Capillary Pressure Saturation Relations for Supercritical CO2 and Brine in Limestone/Dolomite Sands: Implications for Geologic Carbon Sequestration in Carbonate Reservoirs

In geologic carbon sequestration, capillary pressure (P-c)-saturation (S-w) relations are needed to predict reservoir processes. Capillarity and its hysteresis have been extensively studied in oil-water and gas-water systems, but few measurements have been reported for supercritical (sc) CO2-water. Here, P-c-S-w relations of scCO(2) displacing brine (drainage), and brine rewetting (imbibition) were studied to understand CO2 transport and trapping behavior under reservoir conditions. Hysteretic drainage and imbibition PcSw curves were measured in limestone sands at 45 degrees C under elevated pressures (8.5 and 12.0 MPa) for scCO(2)-brine, and in limestone and dolomite sands at 23 degrees C (0.1 MPa) for airbrine using a new computer programmed porous plate apparatus. scCO(2)-brine drainage and imbibition curves shifted to lower P-c relative to predictions based on interfacial tension, and therefore deviated from capillary scaling predictions for hydrophilic interactions. Fitting universal scaled drainage and imbibition curves show that wettability alteration resulted from scCO(2) exposure over the course of months-long experiments. Residual trapping of the nonwetting phases was determined at P-c = 0 during imbibition. Amounts of trapped scCO(2) were significantly larger than for those for air, and increased with pressure (depth), initial scCO(2) saturation, and time. These results have important implications for scCO(2) distribution, trapping, and leakage potential.