Adsorption Characteristics Of Supercritical CO₂/CH₄ In Micro-nano Porous Media

The global shale gas recoverable reserves of 200 trillion cubic meters,China's shale gas resources are widely distributed,accounting for 20%of the world's shale gas recoverable resources,ranking the first in the world,but the complexity of shale reservoir causes the particularity of its occurrence form and production mode.Shale reservoir has the characteristics of ultra-low porosity and low permeability,and the pore structure of the reservoir is in micro-nano scale,and the shale gas existing in adsorption state in the reservoir can reach 80%.At the same time,the huge CO₂ storage capacity of shale matrix has become one of the effective ways to alleviate the greenhouse effect.Therefore,CO₂ geological storage and enhanced shale gas recovery(CS-EGR)project has been widely concerned.In this context,it is of great theoretical and practical significance to explore the adsorption characteristics of supercritical CH₄,CO₂ and their binary mixtures in shale matrix.In this paper,graphene slit pores were constituted by molecular dynamics method to study the adsorption behavior of supercritical CH₄,CO₂ and their binary mixtures in micro–nano pores media under the flow state,the influence mechanism of CH₄ adsorption characteristics and CO₂/CH₄binary mixture displacement and competitive adsorption characteristics are described from the micro perspective.In the research of adsorption behavior of CH₄ under the flow state,the thickness of adsorption layer and adsorption quantity were explored.Conclusions are as follows:(1)Pressure,temperature,pore size and differential pressure affect the thickness of the adsorption layer,which varies with the repulsive force between the adsorption layers.(2)The methane molecules in the adsorption layer are still monolayer structure,and the thickness of the single-wall adsorption layer is 1?2 times of the molecular dynamic diameters of methane.(3)The thickness of adsorption layer increases with differential pressure obviously increases under the low pressure.With the decrease of pressure or increase of temperature,the thickness of adsorption layer increases.When the pore>7nm,the thickness of adsorption layer increases with pore size in the flow state.(4)According to importance analysis,temperature and differential pressure are the most main factors affecting the thickness of the adsorption layer.(5)The relative adsorption capacity is negatively correlated with pressure,and positively correlated with pore size and differential pressure.The effect of temperature on the relative adsorption capacity under various differential pressures were 2.82%,6.76%,9.64%and13.06%,respectively.(6)The differential pressure can promote desorption of the adsorbed methane molecules.In the research of adsorption behavior of CO₂/CH₄ binary mixture under the flow state,the displacement characteristics,competitive adsorption characteristics and adsorption layer thickness of binary mixture were explored.Conclusions are as follows:(1)Injection pressure(<14MPa)is positively correlated with displacement time,while injection temperature(>330K),injection velocity and pore size are negatively correlated with displacement time.The displacement efficiency of CO₂ is positively correlated with injection pressure and injection velocity,and negatively correlated with injection temperature and pore size,when pore size is greater than 3nm,the displacement efficiency tends to be stable.Under the research conditions,the displacement efficiency of CO₂ is more than 75%,which indicates that CO₂ injection can effectively recover shale gas.(2)The existence of differential pressure reduces the competitive adsorption selectivity of CO₂.Under the flow state,the selectivity coefficient of CO₂/CH₄ is positively correlated with injection pressure and negatively correlated with injection temperature.With the increase of injection velocity,the adsorption selectivity increases slightly,and the effect of pore size on the adsorption selectivity first increases and then decreases.(3)The existence of differential pressure makes the adsorption layer thickness of binary mixture molecules decrease;the adsorption layer thickness has a negative correlation with injection pressure and a positive correlation with injection temperature,the pore size independent on the adsorption layer thickness,with the increase of injection speed,the adsorption layer thickness increases slightly,and then tends to be stable.The above results show that molecular flow has crucial influence on the adsorption behavior of CO₂,CH₄ and their binary mixtures.The obtained results in this paper are helpful to understand the adsorption characteristics of CO₂,CH₄ and their binary mixtures in flowing state,and provide theoretical guidance and technical support for accurately evaluating shale gas reserves and improving shale gas recovery by CO₂injection.