Supercritical CO2 dissolution and mass transfer in low-permeability sandstone: Effect of concentration difference in water-flood experiments

Abstract

Core-flood experiments of supercritical CO2 (scCO2) and water were conducted under pressures higher than 10.00MPa and a temperature of 40°C to investigate scCO2 dissolution and mass transfer in water and their effects on displacement and imbibition. In these experiments, two representative sandstone cores of low permeability obtained from Shenhua Group CCS site in the Erdos Basin in China were used. On each core sample, five water-flood experiments were performed on the scCO2-saturated core by injecting water of a wide range of dissolved CO2 concentration (from CO2-free to CO2-saturated) for ∼37h, with a focus on the effect of differential concentration (ΔC) (i.e., the difference between the concentration of solubility and that of injected water) on dissolution and mass transfer, as well as displacement. An additional experiment was conducted on the first sample by water flood using CO2-free injected water into the scCO2-flooded core initially saturated by water free of dissolved CO2. In each experiment, the mass flow rate of scCO2 as a free phase and the net mass rate of dissolved CO2 were measured and calculated for core effluent. For the first sample, the residual CO2 saturation ranges from 0.52 in the case of ΔC=0–0.67 in the case of ΔC=solubility, after two displacement periods of stable and unstable scCO2 mass rate. The difference in residual saturation indicates that scCO2 dissolution during displacement may enhance snap-off, thus trapping more scCO2. At the end of the four unsaturated experiments, nearly all the residual CO2 mass is dissolved into the injected water through local CO2 dissolution and mass transfer between the dissolved CO2 and displacing water. In each experiment, the measured mass rate of dissolved CO2 decreases significantly over the time from that of ΔC in period III to more than two orders of magnitude smaller in period V. The dependence on ΔC of mass flow rate and net effluent concentration in periods III and V indicates that local CO2 dissolution in large pores in period III and in small and fine pores over all the time is at equilibrium as scCO2 residence time is sufficiently long. In periods IV and V, the overall mass transfer is non-equilibrium, leading to a lengthy depletion process, in comparison to an equilibrium case. The contribution of equilibrium dissolution in small and fine pores is also demonstrated by one order of magnitude reduction in effluent concentration in the additional experiment as these pores are not accessible by scCO2. The similar behavior of dissolution and mass transfer and their effect on displacement in both samples are obtained. A logistic model developed can reasonably represent the S-shaped function between mass transfer rate and core mass ratio as a function of scaled ΔC.