Dissecting Dispersion Mechanisms in CO2–CH4 Systems: Channeling, Gravity, and Water Saturation Effects in Depleted Gas Reservoirs

Summary In this study, we investigate the complexities of carbon dioxide (CO2) injection/storage associated with dispersion in depleted natural gas reservoirs, examining the role of reservoir heterogeneity, permeability distributions, autocorrelation lengths, and variations in residual and mobile water saturations. These reservoir conditions are crucial for understanding the dispersive characteristics and their impact on the efficiency of CO2 storage in natural gas reservoirs. We used 2D and 3D stochastic numerical models to capture various reservoir heterogeneity configurations by investigating the impact of Dykstra-Parsons heterogeneity coefficients and vertical-to-horizontal permeability ratios. The impacts of residual water (Swr) and mobile water saturations (Sw), with and without the effect of CO2 and methane (CH4) solubility, were also studied. Approximate Péclet numbers were calculated to quantify the interplay between diffusive and advective transport while exploring gravity-dominated and channeling flow that ultimately influences CO2 transport and produced gas composition. All analyses are performed with a compositional reservoir simulation with the capability to account for the impacts of molecular diffusion and mechanical dispersion. Our findings reveal that higher Dykstra-Parsons coefficients (i.e., greater permeability heterogeneity) resulted in larger mixing zones (more dispersion), signifying the prevalence of advection in these cases. The analysis further indicates that in models with large vertical-to-horizontal permeability ratios, the advection-dominated flow regime is especially sensitive to variations in permeability distribution, with reservoirs displaying greater heterogeneity and exhibiting more channeling and convective spreading. In addition to this, large residual water saturation has no impact on dispersion unless CO2 and CH4 solubilities in the formation brine are significant. Variations in mobile water saturation (Sw) similarly alter CO2 flow behavior, with solubility effects found to generally reduce the size of the CO2-CH4 mixing zone because of solubilization of the downstream CO2, thereby affecting the spreading of the injected CO2. The insights gained from this work highlight the importance of understanding reservoir heterogeneity to optimize CO2 injection and storage in depleted gas reservoirs. By recognizing and quantifying the impact of these heterogeneities on gas flow and mixing, this research offers a way to enhance CO2 sequestration efficiency, aiding in the successful execution of CO2 storage initiatives in line with global energy transition goals.