Capillary pressure - saturation relations for supercritical CO2 and brine: Implications for capillary/residual trapping in carbonate reservoirs during geologic carbon sequestration

Abstract:

In geologic carbon sequestration (GCS), data on capillary pressure (P_c) - saturation (S_w) relations are routinely needed to appraise reservoir processes. Capillarity and its hysteresis have been often experimentally studied in oil-water, gas-water and three phase gas-oil-water systems, but fewer works have been reported on scCO₂-water under in-situ reservoir conditions. Here, P_c-S_w relations of supercritical (sc) CO₂ displacing brine, and brine rewetting the porous medium to trap scCO₂ were studied to understand CO₂ transport and trapping behavior in carbonate reservoirs under representative reservoir conditions. High-quality drainage and imbibition (and associated capillary pressure hysteresis) curves were measured under elevated temperature and pressure (45 ºC, 8.5 and 12 MPa) for scCO₂-brine as well as at room temperature and pressure (23 ºC, 0.1 MPa) for air-brine in unconsolidated limestone and dolomite sand columns using newly developed semi-automated multistep outflow-inflow porous plate apparatus. Drainage and imbibition curves for scCO₂-brine deviated from the universal scaling curves for hydrophilic interactions (with greater deviation under higher pressure) and shifted to lower P_c than predicted based on interfacial tension (IFT) changes. Augmented scaling incorporating differences in IFT and contact angle improved the scaling results but the scaled curves still did not converge onto the universal curves. Equilibrium residual trapping of the nonwetting phase was determined at P_c =0 during imbibition. The capillary-trapped amounts of scCO₂ were significantly larger than for air. It is concluded that the deviations from the universal capillary scaling curves are caused by scCO₂-induced wettability alteration, given the fact that pore geometry remained constant and IFT is well constrained. In-situ wettability alteration by reactive scCO₂ is of critical importance and must be accounted for to achieve reliable predictions of CO₂ behavior in GCS reservoirs.