Adsorption and Desorption Characteristics of CH4 on Coal with Different Depths in Gujiao Mining Area of Xishan Coalfield

The adsorption and desorption characteristics of CH4 on coal with different depths are significant of development effect and productivity distribution of coalbed methane in Gujiao Mining Area, Xishan Coalfield. Thus, as the No.2 coal seam from Shanxi Formation and the No.8 coal seam from Taiyuan Formation were sampled, the change of adsorption and desorption characteristics of CH4 on dry coal under the conditions of temperature of 40 ℃ and pressure of 8 MPa with the increasing buried depth was analyzed. The control mechanisms of buried depth on CH4 adsorption and desorption were discussed based on the characteristics of nano-pores distribution. The results show that, with the increase of burial depth, both the coal rank and micropores increase, resulting in the increase of CH4 adsorption capacity. However, the influence of mesopores and macropores on the adsorption of CH4 in coal reservoirs with different depths is weak. The buried depth dependent high pressure is another reason favoring for CH4 adsorption. The deeper the coal seam is buried, the higher the degree of hysteresis of CH4 adsorption and desorption. The increasing in ultra-pore with size less than 0.7 nm and the induced enhancement of micropore heterogeneity are reasons for the significant CH4 adsorption hysteresis with the increasing burial depth.