Gas desorption diffusion hysteresis pressure and the matrix fracture mass transfer function of dual porosity coal

Gas desorption,diffusion and seepage make up the sequential gas migration pattern from coal seam to borehole or roadway. Plenty of studies on residual gas content,adsorption desorption residual deformation and sorption hysteresis on coal have revealed that there is non negligible additional resistance in the gas desorption process compared with the gas adsorption process,and the resistance is controlled by the length and complexity of the diffusion path. The additional resistance of desorption diffusion affects the mass exchange between the matrix system and the fracture system,but the widely used double pore coal gas migration model does not consider its effect,which will make the gas content underestimated when used in the theoretical analysis of gas drainage flow field. The mass exchange between the matrix system and the fracture system is also known as interflow,and the matrix fracture mass transfer function is the key function to calculate the mass exchange capacity/rate between the two systems. To modify the coal seam gas migration model,the coal particle (block) adsorption and desorption isotherm tests were used to simulate the interflow with constant fracture pressure,the gas desorption diffusion hysteresis pressure was proposed to quantitatively evaluate the additional resistance,and the matrix fracture mass function was modified and verified by comparing with the test results. The following results were obtained:the concept and physical meaning of gas desorption diffusion hysteresis pressure were clarified. When the maximum adsorption equilibrium pressure is the same,the gas desorption diffusion hysteresis pressure increases with the increase of particle size in the depressurization process. For the same sample,the gas desorption diffusion hysteresis pressure decreases with the increase of pressure drop. The simulation result of the gas desorption process of coal block based on the modified model can well match the isothermal desorption data,whose R2 value can reach 98.2%. As the in situ stress increases with the increase of mining depth,the matrix size of coal will be greatly increased,and the desorption diffusion hysteresis pressure of matrix fracture mass transfer will generate a more significant impact on gas migration in a coal seam. Reducing the hysteresis pressure of matrix fracture mass transfer by technical means should be one of the problems to be studied in the deep gas control or CBM development.