CO2 Dissolution Trapping in Depleted Gas Reservoirs with Residual-Gas Mixtures and Bottom Water

Abstract

Abstract The depleted gas reservoir with bottom water exhibits great potential for long-term CO2 sequestration. Due to the reservoir heterogeneity and technical limitations, a considerable amount of natural gas resides in the reservoir. The residual gas affects the mass transfer within and between phases, further impacting the behavior of CO2 dissolution and convection in the porous media. Quantitatively characterizing the mass transport behavior of CO2-residual gas mixtures is of great significance for CO2 storage capacity evaluations. In this work, a series of numerical simulations were performed to investigate the effect of residual-gas mixtures on CO2 dissolution trapping. We first adopted the modified cubic equations of state and fugacity-activity model to calculate the phase behavior of CO2 with different compositions. Then, the Sherwood number of the 2D synthetic model with a capillary transition zone (CTZ) was calculated to explore the mass transfer for different gas mixtures. Besides, we compared CO2 dissolution rate at each stage and characterized the onset and decay time of convection as a function of gas composition to quantify the effect of residual-gas mixtures. The results indicate that the variation trend of the Sherwood number resulting from the synthetic model with CTZ is similar to that from the single-phase model. The presence of CTZ enhances gravity-induced convection and accelerates CO2 dissolution, whereas the residual-gas mixtures have a negative effect on mass transfer. The increasing residual gas reduces the partial pressure and solubility of CO2. As a result, the concentration and density difference between saturated water and fresh water are mitigated, leading to the suppression of the driving force for CO2 diffusion and convection. Accordingly, the rate of CO2 dissolution, dominated by gravity-induced fingering, is significantly decreased, resulting in a delay in the onset and decay time of convection and a significant decrease in the maximum Sherwood number. Considering the negative influence of residual gas on the solubility trapping of CO2, it is necessary to produce natural gas with CO2 injection in the depleted gas reservoir as possible along with carbon sequestration. This work explores the relationship between the Sherwood number and dissolution time under more realistic reservoir conditions. In addition, the influence of the capillary transition zone and residual gas concentration on the dissolution trapping is well analyzed. It can provide a reference for practical carbon capture and storage (CCS) projects.