Advancing CMM drainage, quality, and utilization with horizontal directional-drilled boreholes in Duanshi Coal Mine, southern Qinshui Basin, China

Abstract

The Duanshi coal mine is a private, small-size producer and one of >1000 Chinese underground coal mines that pre-drain CMM prior to mining to prevent explosions, capture, and utilize fugitive gas. The coal mine opened in 2008 and employed in-seam auger-drilled boreholes to capture CMM and use as required by the China Emission Standard to curb CH4 emission, which is one of the most potent GHG. As drilling technology advanced, the coal mine used in-seam horizontal directional drilling in 2017 to boost capture of higher quality CMM for utilization but was hampered by technical and geological challenges. Deformation of Pennsylvanian–Permian coal-bearing rocks in the southern Qinshui Basin restricted horizontal directional drilling in the coal mine. The target No. 3 coal seam of the Permian Shanxi Formation is tightly folded into an asymmetrical, northward plunging syncline in the mine area, which requires designing innovative strategies for implementing drilling. Also, the “tight reservoir” (impermeable) property of the No. 3 coal makes recovery of high-volume and -quality CMM difficult. These technical and geological conditions were revealed as causes of the increasing trend in China's total CH4 emission, which is 1.1 ± 0.4 Tg CH4/yr from 2010 to 2015, specifically attributed to underground coal mining (Miller et al., 2019). The increase indicates that CMM drainage and utilization regulations have not curbed growing CH4 emission from underground coal mines. We conclude that our study of advancing CMM drainage technology and use in the Duanshi coal mine could serve as an example for China's small and medium mine operators of underground coal mines that suffer similar technical and geological challenges.