Dynamic displacement and non-equilibrium dissolution of supercritical CO2 in low-permeability sandstone: An experimental study

Abstract An experimental setup was developed for conducting core-flood experiments of supercritical CO2 and water under pressures higher than 8.00 MPa and a temperature of 40 °C. Two representative low-permeability sandstone cores were obtained from the Shenhua Group CCS site in the Erdos Basin in China and the experimental study was in support of China's first field test. Unlike most laboratory experiments with two-phase CO2–water flow, dry CO2 was injected in the CO2-flood experiments, and deionized water (without dissolved CO2) was used in the water-flood experiments. In the CO2-flood experiments, dynamic displacement of water by injected CO2 was investigated using transient inlet and outlet pressures and transient flow rates of outflowing CO2 and water. The residual water saturation estimated at the end of the experiments (with an injection rate of 1.2 mL min−1) for both cores is 0.52. The higher residual water saturation can be attributed to the high CO2/water viscosity contrast and non-uniform displacement. The estimated relative CO2 permeability at residual water saturation varies from 0.13 to 0.23. During the water-flood experiments, non-equilibrium CO2 dissolution at the core scale was observed using the transient concentration of total dissolved CO2 in outflowing water. The non-equilibrium dissolution possibly results from non-uniform distribution of water and CO2 caused by sub-core heterogeneity. The endpoint CO2 saturation estimated varies from 0.17 to 0.10 for a low injection rate of 0.2 mL min−1. Additional experiments indicate that higher water injection rate (up to 2 mL min−1) drive more free-phase CO2 out of the cores, with less CO2 mass stored, because of CO2 density change with elevated pressure and weaker capillarity for low-permeability sandstone. Additional experiments with injected water of varying dissolved CO2 concentration (including CO2-saturated conditions) indicate that CO2 dissolution mobilizes additional free-phase CO2 out of the cores by enhanced displacement, and increases relative water permeability after CO2 displacement is complete, even though dissolution only accounts for 6–7% of the total CO2 mass initially in the cores before the experiments.