Wettability of reservoir minerals and rocks is a critical factor controlling CO 2 mobility, residual trapping and safe-storage in geologic carbon sequestration, and currently is the factor imparting the greatest uncertainty in predicting capillary behavior in porous media. Very little information on wettability in supercritical CO 2 (scCO 2)-mineral-brine systems is available. We studied porescale wettability and wettability alteration in scCO 2-silica-brine systems using engineered micromodels (transparent pore networks), at 8.5 MPa and 45 °C, over a wide range of NaCl concentrations up to 5.0 M. Dewetting of silica surfaces upon reactions with scCO 2 was observed through water film thinning, water droplet formation, and contact angle increases within single pores. The brine contact angles increased from initial values near 0˚ up to 80˚ with larger increases under higher ionic strength conditions. Given the abundance of silica surfaces in reservoirs and caprocks, these results indicate that CO 2 induced dewetting may have important consequences on CO 2 sequestration including reducing capillary entry pressure, and altering quantities of CO 2 residual trapping, relative permeability, and caprock integrity.

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[1] In geologic carbon sequestration, reliable predictions of CO 2 storage require understanding the capillary behavior of supercritical (sc) CO 2 . Given the limited availability of measurements of the capillary pressure (P c ) dependence on water saturation (S w ) with scCO 2 as the displacing fluid, simulations of CO 2 sequestration commonly rely on modifying more familiar air/H 2 O and oil/H 2 O P c (S w ) relations, adjusted to account for differences in interfacial tensions. In order to test such capillary scaling-based predictions, we developed a high-pressure P c (S w ) controller/meter, allowing accurate P c and S w measurements. Drainage and imbibition processes were measured on quartz sand with scCO 2 -brine at pressures of 8.5 and 12.0 MPa (45 C), and air-brine at 21 C and 0.1 MPa. Drainage and rewetting at intermediate S w levels shifted to P c values that were from 30% to 90% lower than predicted based on interfacial tension changes. Augmenting interfacial tension-based predictions with differences in independently measured contact angles from different sources led to more similar scaled P c (S w ) relations but still did not converge onto universal drainage and imbibition curves. Equilibrium capillary trapping of the nonwetting phases was determined for P c ¼ 0 during rewetting. The capillary-trapped volumes for scCO 2 were significantly greater than for air. Given that the experiments were all conducted on a system with well-defined pore geometry (homogeneous sand), and that scCO 2 -brine interfacial tensions are fairly well constrained, we conclude that the observed deviations from scaling predictions resulted from scCO 2 -induced decreased wettability. Wettability alteration by scCO 2 makes predicting hydraulic behavior more challenging than for less reactive fluids.Citation: Tokunaga, T. K., J. Wan, J.-W. Jung, T. W. Kim, Y. Kim, and W. Dong (2013), Capillary pressure and saturation relations for supercritical CO 2 and brine in sand: High-pressure P c (S w ) controller/meter measurements and capillary scaling predictions, Water Resour. Res., 49,[4566][4567][4568][4569][4570][4571][4572][4573][4574][4575][4576][4577][4578][4579]

a b s t r a c tThe current ambiguity on wettability of minerals in CO 2 -brine systems under the geological CO 2 sequestration (GCS) reservoir conditions imparts the greatest uncertainty in predicting capillary behavior controlling safe-storage of CO 2 . To address this issue we conducted a series of experiments using muscovite as a representative of common aluminosilicate minerals. Based on new experimental results we identified several possible causes of the ambiguity problem in contact angle (CA) measurements. We also found that reaction with water-saturated supercritical (sc) CO 2 (but not with scN 2 ) phase severely roughened the muscovite surfaces, largely increased CA hysteresis and CO 2 adhesion. Although some methodological influences on contact angle uncertainty can be reduced, the high surface-energy of clean and pristine aluminosilicate minerals have strong tendency to adsorb oppositely changed molecules and particles to reduce their surface energy, resulting in less reproducible CA values. Giving the fact that such clean and pristine mineral surfaces do not exist in real reservoirs, our future investigations shall focus on improving understanding of the effect of long-term CO 2 -mineral-brine reactions on reservoir wettability under realistic reservoir geochemical conditions.

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1 mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100 mM) and Accusand (1 mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100 mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100 mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.