Coal rank-pressure coupling control mechanism on gas adsorption/desorption in coalbed methane reservoirs

Systematic understanding of the nature of gas adsorption/desorption in coal matrix is the basis for efficient development of coalbed methane (CBM). In this paper, firstly, the coupling control mechanism of coal rank-pressure on gas occurrence is studied. Secondly, a novel concept of surface control field is proposed, and the influence mechanism of coal rank on methane adsorption capacity is revealed. Thirdly, the difference of energy conversion mechanism between adsorption and desorption processes is clarified. Lastly, a new concept of adsorption potential is proposed on the basis of a novel and useful coal rank-pressure linkage model of energy conversion. It is found that: (a) The surface control field generated by two sidewalls of the pore can overlap, resulting in mutual cancellation of gravitational forces from the two sidewalls. (b) The isosteric heat of adsorption calculated on the basis of desorption curve can reflect the real adsorption performance. (c) Although the density of oxygen-containing functional groups in the microscale pores of the high rank coal matrix is low, the mutual cancellation of gravitational forces leads to a high adsorption saturation but a low adsorption potential.