Capillary pressure and saturation relations for supercritical CO sub(2) and brine in sand: High-pressure P sub(c)(S sub(w)) controller/meter measurements and capillary scaling predictions

In geologic carbon sequestration, reliable predictions of CO sub(2) storage require understanding the capillary behavior of supercritical (sc) CO sub(2). Given the limited availability of measurements of the capillary pressure (P sub(c)) dependence on water saturation (S sub(w)) with scCO sub(2) as the displacing fluid, simulations of CO sub(2) sequestration commonly rely on modifying more familiar air/H sub(2)O and oil/H sub(2)O P sub(c)(S sub(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 sub(c)(S sub(w)) controller/meter, allowing accurate P sub(c) and S sub(w) measurements. Drainage and imbibition processes were measured on quartz sand with scCO sub(2)-brine at pressures of 8.5 and 12.0MPa (45 degree C), and air-brine at 21 degree C and 0.1MPa. Drainage and rewetting at intermediate S sub(w) levels shifted to P sub(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 sub(c)(S sub(w)) relations but still did not converge onto universal drainage and imbibition curves. Equilibrium capillary trapping of the nonwetting phases was determined for P sub(c)=0 during rewetting. The capillary-trapped volumes for scCO sub(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 sub(2)-brine interfacial tensions are fairly well constrained, we conclude that the observed deviations from scaling predictions resulted from scCO sub(2)-induced decreased wettability. Wettability alteration by scCO sub(2) makes predicting hydraulic behavior more challenging than for less reactive fluids. Key Points * equilibrium between CO sub(2) and brine in reservoirs differs from nonreactive fluids * wettability alteration from CO sub(2) exposure is important * residual trapping of CO sub(2) can be higher than expected