Measurement and modeling of coal adsorption-permeability based on the fractal method

Abstract

With the development of coal-bed methane (CBM) mining technology, deep coal-beds have become the target of CBM development. In order to simulate changes in coal adsorption and permeability characteristics caused by increased mining depth, low-temperature liquid nitrogen adsorption tests, isothermal adsorption tests and triaxial seepage tests under rising gas pressure have been conducted. In this study, a fractal Langmuir model that considered the effects of excess quantity and temperature was constructed. From this premise, a fractal permeability model for coal that considered effective stress and gas pressure was constructed. The results indicated that under low pressure conditions, excess adsorption quantity in coal increased under increasing gas pressure. As gas pressure increased, the density during the gas free phase gradually increased. This revealed a difference between excess coal adsorption, and a gradual increase in absolute adsorption. Secondly, as the quantity of gas adsorption gradually increased, the isosteric heat in the adsorption of coal gradually decreased. The results revealed that the coal surface was rough. We used fractal dimension to represent the pore distribution characteristics on the coal surface. With an increase in the fractal dimension D, the adsorption constants a and b of coal showed an upward trend. Based on this, the fractal Langmuir model that considered the effects of excess quantity and temperature had a better applicability than other models. In addition, with a rise in gas pressure, the permeability of coal demonstrated a tendency to decline sharply at first and then to level off. However, the permeability of coal when filled with CO2 gas was always lower than that when filled with CH4 gas. Compared with Lu's et al. research, and Shi and Durucan's (S&D) model, this new permeability model had better applicability in terms of calculation accuracy or parameter rationality.