Effect of Trajectory Deviation of Borehole Group on Gas Extraction and Its Underlying Mechanism

In the process of mine gas extraction, borehole trajectory deviation could result in the formation of gas extraction gap zones, which significantly lower the efficiency of gas extraction. To study the influencing mechanism of the deviation of a borehole group on the gas extraction effect, a gas extraction model from cross-measure boreholes was established based on field monitoring and statistical data of deviated boreholes in the floor gas drainage roadway of the haulage gateway in the 11,308 working face in Sangbei Coal Mine, which was used to quantify and characterize the influence of the borehole deviation degree on the gas extraction effect. On this basis, the evolution of the area that reached the gas extraction standard was compared under conditions with and without deviation of a borehole group. The underlying mechanism of deviation of the borehole group on gas extraction was determined. It is shown that the wider the deviation distance is after the same gas drainage duration, the smaller is the effective gas extraction area. The weakening degrees of gas extraction based on the elimination time of blank zones and based on the effective gas extraction area were defined. The weakening degrees can be used to measure the influences of the borehole deviation degree on the gas extraction effect. As the borehole deviation degree increases, the deviated boreholes exert a growing influence on the gas extraction effect, both in spatial scope and in the temporal dimension. The pressure-relief effects of deviated and normal borehole groups in the gas extraction process both exhibit three-stage characteristics (slow, rapid, and slow stages). The deviated borehole group is slightly superior in the initial stage, but its gas extraction effect is poorer in the middle and late stages, indicating delayed gas extraction. On the whole, the deviated boreholes weaken the gas extraction effect. The appearance of blank zones in the middle stage is the main cause of the deviated boreholes delaying and weakening the gas extraction effect. The research results of this study can provide theoretical support for designing borehole spacing and angle in gas extraction engineering.