Effect of Primary Fracture Orientation on CO2 Fracturing in Coal Seam Stress Relief

CO2 fracturing (CO2-Frac) is a novel technology for coal mine gas control, which is distinct from CO2 Enhanced Coalbed Methane, and has been applied to alleviate in situ stress concentration and to eliminate coal and gas outbursts in coal mines. However, the reasons for the greatly varying effects of CO2-Frac application among different regions remains largely unknown, and the influence of geological structures, particularly pre-existing fracture orientations, remains poorly understood. The equipment system of phase fracturing and permeability improvement of low-permeability coalbed methane and the gas phase fracturing and permeability improvement technology are studied and analyzed, and the engineering application is carried out in the head face of Xinyuan Coal Mine. This study conducted three CO2-Frac experiments in the Xinyuan coal mine in which borehole orientations were varied, with the primary fracture strike of coal seam #3 in the Shanxi Formation ranging from N3°E to N15°E. The characteristics of reservoir stress redistribution after CO2-Frac and its mechanism controlled by the orientation of primary fractures were explored based on the analysis of microseismic focal mechanisms. The results showed that (1) Both the fracturing section and the buffer section determined the stress relief effect of CO2-Frac. While the different experiments showed largely similar stress relief effects of the fracturing section, the effects of the buffer section greatly differed. (2) The microseismic events generated by the CO2-Frac in the borehole with an N–S orientation showed a more concentrated spatial distribution, with higher proportions of tensile and dip-slip events. (3) The range of the stress relief in the buffer section of the borehole with an N–S orientation exceeded those of the other sections. Further geological analysis revealed that higher stress relief was achieved in both boreholes with a N–S orientation and a smaller angle between the borehole direction and the primary fracture orientation (angle BF). An improved numerical calculation model that integrated fracture mechanics and gas reservoir engineering was used in this study; the result showed that an improved CO2-Frac effect was achieved under a BF angle of 0–21°, in good agreement with the field experiment results. The results of this study can help improve the effectiveness of CO2-Frac and reduce the occurrence of coal and gas outbursts.