Abstract
Methane flow in coal is associated with the content of both organic matter (OM) and inorganic matter (IOM). Coal matrix contains nanopores ranging in size from a few to hundreds of nanometers, which leads to a non-Darcy effect where the measured permeability of a gas (apparent permeability) is higher than that of a liquid (intrinsic permeability). In this study, a generalized Lattice Boltzmann model (GLBM) is employed for gas flow through the reconstructed coal matrix consisting of OM, IOM, and fractures. The apparent permeability model is proposed to calculate the total flow flux accounting for multiple transport mechanisms including viscous flow, slip flow, transitional flow, and the Knudsen diffusion. The impact of effective pore radius and gas surface diffusion on permeability in the gas adsorption-desorption process is also considered in the model. What’s more, the weighting factors are adopted to adjust the contribution of the viscous flow and the Knudsen flow. The effect of total organic/inorganic content and the development of fractures on the apparent permeability of the reconstructed coal matrix is also studied. It is found that the apparent permeability is extremely low when a fracture is nonexistent, and varies almost linearly with the total organic/inorganic content. A fracture plays a significant role in determining apparent permeability and the velocity distribution of the coal matrix.
Highlights
Permeability is a key factor in the exploitation of coalbed methane, which directly determines whether the target coal reservoir is valuable for mining
(2) The fully coupled viscous flow, slip flow, transition flow, and the Knudsen diffusion were considered in the simulation, and the contribution of these flow mechanisms to the total flow flux was automatically adjusted by the weight factor
The results show that the content of organic matter (OM) and inorganic matter (IOM) in coal is markedly different, and IOM has a wide range of contents
Summary
Permeability is a key factor in the exploitation of coalbed methane, which directly determines whether the target coal reservoir is valuable for mining. The model only considered the effect of gas slippage, and other involved flow mechanisms were not taken into account Based on this model, Xiong et al [18] proposed a fully coupled, free and adsorptive phase transport permeability model taking into consideration the impact of the effective pore radius and the non-Darcy gas flow. Xiong et al [18] proposed a fully coupled, free and adsorptive phase transport permeability model taking into consideration the impact of the effective pore radius and the non-Darcy gas flow This model linearly superimposed the flow flux contributed by gas slippage and surface diffusion, and took into account the effect of adsorbed gas molecules on the pore space, but did not regard the influence of the Knudsen diffusion. This model included the following sections: (1) The generalized Lattice Boltzmann method was employed to solve the generalized Navier-Stokes equations, which was applied to the calculation and prediction of local permeability at representative elementary volume (REV) scale. (2) The fully coupled viscous flow, slip flow, transition flow, and the Knudsen diffusion were considered in the simulation, and the contribution of these flow mechanisms to the total flow flux was automatically adjusted by the weight factor. (3) The impact of adsorbed gas molecules on the pore space and the contribution of surface diffusion to the total flow flux were taken into consideration. (4) The separate evolution of the local permeability of OM and IOM in the coal matrix was taken into consideration
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