these results. The contact angle values from the one pressure-one CO2 droplet-one plate method (One-P method) under equilibrium conditions are shown in Figure 3 as black solid symbols, on top of the data from the Step-P method (the data points are in hollow symbols connected with dashed lines). The two independent methods yielded nearly the same contact angle values under all the tested conditions. All the contact angle values presented so far were measured through generating CO2 droplets in pre-equilibrated silica−brine systems. We also
4. DISCUSSION This paper is designed to report the measured equilibrium contact angles under varied pressure and salinity. However, the observed kinetics of contact angle and drop-size changes upon pressure increase are worth describing as well. (1) We observed that rapid contact angle changes occurred at the very beginning when a CO2 droplet was just positioned on the substrate, within seconds to less than a minute. The contact angle remained about constant afterward. (2) We also observed that the drop-size decreased relatively slowly with a decreased rate and reached the stable value in around two hours after raising the pressure. As shown in Figure 2c, the contact angle changed from 39° to 51° after the pressure was raised from 7.0 to 8.0 MPa in less than a minute, and no significant changes occurred after, but the drop-size changed progressively over time. These 6055
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plateau δθ = 17.6° ± 2.0°. Thus, we conclude from our study that the maximum impact of CO2 pressure increase alone on contact angles on silica ranges from 15° to 20°. This provides a practical and relatively quantitative assessment of silica wettability alteration due to reactions with scCO2 under GCS relevant conditions. The variability in the maximum δθ (±2°) is partly attributable to measurement uncertainties, especially the θ values measured at 0.1 MPa at different ionic strengths, which were subtracted to calculate the δθ values. Implications of our findings on the pressure-dependence of θ on interfacial tensions at the silica surface can be examined through considering the Young equation. For this purpose, we rearrange the Young equation force balance at the three phase contact line to give
observations suggest that the decreased drop-size is not the main cause of the increased contact angle. Physicochemical reactions of CO2 with the substrate in brine are likely to be important; including protonation of silanol groups, CO2 capping, and brine-film thinning. As Kim et al. observed, these CO2-induced processes occurred rapidly within seconds.13 4.1. Effect of CO2. To understand the effect of CO2, we compared CO2 with N2 as the nonaqueous fluid. Using N2, the measured contact angles remained fairly constant as pressure increased (tested up to 12 MPa in Figure 3), with an average θ value 32.6° ± 0.4°. However, when CO2 was used as the nonaqueous phase (other conditions remained the same), the θ values are 37.5° ± 1.8°, 39.0° ± 1.3°, and 56.1° ± 0.8° at the same pressures, 0.1, 7.0, and 12.0 MPa, respectively. This indicates that the increased contact angle as pressure increase is due to CO2. Contact angle increase became significant when gCO2 became scCO2; as shown in Figure 4, the steep rise of δθ (θP − θatm) occurred at pressure slightly over 7.0 MPa.
γwc cos θ = γsc − λsw
(2)
where γwc is the interfacial tension (IFT) between water and CO2, γsc is the IFT between the solid and the nonaqueous fluid (CO2 here), γsw is the IFT between solid and water. Among the three IFTs, only γwc is readily determined experimentally. As pressure increases, γwc decreases.4,6,8,10 The γwc decrease occurs steeply as pressure increases in the pressure range P < Pc, decreases gradually for P > Pc, and become nearly constant at very high P.4−6 Note that cos θ must generally decrease with increased pressure because we have found that θ generally increases with pressure. Thus, from eq 2, γsc − γsw must decrease with increased pressure. In the context of Young’s equation, this decrease in γsc − γsw with increased pressure is responsible for the increased θ and decreased wettability. However, it should be noted that interpretation of interfacial force balances through Young’s equation remains controversial because γsc and γsw are not directly measurable.29 The presence of an adsorbed water film between the silica surface and CO2 droplet further complicates the simple application of eq 2. Recent experimental studies have demonstrated the presence of adsorbed water at scCO2− mineral interfaces.30,31 The stability of adsorbed water films between silica surfaces and CO2 was recently explored within the framework of the Derjaguin−Landau−Verwey−Overbeek theory, where pH-dependent surface electrostatic potential and van der Waals interactions appear important.32 The specific density of the function groups (silanol and silicilic acid groups) on the silica surface influences the water film thickness or the hydrophilicity,33 as described in eq 3
Figure 4. Net effect of pressure on contact angles in silica−CO2−brine systems. The brine contained 0−5 M NaCl at 45 °C. Here, θatm denotes the measured contact angles at atmospheric pressure under different ionic strength conditions; θP stands for each of the measured contact angles at pressures > atmosphere P (0.1 MPa) and under all the measured ionic strength conditions.
