Molecular anatomy and macroscopic behavior of oil extraction from nanopores by CO₂ and CH₄

Oil production from unconventional reservoirs dominated by nanoscale pores contributes substantially to the liquid petroleum production in the US, but its recovery factor remains very low. Gas injection is a promising method to enhance oil production, but its underlying processes at the nanoscale are not well understood. This study focuses on the extraction of decane from single calcite nanopores under reservoir conditions. CO₂ and CH₄ with very different wall adsorption strengths are chosen as the injection gases for comparison. The results demonstrate that decane and injected gas exist as two populations (adsorbed and free phase) inside the pore. When CO₂ is injected, the transport of the adsorbed populations initially dominates the oil extraction, but the transport of the free populations eventually governs oil extraction; the opposite trends are observed when CH₄ is injected. Despite the evolving dominance by different populations, the overall decane extractions and gas accumulations in both cases can be described by a diffusive law. The effective diffusivities of decane extractions and gas accumulations, however, do not always align well with their self-diffusivities.