Theoretical model of coal swelling deformation under supercritical carbon dioxide action

In order to study the law of the coal swelling deformation in the process to enhance coal bed methane recovery of the low permeability coal seam by injecting supercritical carbon dioxide, this paper solved the problem that the classical adsorption theory established by absolute adsorption is no longer applicable in the supercritical state. According to Elliott-Suresh-Donohue state equation and the Simplified Local Density model, this paper can determine the excess adsorption with supercritical carbon dioxide. Based on the thermodynamic theory and the internal energy relationship of the adsorption system, the swelling strain of supercritical carbon dioxide can be obtained. Considering the influence of the compressive strain generated by supercritical pressure and the thermal expansion strain caused by supercritical temperature on the deformation of coal, the theoretical model of coal swelling deformation under supercritical carbon dioxide was established. This model included the thermal strain and the strain and absorption strain effect caused by pore pressure. The numerical solution of the theoretical model under the different conditions of temperature and pressure, relatively ranging from 303.15 K to 330.65 K and from 7.5 MPa to 9.5 MPa, was found out by programming. Comparing the numerical solution with the measured results of the independently-developed volume expansion strain measurement device with heat flow loading function, the results showed that firstly, the calculated data of the theoretical model coincides with the experimental data to significant extent; secondly, the theoretical model based on excess adsorption can better reflect the law of coal swelling deformation under supercritical carbon dioxide with different temperature and pressure conditions; finally, under the action of supercritical carbon dioxide, the coal volumetric strain, which varies with the supercritical temperature and pore pressure, presents S-shaped Logistic function pattern. Therefore, when the supercritical carbon dioxide is used to improve the permeability of coal seam, the appropriate supercritical temperature and pressure conditions should be selected according to the expansion curve to avoid the adverse impact of coal swelling on the permeability of coal seam. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.