Molecular Simulation Study On The Influence Of Shale Pore Characteristics On CH₄ Adsorption And Recovery

Increasing energy demand has brought new challenges to the energy shortage problem.At the same time,a great deal of CO₂ emissions caused by the consumption of traditional fossil energy has exacerbated the global greenhouse effect.Shale gas is an unconventional natural gas resource with great development prospects.It has attracted extensive attention with the advantages of large carbon content,low carbon content,and high energy efficiency.The carbon sequestration with enhanced gas recovery(CS-EGR)has been a potentially feasible win-win solution to solve the energy shortage and greenhouse effect.However,due to the extremely low permeability of shale nanopores,the initial gas production of shale gas only accounts for 5%-15%of shale gas reserves.The focus of shale gas production research has shifted to the improvement of recovery efficiency,and the extremely low permeability of shale nanopores.The low permeability is related to the shale pore structure characteristics.In the work,based on the model of the actual molecular structure of shale and the actual environmental conditions of the shale reservoir,the influence of shale pore structure characteristics on shale gas adsorption and recovery is studied,mainly considering the factors of pore size,pore structure shape and water content.The study explained the mechanism of CO₂ and CH₄adsorption in shale reservoirs and the mechanism of CO₂ replacing CH₄ from a microscopic perspective,so as to provide a technical reference for the assessment of shale reserves and the formulation of a reasonable shale gas recovery scheme.In view of the nano-scale pore characteristics of shale,molecular dynamics simulation and Grand Canonical Monte Carlo simulation methods are used.Firstly,appropriate research models are selected combining with the research objectives.For the study of pore size and pore water content,sodium-saturated montmorillonite model are selected.As for the study of pore structure shape,kerogen molecules are used to construct cylinder-shaped,bottleneck,wedge-shaped and slit-shaped pores,and then Grand Canonical Monte Carlo simulations are used to investigate the influence of these three pore structure characteristics factors.Then the Grand Canonical Monte Carlo simulation is used to study the effect of pore structure characteristics on the adsorption properties of CH₄and CO₂ single components and their mixtures.The dynamics characteristics of CH₄or CO₂and the process of injection CO₂ dispalcing CH₄in shale pores are studied using molecular dynamics simulation methods.The results show that pore size,pore structure shape and pore water content have significant effects on the adsorption of CH₄ and CO₂in shale.Under the same conditions,the excess adsorption capacity of CH₄ increases at initial and then decreases as the pore size increases.The excess adsorption of CO₂increases with the pore size,the excess adsorption capacity of CH₄ and CO₂ in the pores with different structure shapes follows in the order of cylinder-shaped>neckbottle-shaped>wedge-shaped>slit-shaped,water content will reduce the adsorption of CH₄ and CO₂.The adsorption capacity of H₂O,CO₂ and CH₄ in the montmorillonite pores sequentially decreases.The adsorption sites of CO₂ and CH₄ are different.CO2 molecules have a preferential to accumulate near the Na+cations.On the contrary,CH4 molecules have a preference to adsorb in the hollow site of the six-membered oxygen ring.There is a strong electrostatic interaction between H₂O and montmorillonite,which leads to the formation of a water film on the surface of montmorillonite.The presence of the water film weakens the interaction between CH₄,CO₂ and montmorillonite,thereby reducing the amount of adsorbed CH₄ and CO₂ molecules in the pores of montmorillonite.For competitive adsorption of CH₄ and CO₂,the increase in pore size and water content will reduce the selectivity of CO₂/CH₄,but pore structure shape has little effect on the selectivity.The diffusion coefficients of CH₄ and CO₂ in the pores of montmorillonite increase with the pore size.In the montmorillonite pores with different water content,the diffusion mechanisms of CH₄ or CO₂ are different.With the increase of water content,the diffusion mechanism of CO₂ or CH₄ in the pores of montmorillonite changed from surface diffusion,Knudsen diffusion to surface diffusion,Knudsen diffusion and viscous flow.At the same time,due to the high energy barrier and low diffusion coefficient in the montmorillonite pores with small pore size,CH₄ is difficult to be displaced.The"water locked"phenomenon caused by the increase in water content has a significant inhibitory effect on the recovery speed of CH₄,but does not affect the displacement efficiency of CH₄.The shape of the pore structure has a significant effect on the recovery speed of shale gas.The difficulty of CH₄ molecules being displaced in various pore structures is ranked as cylinder-shaped>neck-bottle>slit-shaped>wedge-shaped,which is consistent with system resistance.It is hoped that this study will help to better understand the microscopic state of shale gas molecules and provide guidance for the research of CS-EGR.