Molecular simulation of the adsorption mechanisms of CH4 and CO2 in calcite-dolomite nanopore
To clarify the enrichment characteristic and recovery mechanism of shale gas in carbonate rocks,the adsorption behaviors of CH4 and CO2 in calcite-dolomite nanopore were investigated by grand canonical Monte Carlo(GCMC)and molecular dynamics simulations and density function theory.The adsorption heat of CO2 in the calcite-dolomite nanopore ranges in 38～48 kJ/mol.The adsorption energy of CO2 on dolomite surface is greater than 44 kJ/mol,suggesting that CO2 can be chemically adsorbed on dolomite surface.The adsorption density peak of CO2 on dolomite surface is higher than that on calcite surface.Dolomite is therefore more beneficial to the underground CO2 storage than calcite.With the increase of geological depth,the excess adsorption capacities of pure CO2 and CH4 exhibit a maximum at 600 and 1600 m,respectively.The loading of CH4 increases rapidly at first and tends to level off after 3000 m.It indicates that the favorable geological depth for CO2 storage in adsorbed state is at 600 m,while the enrichment region of CH4 is at over 3 000 m.The recovery rate of CH4 is positively correlated with the injection pressure of CO2,which increases gradually with the increase of geological burial depth,and tends to level off at over 1 000 m.These findings can provide important implications for the assessment of shale gas reserve and the application of CO2 sequestration with enhanced gas recovery technology in carbonate rocks.
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