Accessibility of methane at micro-pore passage and its effect on the methane desorption in coal

Coal is a natural porous medium with a large number of fractured coal blocks and various types of pore. The accessibility of methane at micro-pore passage has effect on the methane desorption in coal. In this paper,the blocking mechanism of potential well and barrier at micro-pore passage to methane migration is analyzed,the inhomogeneous micro-pore passage model of coal is established,the methane adsorption and desorption rate as well as methane desorption hysteresis characteristics in coal are studied. It is found that when R / r0(pore passage radius / methane diameter) is less than 0. 89,there is a potential barrier at the pore passage plane,and there are potential well on both sides of the pore passage plane. When R / r0 is greater than 0. 89,there is only a potential well at the pore passage plane,and the depth of the potential well decreases gradually with the increase of the pore passage radius. The temperature,the pres-sure difference and the pore passage diameter are the main factors affecting methane passing through the pore passage. By numerical experiment results,it is found that the blocking effect of micro-pore passage potential well and barrier is the fundamental reason for the slow process of methane desorption and the failure reach to 100% of desorption rate. The blocking effect of micro-pore passage on methane migration in coal micro-pores results to methane desorption hys-teresis under different pressures. In the process of coalbed methane exploitation,the methane desorption rate and meth-ane desorption proportion should be effectively improved by changing the distribution characteristics of micro-pore pas-sage in coal through fracturing. The research results have important theoretical guiding significance for the evaluation of CBM recovery efficiency and the enhancement of CBM recovery by coal seam fracturing and permeability enhance-ment.