Molecular insights into carbon dioxide enhanced multi-component shale gas recovery and its sequestration in realistic kerogen

Abstract

Understanding the governing processes of CO2 huff-n-puff in shales is essential for enhancing shale gas recovery and CO2 sequestration. However, existing studies have not fully accounted for the chemical composition and geometry of kerogen nanopores and the reality that natural gas is a multi-component mixture. We used grand canonical Monte Carlo (GCMC) simulations to study the competitive adsorption between CO2 and typical hydrocarbon components (CH4, C2H6, and C3H8). We further studied the recovery mechanisms of CO2 huff-n-puff within kerogenic circular nanopores at reservoir conditions. We probed the effects of pressure, pore geometry, and size on gas recovery and CO2 sequestration efficiency. Although pressure drop readily exploits CH4 in the adsorption layer, the recovery due to CO2 injection primarily occurs within the kerogen matrix for pure CH4. Injecting CO2 facilitates the recovery of heavier hydrocarbons, whereas pressure drawdown exhibits better performance for lighter components. CO2 huff-n-puff may serve as a promising method for gas exploitation in circular pores, whereas pressure drop favors the production in kerogen slit. The tremendously different gas adsorption and recovery behavior in distinct pore geometries and compositions necessitate the study using realistic shale kerogen models. Moreover, enlarging the pore size improves the recovery of each component during pressure drawdown but restrains the performance of CO2 injection; meanwhile, the total gas recovery and CO2 sequestration efficiency increase. This study provides a more in-depth understanding of multi-component gas recovery mechanisms within realistic shale kerogen nanopores and sheds light on the CO2 sequestration in shale reservoirs.