分子模拟CH_(4)和CO_(2)在方解石-白云石岩层的吸附机理

Molecular simulation of the adsorption mechanisms of CH_(4)and CO_(2)in calcite-dolomite nanopore

为揭示碳酸盐岩层页岩气的富集特征和开采机理,采用巨正则蒙特卡洛、分子动力学和密度泛函耦合方法探究了不同地质埋深条件下CH_(4)和CO_(2)在方解石-白云石岩层的吸附机理。研究表明,CO_(2)在方解石-白云石岩层的吸附热为38~48 kJ/mol,其在白云石表面的吸附能大于44 kJ/mol,从理论上证实CO_(2)在白云石表面可形成化学吸附。白云石表面CO_(2)的吸附密度峰大于方解石表面,白云石更有利于CO_(2)的地质封存。随着地质埋深的增加,纯组分CO_(2)和CH_(4)在方解石-白云石岩层的超额吸附量分别在600、1600 m会出现一个极大值;而CH_(4)的总负载量随着地质埋深的增加先迅速增加,在3000 m后增长缓慢。因此,CO_(2)在方解石-白云石岩层地质封存的有利埋深约在600 m,CH_(4)的富集有利区位于3000 m以深。CH_(4)的采收率与CO_(2)的注入压力成正比,并随着地质埋深的增加先增加,当地质埋深大于1000 m后,采收率趋于平稳。该研究为碳酸盐岩层页岩气的储量评估和CO_(2)强化页岩气开采技术的应用提供重要理论依据。

To clarify the enrichment characteristic and recovery mechanism of shale gas in carbonate rocks,the adsorption behaviors of CH_(4)and CO_(2)in calcite-dolomite nanopore were investigated by grand canonical Monte Carlo(GCMC)and molecular dynamics simulations and density function theory.The adsorption heat of CO_(2)in the calcite-dolomite nanopore ranges in 38~48 kJ/mol.The adsorption energy of CO_(2)on dolomite surface is greater than 44 kJ/mol,suggesting that CO_(2)can be chemically adsorbed on dolomite surface.The adsorption density peak of CO_(2)on dolomite surface is higher than that on calcite surface.Dolomite is therefore more beneficial to the underground CO_(2)storage than calcite.With the increase of geological depth,the excess adsorption capacities of pure CO_(2)and CH_(4)exhibit a maximum at 600 and 1600 m,respectively.The loading of CH_(4)increases rapidly at first and tends to level off after 3000 m.It indicates that the favorable geological depth for CO_(2)storage in adsorbed state is at 600 m,while the enrichment region of CH_(4)is at over 3000 m.The recovery rate of CH_(4)is positively correlated with the injection pressure of CO_(2),which increases gradually with the increase of geological burial depth,and tends to level off at over 1000 m.These findings can provide important implications for the assessment of shale gas reserve and the application of CO_(2)sequestration with enhanced gas recovery technology in carbonate rocks.