Study On Adsorption Mechanism Of Shale Methane Based On Molecular Simulation

The reservoir fluid accumulation mechanism of shale gas reservoirs is quite different from that of conventional gas reservoirs.The nanopores are the main storage space of shale gas,and adsorption gas is one of the main storage forms of shale gas.Therefore,in-depth study of the adsorption behavior of shale gas has important practical significance for the effective development of shale gas.At present,the research on the adsorption mechanism of shale gas is mostly based on indoor physical experiments,in which the actual high temperature and high pressure reservoir conditions are not met,and the microscopic adsorption mechanism of shale gas needs to be further studied.In this thesis,the molecular modeling technique is used to study the adsorption behavior of shale gas in nanopores of reservoirs from the perspective of microscopic molecules,and the adaptability of classical adsorption theories in this field is investigated.Based on the shale structure parameters,the nanoporous models are constructed from organic matters,clay minerals and clay mineral-organic matter composites.The physical properties of these models are in line with that of actual shale gas reservoirs,and the adsorption characteristics are consistent with the laboratory test results.Based on the established shale nanoporous models,the macro-regular Monte Carlo method(GCMC)is used to simulate the shale methane isotherm adsorption under high temperature and high pressure conditions of actual reservoirs,and the microscopic mechanism of shale methane adsorption is studied as follows:(1)Converting simulation results to get shale methane isotherm adsorption curves,and studying the characteristics of these curves.On the bases of shale methane adsorption results under different conditions,analyzing the influencing factors of shale methane adsorption,and studying the adsorption behavior under the influence of reservoir temperature and pressure conditions,organic matter composition and clay mineral types.(2)Calculating the radial distribution function of methane molecules and nanoporous skeleton to study the preferential adsorption behavior of gas molecules,and define the significance of atomic electronegativity to adsorption.(3)Studying the density distribution characteristics of methane in nanoporous models,and the microscopic process of methane adsorption on shale,this process is defined as a multi-component process of single-layer adsorption,multi-layer adsorption and micro-pore filling.(4)Based on molecular dynamics and thermodynamics theory,the changes of characteristic parameters such as adsorption heat,electrostatic force and van der Waals force between gas molecules and nanopore models are studied from the molecular point of view to reveal the intrinsic adsorption properties of methane on shales.(5)According to the mechanism and characteristics of shale gas adsorption,classical theories such as Freudlich model,Langmuir model,BET model and D-R model are selected,and the four adsorption theories are applied to the fitting of shale methane isotherm adsorption curves and the applicability is analyzed.