Effect of damage zone around borehole on carbon dioxide injection promoted gas extraction in soft and low-permeability coal seam

Injecting external CO2 into soft and low-permeability coal seams can improve CH4 extacctinn efficiency, and also benefit in CO2 sequestration. However, the distribution law of damage zone around borehole in soft coal seam and its effect on the efficiency of CO2 injection promoted CH4 extraction are not clear. In this paper, a multi-physics coupling mathematical model considering damage effect is established for simulating the process of CO2 injection promoted CH4 extraction in soft and low-permeability coal seam. The distribution of damage zone and permeability around boreholes under different diameters and coal strengths are analyzed. The gas pressure and gas content in coal seam during CO2 injection promoted CH4 extraction when the model considered damage effect are compared with that of ignored. The results show that small borehole diameter corresponds to narrow damage zone around the borehole in coal seam. The damage zone expands with the increase of the borehole diameter. The damage zone increases exponentially with the borehole diameter, while decreases exponentially with the compressive strength of coal seam. The highest permeability in the damage zone has increased by nearly 300 times under the condition of simulated case. CH4 pressure around the extraction borehole reduces, and the reduction area expands with the increase of time. Compared with the result of considering the damage effect, the reduction area of ignoring it is smaller, and the reducing speed is slower. The integrated effect of CO2 injection and CH4 extraction leads to rapid decrease of CH4 content in coal seam near the boreholes. The CO2 pressure and content increase around the injection borehole, and the increasing area gradually extends to the whole coal seam. In soft coal seams, failure to consider the damage effect will underestimate the efficiency of CH4 extraction and CO2 sequestration, resulting conservative layout of boreholes.