Multiscale Lattice Boltzmann Simulation of the Kinetics Process of Methane Desorption-Diffusion in Coal

The methane desorption and diffusion characteristics in coal are key factors affecting coalbed methane productivity. In this paper, we developed a lattice Boltzmann model for methane migration in the multiscale porous media of coal. In the simulation, the diffusion of methane in macropores/fractures is assumed to follow Fick's law, and that in the coal matrix is treated as Knudsen diffusion. In addition, the Langmuir adsorption kinetics equation is employed to describe the dynamic process of methane adsorption and desorption. The results indicated the following: (1) The specific surface area and fracture proportion of the coal will increase with the employment of hydraulic fracturing, which may prompt the gas desorption-diffusion efficiency. (2) The flow and diffusion of methane are closely related to each other. When the gas diffusivity is poor, the desorption-diffusion can be effectively accelerated by increasing the drainage intensity, but when the gas diffusivity is fine, the flow velocity has little influence on the methane desorption. In practice, if the estimated methane diffusion coefficient is below the order of 10^-5 m²/s, more attention should be paid to its accuracy; otherwise, the obtained results may have a large deviation from the real value. (3) In the typical range of average pore sizes of coal, gas desorption rate growth with the increase of pore size makes the low-rank coal more advantageous in exploitation due to its larger average pore size. With the decline of reservoir pressure, the low- and high-rank coals more easily desorb methane than medium-rank coal. (4) In the kinetic study of the coalbed methane desorption-diffusion process, the accuracy of the obtained results may depend on the adsorption and desorption rate constants if the desorption rate constant is less than 10⁶ 1/s.