Molecular simulation of CO2/CH4 competitive adsorption in organic matter pores in shale under certain geological conditions

摘要

To reveal competitive absorption behavior between CH4 and CO2 in organic matter (OM) nanopores, OM pore structure was first characterized using focused ion beam?scanning electron microscope (FIB-SEM) and pore-size distribution was studied using N2 adsorption, using Lower Silurian Longmaxi shale in Sichuan Basin as sample. Then a simplified pillar-layer model was used to study CH4 adsorption behavior and competitive adsorption effect between CO2 and CH4, using grand canonical Mote Carlo (GCMC) method. Research indicates that nanopores with good connectivity widely exist in OM, offering important storage space for absorbed shale gas. The amount of absorbed CH4 can increase with lower temperature and increased pressure, and overpressure will significantly increase the amount of CH4 absorbed underground; CO2 shows high competitive absorption ability; CO2/CH4 selectivity coefficient decreases dramatically with increasing temperature or pressure, or both, and it corresponds to deeper burial depth. CO2 EGR during shale gas exploration will be more efficient if it is conducted after the pressure drops to a certain degree.