Experimental Research On Coal Gas Adsorption Deformation And Diffusion Characteristics

Coalbed methane reservoir in China has the characteristics of typical low saturation,low permeability and low pressure,so it is difficult to extract.Diffusion plays an important role in the process of coalbed methane extraction.Coal is a kind of complex geological body.In the process of coalbed methane migration,coal will deform and affect the gas migration to some extent.The study of the deformation law and gas diffusion characteristics in the process of coal absorbing gas can provide theoretical basis and application basis for the design of CO₂injection and coalbed methane extraction scheme.It is of great significance to explore the mechanism of coal and gas outburst and guide the safe mining of coal.In this paper,through the experiments of low temperature liquid nitrogen adsorption,mercury injection and adsorption deformation dynamics,the characteristics of coal pore structure are understood,and the thermodynamic parameters,such as temperature and pressure,deformation parameters and diffusion parameters,are obtained in the adsorption process.The law of coal gas adsorption and diffusion is analyzed by using the model.The main research results are as follows:（1）The adsorption process of coal gas is exothermic and non constant temperature process,which affects the diffusion.（1）The single pressure adsorption method produces a large temperature change phenomenon.The temperature of CH₄and CO₂increased by 8.6℃and 14℃,respectively.（2）The process of adsorption diffusion and the process of temperature change are divided into two stages:T_tand T_c.T_cslightly lags behind T_tat the boundary point of adsorption capacity stage.The higher the temperature and the longer the time of T_t,the smaller the corresponding adsorption rate.The adsorption capacity and equilibrium time decreased with the increase of temperature.（2）The pressure is the influence factor of CO₂and CH₄adsorption diffusion stage.（1）The pressure change of CO₂and CH₄in the adsorption process has a corresponding relationship with the change of adsorption capacity,both of which have the same law and show the characteristics of stage change.（2）With the increase of pressure,the higher the temperature rise and T_ttime,the smaller the corresponding adsorption rate.（3）The relationship between adsorption equilibrium state and adsorption capacity and deformation is not a single rule.The equilibrium adsorption and deformation of CO₂and CH₄are linear.In the process of adsorption,the amount of CO₂adsorption and the amount of deformation show a linear relationship with different slopes,while the law of CH₄is the same as that of equilibrium state.A new stage division of the adsorption deformation process is proposed:the stage of temperature and pressure comprehensive influence deformation,the stage of elastic recovery deformation and the stage of adsorption expansion deformation.（4）The expansion and deformation process of coal gas adsorption can restrain the diffusion.The single pore model,the expansion index model and the diffusion relaxation model are used to fit and verify the adsorption rate data.The single pore model has the worst effect and the diffusion relaxation model is the best.The diffusion relaxation model has both Fick diffusion and anomalous diffusion.In the diffusion relaxation model,the effective diffusion coefficient and relaxation coefficient decrease with the increase of deformation,and it is concluded that adsorption deformation has an inhibitory effect on adsorption diffusion.