Study On The Hysteresis Characteristics And Influencing Factors Of Methane Adsorption And Desorption In Shale

Energy security is a strategic issue of national economic development,which is vital to the national economy and people’s lives.In the context of today ’s energy shortage,shale gas exploitation has become the focus of attention.The traditional view is that shale desorption of methane is the reverse process of adsorption,but in the experimental study of shale adsorption and desorption at home and abroad,it is found that the desorption hysteresis of methane is widespread.In this thesis,five groups shales(T-1,T-2,T-3,T-4,T-5)of the marine-continental transitional Taiyuan Formation in the Bohai Bay Basin were used as the research objects,and methane adsorption and desorption experiments were carried out.The elemental composition and mineral composition of shale were analyzed by XRF and XRD;the surface morphology of shale was analyzed by infrared spectroscopy and SEM,and the pore characteristics of shale were tested by liquid nitrogen adsorption method.On this basis,the adsorption and desorption characteristics and thermodynamic properties of methane were explored,and the adsorption and desorption hysteresis characteristics and its influencing factors were analyzed.The results of this thesis are as follows :(1)The total organic carbon(TOC)content of the 5 groups of shale is between1.990 wt% and 4.276 wt%.The elements in shale are mainly Si,Ca,Al,Fe,K,S and Ti,and the content of other trace elements is less than 1%.Quartz and clay minerals are the most abundant in shale,with quartz content of 20.22-40.52 wt% and clay content of26.75-33.51 wt%.The pore size distribution of the samples is quite different.The micropore volume of the five samples is small,and the mesopores are dominant.(2)The adsorption and desorption isotherms at different temperatures(318 K,328 K,338 K)and different maximum adsorption pressures(4 MPa,6 MPa,8 MPa,9 MPa)were tested by the volumetric method,and the L-F adsorption model was used for simulation.Four thermodynamic parameters,Gibbs free energy ΔG,surface potential energy Ω,isosteric heat of adsorption Hst,and entropy change ΔS were calculated based on the adsorption and desorption data.The experimental results showed that at the same temperature,the amount of methane adsorbed increases with the pressure.There was a significant positive correlation between total organic carbon(TOC)content and adsorption capacity of shale.The effect of pore structure on adsorption performance of shale is complex.By analyzing the thermodynamic parameters of methane,it is found that the adsorption behavior of CH4 on shale is spontaneous and belongs to physical adsorption.The adsorption capacity of shale to CH4 is stronger at low temperature,and the Hst of adsorption of CH4 decreases with the amount of adsorption.(3)The influencing factors of shale adsorption and desorption hysteresis were studied through adsorption isotherms under different conditions,and three quantitative evaluation indexes were used to calculate the hysteresis degree.As the temperature increased,the hysteresis coefficients of the five groups of shale samples gradually decreased,indicating that the higher the temperature,the easier it is for the methane gas molecules to break free.The desorption hysteresis increases with the maximum adsorption pressure,and its effect is more obvious than the temperature.The relationship between mineral composition content and desorption hysteresis should consider the comprehensive effect of other factors,and the influence of specific surface area and pore structure on the desorption hysteresis coefficient is more complicated.(4)Based on the hysteresis characteristics of shale adsorption and desorption,it is proposed that the fundamental reason for the hysteresis of methane is that after shale adsorbs methane gas,the shale matrix undergoes adsorption expansion,which makes the pore channel mouth of shale compressed and narrowed,resulting in higher energy required for methane molecular desorption to get rid of the bondage of the shale surface,resulting in different amounts of methane adsorption and desorption,and finally showing the hysteresis of desorption.