A micro-macro coupled permeability model for gas transport in coalbed methane reservoirs

It is of great scientific and engineering significances to reveal the coupling mechanism of microscale and macroscale permeabilities in coalbed methane (CBM) reservoirs. In this paper, firstly, the characteristics of the pore-fracture system of coal matrix, such as gas content, permeability, connectivity, geometric size difference, and direction of micro-fracture development, are revealed. Secondly, a coupled permeability model is established, and the coal rank control mechanism on the transformation of microscale and macrosclae permeabilities is clarified. Thirdly, based on the microscale gravitational field model, the coal rank-pressure coupling influence mechanism on slippage/diffusion/jump of adsorbed gas in the surface control field is clarified. Results show that: (a) under a certain temperature, the microscale permeability and macroscale permeability of coal matrix have a statistical correlation, and the degree of coal metamorphism is the only variable controlling the transformation of micro-macro permeabilities. (b) With the increase of coal rank from 0.5% to 3.5%, the conversion coefficient of bulk gas permeability decreases by 15 orders of magnitude. (c) With the increase of coal metamorphism, the porosity and the density of oxygen-containing functional groups decrease, leading to the proportion of the area of adsorbed water film decreases at first and then increases.