Stress-unloading and gas migration improvement mechanism in the soft and hard interbedded coal seam using directional hydraulic flushing technology

Abstract

To enhance gas drainage in the soft and hard interbedded (SHI) coal seam, it’s necessary to unload the in-situ stress and improve its gas migration performance. In this research, a directional hydraulic flushing (DHF) technology was carried out. The stress-unloading and gas migration improvement mechanism was analyzed through numerical simulation, and systematic engineering tests were conducted to verify the gas drainage effect. The results show that the improvement of gas migration performance in the SHI coal seam is caused by a combined effect of seepage-improving and diffusion-improving. After DHF, stress-unloading and plastic failure could be achieved both in the soft coal (SC) sublayer and in the hard coal (HC) sublayer. However, the gas diffusion capacity improves significantly in the SC sublayer, while the gas seepage capacity improves notably in the HC sublayer. Meanwhile, the stress-unloading and gas migration improvement effect improves with the flushing radius and the thickness of the SC sublayer. Besides, after adopting the DHF technology, the gas drainage effect improved markedly. The borehole number dropped by 49%, the gas drainage ratio increased from 26.0% to 48.2%, and the average coal roadway excavation speed increased from 2.4 to 5.6 m/d.

Keywords

• Hydraulic flushing
• Stress-unloading
• Plastic failure
• Permeability-increasing
• Gas migration