| Large amount of nanoscale pores in shale gas reservoir result in particularity of shale gas accumulation and exploitation.For shale gas production,it is significant to clarify methane adsorption property in nanopores and influence factors which have effect on gas storage potential of shale gas reservoirs.To figure out it,we build nanoscale clay mineral models like illite,montmorillonite and kaolinite,and kerogen model which represented by carbon sheets for simulation work.All nanopore models are slit pore and pore width is the distance between two sheets.In this work,grand canonical Monte Carlo(GCMC)simulation is applied to study methane adsorption in various slit nanopores at different temperature and pressure and analyze different approaches to characterize the absolute adsorption.Also,competition adsorption of methane and carbon dioxide is studied with different molar density in bulk phase.Based on simulation,we find that methane adsorption in micropores is micropore filling,and methane density in effective pore space is greater than bulk density at same temperature and pressure.On the other and,methane adsorption in mesopores is monolayer adsorption,it means that methane density in the middle of pore almost equal to bulk density but is much higher than bulk density near pore wall which indicates that methane forms a strong adsorption layer there.Effective pore space has been divided into adsorbed phase and bulk phase according to density profiles.Adsorbed amount calculations show that it is better to compute absolute adsorption amount by excess adsorption amount and density of adsorbed phase than by excess adsorption amount and volume of adsorbed phase.Compared adsorbed methane amount at various condition,it is obvious that adsorbed methane amount decreases with temperature increasing.For excess adsorption amount,it increases with pressure increasing at relative low pressure,but shows opposite trend when pressure is higher than 15 MPa.For absolute adsorption amount,it increases with pressure increasing even pressure is up to 80 MPa.Due to surface adsorption,both the excess adsorptions and the absolute adsorptions in various clay nanopores are similar.Differ from methane adsorption in nanopores,methane and carbon dioxide competition adsorptions indicate that carbon dioxide adsorption is multilayer adsorption because of its strong quadrupole moment.Selectivity analysis reveals that influences caused by temperature can be ignored during competition adsorption.However,selectivity is sensitive to pressure.It decreases rapidly with pressure increasing at relative low pressure region.When pressure is higher than 15 MPa,selectivity curve tends to be flat.Selectivities of different clay minerals are in order: menomorillonite>illite>kaolinite.Influences caused by pore size and molar fraction also have been studied and simulation work indicates that pore size has negative effect on selectivity as well as molar density of carbon dioxide.By calculating absolute adsorption capacities,influence factors of competition adsorption are also analyzed.As same as methane adsorption,absolute adsorptions of adsorbates in competition adsorption are increases with pressure and decreases with temperature.On account of iron exchange,methane adsorption capacities in competition adsorption are much less in illite and montmorillonite nanopores than that in kaolinite nanopores at same simulation condition.But the discrepancy between kaolinite and illite(or montmorillonite)gets smaller when pressure is higher than 10 MPa,especially at relative high pressure,absolute adsorption capacity in kaolinite model is same as that in montorillonite model.Simulation works provide theoretical fundamental for enhancing shale gas recovery.Considering mass balance,the upper limit pressure recovery for a given molar fraction can be estimated.Furthermore,temperature weakly influences amount of methane replaced by injected carbon dioxide.However,large molar density of carbon dioxide contributes to ratio of replaced methane molecules efficiently.By molecular simulation,parameters of methane distribution in nanopores can be confirmed accurately.It is significantly important for flow regime researches of methane in nanopores.Based on knudsen number calculation,flow regimes in clay minerals are discussed.We find that there is a clear power function relationship between pore size and critical pressure which indicates flow regime transition in nanopores.It shows that molecular diffusion,slippage,transition flow,knudsen diffusion and surface diffusion in adsorbed phase are main flow regimes in clay mineral nanopores.Flow regimes transition is closely related with pore size,pressure and properties of floe fluid.Combined with pore size distribution analyzed by nitrogen adsorption experiments,apparent permeability of various clay minerals have been calculated.Apparent permeability in three kinds of clay minerals are similar for a given temperature and pressure.In relative low pressure region,apparent permeability seems to be insensitive to pressure,and it is in the range of 100-150 nD.But in relative high pressure region,apparent permeability increases obviously and it is in the range of 150-400 nD.Compared with methane adsorption in nanopores,competition adsorption results in smaller volume and density of free gas phase,fluid viscosity,langmuir adsorption amount and langmuir pressure.Thus,apparent permeability at competition adsorption condition is less than that at methane adsorption condition.As a result,enhanced shale gas recovery can be evaluated based on apparent permeability and competition adsorption capacity.For methane adsorption,shale gas recovery is 1.47%-2.36%.By injection carbon dioxide,the percentage of enhanced shale gas recovery can reach 37.84%.It is obviously that carbon dioxide injection is effective method to enhance shale gas recovery. |
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