Molecular Simulation of Hydrocarbon Recovery from Calcite Surface with the Implications for CO2 Storage in Nanopores

Carbon neutrality is increasingly becoming a global consensus. To reduce CO2 emissions and enhance oil-in-place (OIP) recovery, sequestration of CO2 into reservoirs is an effective solution. However, the microscopic processes involved in the stripping of hydrocarbon molecules by CO2 and geological storage in carbonate reservoirs are not fully understood. This study utilizes molecular simulations to analyze the recovery of hydrocarbons from calcite surfaces and the behavior of CO2 storage at the nanoscale. Molecular dynamics simulations were utilized to investigate the characteristics of n-octane adsorption, detachment, and migration from the surface of calcite, as well as the effects of CO2 under reservoir circumstances. The influence of temperature and CO2 density on the detachment of n-octane by CO2 was investigated over a range of 293.15–383.15 K and 0.1–0.9 g/cm3. Grand Canonical Monte Carlo (GCMC) technique is used to examine how the presence of residual oil affects the storage of CO2 in calcite nanopores. The radial distribution function, diffusion behavior, and interface properties of n-octane and CO2 are thus analyzed. The density of N-octane decreases as the distance from the surface increases. This is because N-octane molecules create double adsorption layers on the CaCO3 wall, and the oil layer formed on the calcite surface becomes looser at higher temperatures. The primary process of CO2 stripping the n-octane molecules is through dissolving, diffusion in the crude oil and adsorption on the CaCO3 wall. The volume and mobility of n-octane are improved by the dissolution and diffusion of CO2 in the octane molecules. Since CO2 interacts with calcite more strongly, CO2 can substitute octane molecules and adsorb on the surface of calcite, thereby improving oil recovery. The recovery could be efficiently enhanced by raising the temperature and CO2 density to intensify the interaction between carbon dioxide and n-octane. Residual oil will inhibit CO2 adsorption trapping in calcite nanopores while it has very little impact on the CO2 adsorption layer on the calcite wall. This is because the octane, which accumulates in the middle of the pores, compresses the space available for CO2 molecules, thereby decreasing their storage quantity. This work sheds light on the fundamental mechanisms underlying the microscopic process of CO2-stripping oil molecules, as well as the sequestration behaviors. The findings can serve as a guide for OIP recovery and CO2 geological sequestration in carbonate reservoirs.