Promising combination of CO2 enhanced oil recovery and CO2 sequestration in calcite nanoslits: Insights from molecular dynamics simulations

CO2-Enhanced recovery has attracted extensive attention, meanwhile reducing greenhouse gas emissions via CO2 geological sequestration is causing global concern in recent years, combining CO2-EOR and CO2 sequestration has been recognized as one of the CCUS approach with high potential, and tight reservoirs such as shale oil are the most promising resource in this respect, which has become one of the hot topics unsurprisingly. The un-derstanding about the microscopic molecular behaviors and interaction mechanisms between CO2, shale oil, and reservoirs is critical for the development of the techniques, while still incomplete, which makes it challenging for efficient large-scale application in the world. In this paper, molecular dynamics (MD) simulations are used to investigate the displacement and miscible behaviors of CO2/octane in calcite nanoslits under reservoir condi-tions, the effects of temperature, pressure and slit apertures on the molecular behaviors have been studied comprehensively. The interaction energy between CO2 and octane molecules and calcite surface were calculated to quantitatively analyze the mechanisms in the process of CO2 flooding. The significant strong interactions between calcite surface and CO2 on one hand benefit the displacement of octane by CO2 increasing the miscible flooding efficiency, on the other hand ensure the high adsorption capacity of CO2 inside the calcite nanoslits, thereby plenty of CO2 can be stored inside the calcite nanoslits after the flooding. It could be concluded that the carbonate reservoir could be the most promising field combining CO2-EOR and CO2 sequestration. This work provides theoretical guidance for both the optimization of the CO2-enhanced oil recovery and effective CO2 sequestration in unconventional tight reservoirs, especially carbonate reservoir.