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

Injecting gas to enhance oil production from unconventional reservoirs dominated by nanoscale pores has been practiced in past decades with varying success, in part due to the lack of a fundamental understanding of the underlying physical processes. Here, we report molecular dynamics simulations of gas-enhanced recovery of decane from single 4 nm-wide calcite pores under reservoir conditions (383 K and 345 bar). Two gases, CO2 and CH4, are considered due to their different adsorption strength on calcite pore walls and practical considerations such as their availability and benefits for carbon sequestration. We show that, upon entering a pore, both gases form two molecular populations (free and adsorbed molecules), and their accumulation leads to the extraction of corresponding decane populations. The CO2-decane exchange is initially significantly driven by the evolution of the adsorbed populations, but a transition to the dominance by free populations occurs later; For the CH4-decane exchange, the opposite occurs. Despite this difference, the overall gas accumulation and decane extraction behavior follow the same diffusive law for CO2 and CH4 gases. The CH4-decane exchange has higher effective diffusivities than the CO2-decane exchange, i.e., CH4 enables faster decane extraction under the conditions studied here. These effective diffusivities do not always align well with the self-diffusion coefficients of CO2, CH4, and decane in nanopores.