Experiment study on the porosity and permeability of low permeability coal by supercritical CO₂ temperature variation

In order to solve the universal problem of low permeability and low gas recovery in China's high gassy seams, combining the strong diffusion, dissolution-enhancement and other unique advantages of super-critical carbon dioxide. The three axis penetration experimental device was self-made, the macro increasing permeability experiment of coal was carried out after the action of super-critical carbon dioxide with different temperatures, the microscopic scanning imaging experiment of coal was conducted based on macro increasing permeability experiment. The results show:after the action of super-critical carbon dioxide with constant volume stress, constant pore pressure and different temperatures, the permeability is increased by one order of magnitude than that before the increasing permeability experiment, but in the super-critical temperature range of carbon dioxide, the permeability of coal shows a negative exponential change with the increase of temperature. After the action of super-critical carbon dioxide, the micro porosity of coal increases by two orders of magnitude than that before the increasing permeability experiment, the evolution rate of micro pore-fissure slows down with the increase of temperature, the porosity presents a negative exponential changing law with the increasing temperature. The macro-micro experimental data also show that the macro-permeability of coal increases with the increase of micro-porosity. During the process of increasing permeability by the super-critical carbon dioxide, pore pressure plays the main control function in increasing the permeability of low permeability coal seam.