Capillary pressure and saturation relations for supercritical CO₂ and brine in sand: High‐pressure Pc(Sw) controller/meter measurements and capillary scaling predictions

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 = 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.