Molecular Simulation Study On Competitive Adsorption Mechanism Of Multi-components In Shale Gas Reservoir

Shale gas has attracted global focus due to its considerable resource abundance.However,the recovery of most shale gas reservoirs is still below 30 % due to depletion production.Supercritical carbon dioxide possesses excellent properties such as low surface tension and high adsorption ability,which lead to tremendous potential for shale gas production by carbon dioxide injection.Methane coexists with carbon dioxide and water in shale nanopores during carbon dioxide injection.Therefore,the competitive adsorption mechanism of methane and carbon dioxide,and the effect mechanism of water can provide theoretical implications for the evaluation of shale gas reserve and the improvement of carbon dioxide sequestration and enhanced gas recovery.In this work,molecular simulation and experimental measurement were adopted as the research methods.The competitive adsorption mechanism of multi-components in shale gas reservoir was investigated by combining basic theories such as kerogen geochemistry,molecular thermodynamics,adsorption and rock mechanics.Representative porous models for organic matter,clay mineral and shale composite were constructed,which were utilized to explore the competitive adsorption mechanism between methane and carbon dioxide and analyze the underlying mechanism for water effect by using the molecular simulation methods.The dynamic fluid states during shale gas production were also discussed.The key conclusions are summarized as follows.The competitive adsorption mechanisms between methane and carbon dioxide in organic matter and clay mineral are revealed.In the organic nanopores with strong heterogeneity,the adsorption isotherm of methane in the mixtures presents stepwise characteristic,which is caused by the replacement of methane by carbon dioxide from inner surface with high adsorption potential to outer surface.The competitive adsorption is controlled coordinately by gas property differences,porous structures and surface chemical properties.The differences in gas thermodynamic properties dominate the contribution to the preferential adsorption of carbon dioxide in clay mineral,while the contribution of volumetric factor increases as the pore size decreases and the pore heterogeneity strengthens.Gas adsorption capability is determined by the effective porosity,while adsorption selectivity is closely relevant to the content of accessible functional groups contained oxygen in organic matter.Gas adsorption can induce the swelling of organic matter.The swelling induced by carbon dioxide is higher than that by methane at the same adsorption amount.Reserve evaluation without considering the deformation of porous structures can massively underestimate the shale gas resources.The distribution characteristic of water in organic matter and clay mineral and its effect on the competitive adsorption of methane and carbon dioxide are clarified.Water molecules are preferentially adsorbed on the clay surfaces without covered by kerogen and sodium ions.Some water molecules aggregate into clusters due to strong hydrogen bonds.Some water molecules scatter around the functional groups containing sulfur and oxygen.The effect of water on the adsorption of carbon dioxide is larger than that of methane.There are water transportation and cluster extrusion phenomena in organic matter and sodium ion solvation phenomenon in clay mineral at higher water content,which can boost the preferential adsorption of carbon dioxide.The characteristics of fluid states during depletion and carbon dioxide injection in shale gas reservoir are elaborated based on shale composite composed of organic matter and clay mineral.The free state of methane is the major produced component during depletion production,while the dissolved state of methane is the dominated produced component during carbon dioxide injection.In the late stage of shale gas production,the adsorbed state of methane has the minimal producing rate,which is the potential target for improving gas production.During the production by injecting carbon dioxide,the deformation of kerogen structure can enhance the gas solubility in kerogen matrix,water can increase the producing rate of dissolved state of methane,and multi-components can promote the producing rate of adsorbed state of methane.