Recovery mechanisms of hydrocarbon mixtures in organic and inorganic nanopores during pressure drawdown and CO2 injection from molecular perspectives

Competitive adsorption of hydrocarbon mixtures (C-1-C-2-C-3) as well as CO2 in shale nanopores is of critical importance to CO2 enhanced shale gas recovery. While experiments can measure mixture adsorption from macroscopic perspective, the underlying mechanism at nanoscale is still ambiguous. In this work, we use Grand Canonical Monte Carlo (GCMC) simulations to study competitive adsorption of C-1-C-2-C-3 and C-1-C-2-C-3-CO2 mixtures in organic and inorganic pores at reservoir conditions. The dependences of competitive adsorption behavior on rock property, pore size, and fluid composition are explicitly examined. We find that while C-1 in the adsorption layer can be readily recovered during pressure drawdown, C-2 and C-3 are trapped in pores, especially in organic micropores. Injected CO2 can effectively recover each component in the adsorption layer in organic pores, while in inorganic pores, the adsorption layer is dominated by CO2 molecules, displacing all hydrocarbon components. CO2 injection is an effective method to recover the heavier hydrocarbons. While pressure drawdown is more effective in inorganic pores, CO2 injection performs better in organic pores. Our study should provide fundamental understanding about the recovery mechanisms of shale gas mixtures in various rocks and sheds light on the efficiency of CO2 enhanced recovery in shale gas reservoirs.