ATOMISTIC CHARACTERIZATION OF THE PORE-SCALE CO2 IMBIBITION IN SHALES

In this study, molecular dynamics (MD) simulations of CO₂ imbibition, both co-current and the counter-current, in nanoconfined pores are used to study transport phenomena during enhanced shale gas recovery and CO₂ storage in depleted shale gas formations. Supercritical carbon dioxide (scCO₂) is injected into methane-saturated pores and the transport properties that govern the imbibition mechanism in shales are evaluated for different pore sizes, ranging from 3nm to 10nm, as well as for different injection pressures. To realistically represent shale matrix mineralogy and account for the effect of the surface forces, pore wall is modeled as a mixture of quartz, clay, and kerogen molecules that are randomly distributed throughout the walls. The ClayFF force field is used to describe shale-scCO₂ and shale-methane interactions, whereas scCO₂-methane interactions are defined by the Lennard-Jones 12-6 (LJ) potential. For each simulation case, the penetration rate of injected scCO₂ and corresponding displacement of methane are determined. Also, the normalized pressure distribution along the length of the nanopores and the diffusion coefficients for scCO₂ and methane are obtained. The presented study is aimed to demonstrate the ability of MD in addressing challenges related to CO₂ storage and enhanced gas recovery from low-mobility reservoirs. Newly developed MD model can be used as a tool for simulation of nano- and meso-scale flows in ultra-tight porous media, which are otherwise difficult to analyze experimentally.