Risk assessment of dynamic disasters induced by gas injection displacement in coal seams

Abstract

Gas injection displacement technology not only helps to increase gas production, but also serves to store greenhouse gases by reducing CO2 emissions. However, coal seam dynamic disasters caused by gas (CO2) injection displacement are rarely reported. In order to explore the change in coal seam outburst risk after gas injection, the indexes (ΔP) of initial velocity of gas diffusion and the isothermal adsorption curves of coal with different metamorphic degrees were experimentally measured under different concentration ratios of CO2/CH4 mixed gas, and the experimental process of gas injection displacement and the change in pore structure were also investigated. The experiment results indicate that the ΔP of CO2 is nearly three times of that of CH4. Therefore, the accuracy of safety situation judgment of a coal seam would be reduced if outburst risk of the coal seam is only assessed through the ΔP in the pure CH4 environment. The results of isothermal adsorption experiment show that the gas adsorption capacity of three kinds of coal increases with the rise of the CO2 concentration in the mixed gas. The results of gas injection displacement experiment reveal that CO2 plays a significant role in replacing CH4, but a large amount of CO2 has been adsorbed into the coal seam before CH4 can be completely desorbed, which in turn raises outburst risk of the coal seam. After the displacement, the internal pore structures of the three kinds of coal samples become tinier and tinier. Finally, a new comprehensive index K′=αBET×ΔP/f was established to assess the risk of dynamic disasters in a coal seam. Compared with the single index ΔP, the new comprehensive index K′ can more accurately grasp the possibility of outburst danger in a coal seam subjected to gas injection displacement.