Study on gas extraction efficiency using in-seam borehole method considering influence of plastic zone induced by borehole drilling

Abstract

Gas hazards pose a serious threat to the safety of coalmines. Gas extraction through in-seam boreholes is an important technical approach for preventing gas hazards and achieving rational exploitation of gas as a clean resource. Improper extraction design may result in inefficient gas extraction and create the risk of gas emission overrun, or even coal and gas burst during mining process. Gas extraction efficiency is associated with the permeability of the coal seam, which is affected by the integrity and porosity of the coal seam. The change in permeability in plastic zones induced by borehole drilling or roadway excavation is usually not sufficiently considered when designing a gas extraction scheme and predicting the gas extraction efficiency. In view of this, this research investigated gas extraction efficiency using in-seam borehole method considering influence of drilling/excavation induced plastic zone. Based on a gas extraction project in the Huainan mining area of China, site measurements were performed to test the pressure distribution characteristics along the gas extraction pipe. Subsequently, the radius of the plastic zone around the borehole was deduced based on elastic–plastic theory. Furthermore, a gas flow equation under extraction conditions was proposed, and the apparent permeability of the coal seam was analyzed by considering the plastic deformation induced by roadway excavation and borehole drilling. Finally, a three-dimensional model was built, and the gas flow performance was simulated by considering the influence of the plastic zone. The calculated radius of the plastic zone was compared with the analytical results. The evolution characteristics of the gas pressure and the effective extraction radius were obtained. In addition, optimized measures for the design of extraction boreholes are proposed.