Mineralogy impact on pore accessibility of shale nanopores and its implications on gas injection enhanced oil recovery: A molecular perspective

In this work, we employ molecular dynamics (MD) simulations to explain the distinct behaviors of hydrocarbon retention and transport during gas injection enhanced oil recovery in clay-rich and carbonate-rich shales, as observed in small-angle neutron scattering and ultra-small-angle neutron scattering experiments. A series of MD simulations have been conducted to investigate the adsorption behaviors as well as transport mechanisms of methane and carbon dioxide (CO2) based cyclic injection in Permian shales. The simulations reveal that clay-rich shale rock tends to retain greater amount of heavy hydrocarbons due to the water blockage of pore throats caused by the presence of irreducible water adsorbed on clay surfaces. In contrast, carbonate-rich shale facilitates more efficient decane recovery during methane huff-n-puff. In general, CO2 demonstrates a higher sweeping efficiency than methane. The findings highlight the critical roles of pore size distribution, surface properties, and the impact of irreducible water in influencing hydrocarbon accessibility and removability. These insights advance the mechanistic understanding of gas injection processes in heterogeneous shale formations, guiding the development of optimal injection strategies and reservoir screening criteria for unconventional reservoirs.