Gas transport through coal particles: Matrix-flux controlled or fracture-flux controlled?

Abstract

In laboratory measurements, methane ad-/de-sorption behavior on coal is known to be directly related to the particle size. As expected, coal exhibits a relatively high initial desorption rate for smaller coal particles which results in a different gas desorption volume and pressure curves from platter ones for larger coal particles. Both fracture-dominated and matrix-dominated flow theories had been proposed to explain the difference in shapes of desorption curves. These two theories, however, are contradictory to each other and neither of them is completely convincing. Based on the newly developed relationship of apparent diffusion coefficient and apparent permeability, this work uses a dual-permeability concept to explain the different shapes of desorption curves. Numerical simulation solutions indicate that the switching dominance of the fracture and matrix permeability systems produces variable desorption curve shapes. With continuing decrease of coal particle size, the flow will gradually change from fracture-dominated to matrix-dominated mode. The critical apparent permeability ratio dividing the domination of these two systems is in the order of ~10−2, in contrast to the initial hypothesis that if one system dominates the overall gas flow, its permeability must be smaller than that of the other. As particle radius decreases, this parameter first increases and then remain at a certain value. At last, the simulated desorption curves were validated with laboratory desorption experimental data.