Investigation of non-isothermal effect of cyclic carbon dioxide on the petrography of coals for coal mine methane recovery

Abstract

Abstract Effective extraction of methane from underground coal seams is vital to eliminate the potential resource waste and ensure mining safety. Enhanced coal mine methane recovery by injecting CO2 has been proved to be a promising method due to its phase-blasting and flooding effects. The occurrence of non-isothermal kinetics of CO2 flowing in the porous coal media might have some impacts on the pore structure and matrix integrity. A high-pressure and low-temperature experimental system was established to simulate the introduction of liquid CO2 (LCO2) to the coals, and low-field nuclear magnetic resonance and velocity testing device were used to characterize the pore structure change and the crack evolution, and the related elastic parameters were also tested by uniaxial compression test. The T2 spectrum curves showed that the pore structure changed greatly under the impact of LCO2, and the affecting cycles was positively correlated to the pore volume change and was negatively correlated to the wave velocity of coals, which manifested that the cyclic LCO2 effect had positive impacts on the crack volume increase and the improvement of the crack connection. The peak strength σmax of coals all decreased as the LCO2 affecting cycles increased, and the larger the cycles, the greater decrease of coal’s strength. The increased pore volume and the decreased coal strength reflected that the non-isothermal effect of cyclic LCO2 could produce some flow channels for methane seepage by destroying the matrix structure and weakening the intensity. Moreover, a conceptual destruction model was established to explain the fracturing mechanism of non-isothermal effect of LCO2 cyclic injection. The results provide some significant guidelines to enhance methane recovery by producing crack networks and decreasing the coal strength.