Performance enhancement of horizontal underground-to-inseam gas drainage boreholes with double-phase-grouting sealing method for coal mining safety and clean gas resource

Underground to inseam (UIS) borehole gas drainage is of significance for improving mining safety and turning hazardous methane into a clean gas resource. However, in the horizontal UIS borehole sealing by adopting cement-grout-based sealing method, an upper gap frequently occurs in the sealing section after cement-grout solidification, which could potentially be an air-leakage path and consequently decrease drainage efficiency. Despite extensive previous investigations on borehole sealing, very few studies have been reported in addressing the upper gap issue to enhance drainage performance. In this study, the air-leakage flow models in concentric ring and off-center ring seal gap of horizontal borehole were established. Then by adopting new expansive composite sealing (ECS) material, a double-phase-grouting (DPG) sealing method was developed to seal the upper gap. Based on theoretical fundamentals of seal-gap leakage, procedures and explanations of DPG sealing method were discussed. Finally, coalmine field trials were conducted to examine effects of DPG sealing method. Results show that: (1) there are positive correlations between air leakage quantity into borehole and the gap width, borehole diameter, pressure differences and eccentricity ratio. While the air leakage amount is negatively related to fluid viscosity and sealing-section length; and (2) because of adopting ECS material and the second-phase grouting specially designed for upper gap sealing, DPG sealing method is capable of sealing upper gap well to improve sealing quality. It mainly reflects in increasing drained gas concentration and prolonging the period of high-methane-concentration drainage. In comparison with traditional sealing method, the mean gas flow with DPG sealing method averagely rises by 30%. Outcomes of this study are expected to provide an effective way to solve upper gap issue and enhance drainage performance of horizontal UIS boreholes.