−SiOH ⇔ −SiO− + H+
(3)
The reported pKa values of this reaction are in the range between 5.7 and 7.7.34 Therefore, the silica surface is commonly moderately negatively charged (hydrophilic) under near-neutral pH. In the silica−CO2−brine system, dissolution of CO2 acidifies the brine and can decrease pH to as low as 3.0. Such pH lowering shifts the equilibrium shown in eq 3 to the left, and diminishes the silica surface charge.35 It has also been reported that CO2 can directly react with hydroxyl groups on the silica surface via hydrogen bonding.27 Dickson et al. suggested that the reaction between CO2 and silanol group is physical adsorption, capping the silanol group, which causes the contact angle to increase.26 These reactions between scCO2 and the surface function groups transform silica surfaces from negatively charged to neutral charge, thereby reducing the wettability of silica. Gribanova reported that as pH decrease from 6 to 3, contact angle increased from 19° to 23° in the air−brine−silica system
Although ionic strength clearly influences contact angles (Figure 3), the general impact of CO2 pressure on wettability can be examined through subtracting contact angles measured at atmospheric pressure (θatm) from values measured at higher pressures (θP) within each ionic strength set. The CO2 pressure-dependent net contact angle increases relative to values at atmospheric pressure for each ionic strength, δθ = (θP − θatm), are shown in Figure 4. These δθ values emphasize the CO2 pressure effects, although ionic strength still influences the magnitude of the δθ increase. In all of these measurements, δθ had only a slight increase, 0 to 2°, when pressure was increased from 0.1 to 7.0 MPa, a range within which CO2 remained in the gaseous state. Above 7.0 MPa pressure, δθ increased sharply as the critical pressure (7.38 MPa) was traversed and continued to rise up to pressures of about 10 to 12 MPa were a δθ plateau was reached. Up to the 25 MPa limit of our experiments, the 6056
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at ambient pressure and temperature.36,37 Kim et al., visualized the processes of water film thinning and water contact angle increase upon reactions with scCO2 at the pore scale in silica micromodels.13 Increased pressure increases the CO2 solubility and the proton concentrations in the water films, leading to film thinning and increases in the contact angle. 4.2. Effect of Ionic Strength. It has been reported for the air−water−silica system that contact angles increase with ionic strength. At atmosphere pressure and 20 °C, contact angles on glass plates increased from 40° to 55° when the ionic strength was increased from 0 to 4.5 M NaCl.38 Despite the critical importance of the ionic strength impact on wetting properties under CO2 sequestration relevant conditions, currently available literature results from different laboratories are scattered and even conflicting. Espinoza and Santamarina (2010) measured water contact angles on quartz surfaces with gCO2 under two different ionic strengths. Their reported contact angles are 20° and 38° for 0 and 2.58 M NaCl, respectively. Tonnet et al. (2008) reported contact angle values on mica surface for two different pressures and two different NaCl concentrations at 35 °C. At 0.5 MPa, the values are 35° and 27° for 0.08 and 0.8 M, respectively; and at 14 MPa, 34° and 35° for 0.08 and 0.8 M, respectively. It should be kept in mind that ionic strength directly affects the scCO2−brine IFT.4−6 Therefore, in the context of the Young equation, ionic strength must affect θ. It is also worth noting that the presence of divalent cations results in higher interfacial tensions relative to monovalent cations,5 although we only used NaCl in this study. We studied the ionic strength effect on contact angles of silica−CO2−brine system over wider ranges of ionic strengths and pressures than previously reported. In Figure 5, we
effect does not have a significant dependence on pressure is supported by the trends shown in Figure 5 and the relatively small standard deviations obtained at any given ionic strength (shown on the figure). The trend is described fairly well by the linear relationship shown in Figure 5, between δθ (the net increase in θ from 0 M values) and NaCl concentration (M), with a slope of 4.0° per M (r2 = 0.94). 