Apparent Permeability Model of Coalbed Methane in Moist Coal: Coupling Gas Adsorption and Moisture Adsorption

Under specific conditions,moisture in natural coal seams can beadsorbed in the pores of the coal matrix, reducing the amount of methaneadsorption sites and the effective area of the transport channels.This makes the prediction and evaluation of permeability in CBM exploitationmore challenging. In this paper, we developed an apparent permeabilitymodel of coalbed methane coupling viscous flow, Knudsen diffusion,and surface diffusion which considers the effects of adsorbed gasand moisture in the pores of the coal matrix on the permeability evolution.The predicted data of the present model are compared with those ofother models, and the results show good agreement, verifying the accuracyof the model. The model was employed to study the apparent permeabilityevolution characteristics of coalbed methane under different pressureand pore size distribution conditions. The main findings are as follows:(1) moisture content increases with saturation, with a slower increasefor smaller porosities and an accelerated non-linear increase forporosities greater than 0.1. (2) Gas adsorption in pores decreasespermeability, further weakened by moisture adsorption under high pressurebut negligible at pressures below 1 MPa. (3) Higher water saturationweakens gas transport capacity, especially with pore sizes smallerthan 10 nm. (4) The non-Darcy effect weakens with higher initial porosity,and neglecting moisture adsorption may significantly deviate fromactual values in modeling methane transport in coal seams. The presentpermeability model can capture the transport characteristics of CBMin moist coal seams more realistically and is more applicable forpredicting and evaluating the gas transport performance under dynamicvariations of pressure, pore size, and moisture. The results in thispaper can explain the transport behavior of gas in moist, tight, porousmedia and also provide a foundation for coalbed methane permeabilityevaluation.