RESUMO
The occurrence of coalbed methane adsorption-desorption hysteresis has been widely observed, but a unified understanding of its mechanism is lacking, and the factors affecting its degree are unclear. This study introduces a microscale LB model for gas diffusion-adsorption-desorption in porous media that also accounts for the adsorption-desorption hysteresis effect. The accuracy of the model has been validated using previous experimental data, and the primary controlling factors of adsorption-desorption hysteresis were analyzed. The findings are as follows: (1) In the process of methane diffusion-adsorption-desorption, Knudsen diffusion dominates in micro- and mesopores, while viscous flow prevails in macropores; our model can adaptively adjust gas transport regimes across a broad range of pore sizes and pressures. (2) The desorption amount and rate are close relative to the correction factors α and ß. A higher α value corresponds to greater initial adsorption as well as longer desorption time, whereas a lower ß value implies weaker desorption capacity and a slower desorption rate. (3) Pore size can affect gas diffusion-adsorption-desorption kinetics, where larger pore size corresponds to efficient gas diffusivity; when r < 10 nm, the desorption process is mainly controlled by the desorption rate. Overall, this study has offered new insights into the mechanism behind methane adsorption-desorption hysteresis at the microscale, identified primary controlling factors of methane diffusion-adsorption-desorption process, and provided a foundation for numerical simulations and experiments related to the adsorption-desorption hysteresis.
RESUMO
Under specific conditions, moisture in natural coal seams can be adsorbed in the pores of the coal matrix, reducing the amount of methane adsorption sites and the effective area of the transport channels. This makes the prediction and evaluation of permeability in CBM exploitation more challenging. In this paper, we developed an apparent permeability model of coalbed methane coupling viscous flow, Knudsen diffusion, and surface diffusion which considers the effects of adsorbed gas and moisture in the pores of the coal matrix on the permeability evolution. The predicted data of the present model are compared with those of other models, and the results show good agreement, verifying the accuracy of the model. The model was employed to study the apparent permeability evolution characteristics of coalbed methane under different pressure and pore size distribution conditions. The main findings are as follows: (1) moisture content increases with saturation, with a slower increase for smaller porosities and an accelerated non-linear increase for porosities greater than 0.1. (2) Gas adsorption in pores decreases permeability, further weakened by moisture adsorption under high pressure but negligible at pressures below 1 MPa. (3) Higher water saturation weakens gas transport capacity, especially with pore sizes smaller than 10 nm. (4) The non-Darcy effect weakens with higher initial porosity, and neglecting moisture adsorption may significantly deviate from actual values in modeling methane transport in coal seams. The present permeability model can capture the transport characteristics of CBM in moist coal seams more realistically and is more applicable for predicting and evaluating the gas transport performance under dynamic variations of pressure, pore size, and moisture. The results in this paper can explain the transport behavior of gas in moist, tight, porous media and also provide a foundation for coalbed methane permeability evaluation.
RESUMO
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.
