Effect of temperature and pressure on energy parameters ofmethane supercritical adsorption

Coalbed methane mainly exists in supercritical adsorption state in high pressure and temperature environment. Pressure and temperature are the most important external factors that control the gas content of coal reservoirs. The changes in pressure and temperature indicate the breaking of initial equilibrium state of coal and methane adsorption system and the changes of system energy. In this paper, the difference between supercritical adsorption and subcritical adsorption was analyzed through the Gibbs equation using data from isothermal adsorption experiments. The effect of temperature and pressure on energy parameters of methane supercritical adsorption was analyzed from the aspects of adsorption thermodynamics and adsorption kinetics. The results show that the absolute adsorption volume of the coal-methane adsorption system is larger than the excess adsorption volume under supercritical conditions. The difference between the absolute adsorption volume and excess adsorption volume increases with the increase of equilibrium pressure and decreases with the increase of temperature. For a certain coal methane adsorption system, the relationship between adsorption potential and adsorption space remains the same under any pressure and temperature conditions. The increase of equilibrium pressure causes a decrease of adsorption potential and an increase of adsorption space,while the increase of temperature results in an increase of adsorption potential and a decrease of adsorption space. The diffusion mode of methane molecules in coals is Fick diffusion. The diffusion coefficient decreases with the increase of pressure and temperature as the average free path of gas molecules is affected by pressure and temperature. The adsorption activation energy of gas molecules is essentially kinetic energy, which is not controlled by pressure but is affected by temperature. The higher system temperature is associated with larger adsorption activation energy.