Study On Adsorption Mechanism And Thermodynamic Model Of Shale Gas In Micro-Nano Pore

With the proposal and rapid advancement of the “carbon neutral”and “carbon peaking” in China,the demand for natural gas,as a clean fuel,will continue to increase.Shale gas is a kind of unconventional natural gas.There are huge resources potential and development prospect of shale gas in China,and shale gas has become an important strategic choice for energy security supply,clean and low-carbon,and diversified development in China.However,shale gas development is still in the early stage in China,and the understanding of the occurrence characteristics and mechanism of shale gas in China is still unclear,which leads to the lack of a clear basis for the design of shale gas production technology and insufficient understanding of the mechanisms.This paper focuses on the complex molecular interactions in the micro-nano pores.Based on the fluid phase equilibrium relationship between the adsorbed gas and free gas phases of shale gas,a novel occurrence mechanism of shale gas is proposed and the thermodynamic models are scientifically constructed.In-depth research is carried out on the occurrence characteristics,adsorption mechanism,parameter influence mechanism and recovery characteristics.Firstly,focusing on the fluid-wall molecules interactions,an advanced equation of state model is proposed by systematically coupling the original SRK equation of state with Lennard-Jones potential model and introducing a wall-fluid molecular interaction on the basis of the original SRK equation of state in this work,and the reliability of the model is verified.The density deviations between the model predictions and the molecular simulation calculations range from 0.24%-1.73%.Furthermore,the distribution characteristics of thermodynamic properties of non-polar fluids are predicted and analyzed,and a systematic parameter analysis of shale systems is carried out.Besides,the variation patterns of shale gas thermodynamic properties with the change of pore,fluid and reservoir environment parameters are revealed.The fluid density distribution in the nanopores is found to be nonuniform,and the density gradually decreases from the pore wall to the center.The pore wall components,fluid components,pore size,temperature,and pressure will be greatly influential on the fluid distribution in the pores.Subsequently,based on the fluid phase equilibrium relationship,the phase equilibrium model of fluid in micro-nano pore is constructed by coupling the intermolecular interaction model(including the wall-fluid molecular interaction model and the fluid-fluid molecular interaction model)and the capillary effect model,taking the intermolecular interaction and capillary condensation into account.The wall-fluid molecule interaction model is derived from the relevant part of the modified thermodynamic equation of state model.With the help of genetic algorithm,the thickness and the molar concentration of the adsorption layer that satisfy the equilibrium conditions of the model are obtained.Based on the above phase equilibrium model,the pore adsorption characteristics,adsorption mechanism and mining characteristics are systematically studied,and the influence of key parameters on the pore adsorption capacity is investigated by means of parametric analysis.The results show that the isothermal adsorption of the fluid in the pores is distributed in stages with the change of pressure.The first stage is controlled by the fluidwall molecules interactions,while the second stage is dominated by capillary effect.From the first stage to the second stage,the isothermal adsorption curve makes a sudden jump.The thickness of the adsorption layer is kept at 0.7-0.9 nm in the first stage,and finally the adsorbed layer fluid occupies almost the whole pore space.Increasing reservoir pressure,decreasing reservoir temperature and pore size are conducive to the increase of pore adsorption capacity.In the process of decompression production,the conversion of adsorbed gas increases with the increase of pressure drops,which in turn leads to the increase of shale gas recovery.The advantages of the solution proposed in this project lie in its solid theoretical basis,high accuracy,good applicability and simple calculation.The comprehensive thermodynamic model provides a theoretical basis for breaking through the bottleneck of shale gas reservoir evaluation and production process optimization from a new perspective.