Methane desorption as a source of gas flow in coal beds

Desorption exhibited by the diffusion from micropores (of the matrix) to macropores (fractures) system occurs in dual porosity coal system due to the drainage process in coal beds. Gas in the micropores, close to the fractures is in equilibrium with a free gas, volume of which depends on the pressure in micropores. This dependence fits in with Langmuir's theory of adsorption isotherms. To estimate the volume of gas adsorbed in the micropores far from fractures, a model of quasi-steady state has been used. This model is based on the first-order kinetics equation. The author demonstrates how to interpret the desorption source formula of mass balance in physical terms. The role of diffusivity in delayed gas supply is pointed out. The desorption source in the micropore gas flow model is presented in the characteristic form of integral. Numerical integration in successive time steps of coal bed methane production has been performed. In this way the formula for the discharge of adsorbed gas has been obtained.