Implementation of underground longhole directional drilling technology for greenhouse gas mitigation in Chinese coal mines
Many Chinese coal mines are highly gassy with abundant coal mine methane occurring in complex geological conditions and emitted as a greenhouse gas into the atmosphere during mining operations. Major accidents due to coal and gas outbursts and gas explosions also frequently occur leading to fatalities and financial losses. Conventional technologies using specialized roadways and extensive short drilling holes are unable to meet the demand of today's high production mines with strict safety requirements and environmental regulations such as high carbon emissions. Longhole directional drilling technology has been widely employed in gassy mines due to its obvious advantages such as the high proportion of in-seam boreholes, high efficiency of gas drainage for gas mitigation and reduced cost associated with production down time. The effects were analysed and evaluated based on the application of introduced directional drilling equipment and techniques in Chinese coal mines. These results indicate that the complicated conditions involving soft broken coal, high in situ stress and the presence of faults and folds, present ongoing difficulties and challenges. Longhole directional drilling technology has been successfully applied in some Chinese coal mines for gas exploitation and coal mine gas emission mitigation. Daning Coal Mine (located in the southern Qinshui coalfields) and Baijigou Coal Mine are respectively extracting a thick coal seam. Both coal mines have deployed underground longhole in-seam directional drilling technology for seam gas extraction prior to mining operations. This technology has successfully achieved a high rate of gas recovery therefore reducing mine exhaust ventilation air methane (VAM). However there are still several limitations with the application of longholedirectional drilling technology. These include drilling difficulties in complex conditions, insufficient drill rod strength, poor gas drainage efficiency in low permeability coal seams and the inconsistency of the local Chinese-made equipment. Further studies include systematic evaluation of the drilling applicability within different geological conditions, improving equipment reliability, enhancing the permeability through protective seam mining, hydraulic fracturing and longhole pre-splitting blasting. Applications of directional drilling technology in Chinese gassy mines will make a significant contribution to the overall reduction of greenhouse gas emissions from the coal mining industry
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