4.3. Contact Angle Hysteresis. Increased surface roughness can cause the contact angle to increase or decrease.7,29 We measured the surface roughness of the silica plates before and after an experiment using a 3D optical profiler. The measured roughness values before and after a test were in the range from 25 to 33 nm (measured over 710 μm by 530 μm areas), well within the experiment uncertainty of ±10 nm. Thus, although surface roughness may influence measurements, there were no measurable roughness changes on the silica surfaces after reaction with scCO2 and brine. CO2 drop size is another factor that may affect the contact angle values, especially in our Step-P approach, where CO2 drop size decreased up to 50% from the starting point. We did not have the laboratory capability to generate scCO2 droplets with well-controlled sizes, so were not able to conduct a systematic study on drop size effect. However, we were successful in one attempt; we generated two CO2 bubbles simultaneously attached onto one silica plate and monitored their changes in one sight of view. The measured equilibrium contact angles were 36° and 37°, for the drop sizes 1.0 and 2.1 mm, respectively, that is, within the measurement uncertainty. Li and Neumann reported that as the radius of the three-phase contact line increased from approximately 1 to 5 mm, the contact angles were decreased by 3−5°.39 If we simply use Li and Neumann’s results predicatively, the 50% of drop size reduction would cause about 2−3° of contact angle increase. Based on our negligible measured change (Figure 2c), we believe that the effect of drop size on our measured contact angle values is not significant. Contact angle hysteresis has important practical implications in CO2 residual trapping and enhanced oil recovery.14,40−42 In our Step-P approach, an important phenomenon related to contact angle hysteresis of silica surfaces was observed. When the CO2 drop was positioned on the substrate (Figure 2b, c) at an initial pressure of 7.0 MPa (CO2 is a dense gas), the droplet displaced water at its largest drop-size and largest droplet-base (contact area). Upon pressure increase, the CO2 droplet shrank as the result of CO2 dissolution and phase change from gaseous CO2 to scCO2. Depending on the surface energy of the substrate, the shrinking can occur in two different ways, as described by Semenov et al. and Kulinich et al.43,44 for an evaporating droplet. One is shrinking with a constant radius of the droplet-base but decreased CO2 contact angle (increased water contact angle), and another is shrinking with a constant contact angle and shrinking droplet-base. For the silica surfaces tested in our experiments, shown the latter (Figure 2b, c), the radius of the droplet-base decreased proportionally as the droplet shrank, and the contact angle remained constant, indicating that the silica surfaces have negligible hysteresis.
Figure 5. Net effect of ionic strength on contact angles in silica− CO2−brine systems, with pressure varied from 0.1 to 25 MPa at 45 °C. Here, θ0M denotes the measured contact angles under the conditions where the aqueous phase contained no salt (0 M); θi stands for the measured contact angles at different ionic strength conditions, including data from all the pressures measured.
5. CONCLUSIONS We studied wettability alterations of silica surfaces under conditions relevant for GCS, through carefully measuring equilibrium water contact angles using the pendent drop method at 45 °C, under pressures from 0.1 to 25 MPa, and ionic strength from 0 to 5.0 M (NaCl). Two independent
summarized the net impact of ionic strength on contact angles based on the raw data presented in Figures 2 and 3. The values of net ionic effect on contact angle were simply calculated by subtracting θ0M (measured under zero salt concentration at a giving pressure) from θi (measured at different ionic strength at the same pressure). The approximation that the ionic strength 6057
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approaches were applied for the same measurements under otherwise identical conditions, and the results were fairly consistent. CO2 was also compared with N2 as the nonaqueous phase. We found that dewetting (contact angle increase) of silica surfaces occurred upon reaction with scCO2, with a maximum net θ increase of 17.6° ± 2.0°, under any constant ionic strength. Within the pressure range of gas phase CO2 (up to about 7 MPa) the CO2 effect on contact angles is insignificant. When the pressure reaches and excesses the supercritical pressure (7.38 MPa) θ increases steeply as the pressure increases up to about 10 MPa. The contact angle remains fairly constant at P ≥ 10 MPa. Ionic strength has profound impact on the wetting behavior. From pure water to 5 M NaCl, the contact angle on silica increased 19.6 ± 2.1°. A nearly linear relationship between the measured contact angles and ionic strengths (up to the tested maximum of 5 M NaCl) was obtained, with a slope of 4.0° per M (r2 = 0.94). Pressure has only a minor influence on the ionic strength effect. Thus, the new information reported in this paper can be useful for obtaining improved predictions of CO2 distributions in reservoirs.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 510-486-6004. Fax: 510-486-7152. E-mail: jwan@lbl. gov. Present Address †
Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported as part of the Center for Nanoscale Control of Geologic CO2, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-AC02-05CH11231. We thank Dr. Tim Kneafsey for letting us borrow the high-pressure chamber for the contact angle measurements. Helpful comments from the anonymous reviewers are gratefully acknowledged.
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