Study on effective extraction radius of bedding borehole under the impact of normal fault

The reasonable effective extraction radius of borehole is the key to ensure gas extraction effect. However, the research of effective extraction radius mainly focuses on the measurement method of effective extraction radius of borehole, and there is a lack of research on the effective extraction radius of borehole under special geological conditions of normal fault. Taking a coal working face in Guizhou as the engineering background, the fluid-solid coupling model is established, and the variation law of effective extraction radius of bedding borehole without fault impact is analyzed by means of mutual verification between numerical simulation and borehole gas extraction method. The results show that when the gas is extracted for 120 days, the effective extraction radius of the numerical simulation is 3.10 m, and that of the field test is 2.93 m. The field results are basically consistent with the numerical simulation results, which verifies the reliability of the fluid-solid coupling model. By using multi-physics field coupling simulation software, the variation law of effective extraction radius of bedding borehole under the impact of normal fault is analyzed. The results show that within a certain extraction time range, when the borehole is at a certain distance from the normal fault, the gas pressure curve will have a hump-like distribution, and the peak interval is about 3.5 m. The effective extraction radius of the borehole near the normal fault increases with the increase of the distance from the fault. The impact of the normal fault on the effective extraction radius of the borehole is within 70 m from the fault. Combined with the analysis results under the impact of normal faults, the spacing of gas drainage boreholes near the normal fault is arranged in sections under the premise of ensuring the extraction effect and not prolonging the extraction time. Within 30 m from the fault, the boreholes are densely arranged, and the spacing between the boreholes should not be greater than the effective extraction radius (1.83 m) when the distance from the fault is 10 m. Within the range of 30-70 m from the fault, under the condition of ensuring safety, the spacing of boreholes can be appropriately increased. After 70 m from the fault, the spacing can be gradually restored to the borehole spacing layout of bedding boreholes without the impact of the normal fault.