Abstract
Hydraulic fracturing is an effective technology for enhancing the extraction of reservoir methane, as proved by field experience and laboratory experiments. However, unlike conventional reservoirs, coal seams had high stress sensitivity and high anisotropy. Therefore, the efficiency of hydraulic fracturing in coal seams needs to be investigated. In this study, hydraulic fracturing was performed at Nantong mine in the southeast Sichuan basin, China. The field investigation indicated that the hydraulic fracturing could significantly enhance the methane extraction rate of boreholes ten times higher than that of normal boreholes in one of the minable coal seams (named #5 coal seam). The performance of hydraulic fracturing in three districts revealed that compared with south flank, the fluid pressure was higher and the injection rate was lower in north flank. The methane extraction rate of south flank was inferior to that of north flank. It indicated hydraulic fracturing had less effect on #5 coal seam in south flank. Moreover, the injection of high-pressure water in coal seams could also drive methane away from boreholes. The methane extraction rate of the test boreholes demonstrated the existence of methane enrichment circles after hydraulic fracturing. It indicated that hydraulic fracturing did act on #5 coal seam in south flank. However, due to the high stress sensitivity of coal seams and the high geo-stress of south flank, the induced artificial fractures in #5 coal seam might close with the decline of the fluid pressure that led to a sharp decline of the methane extraction rate.
Highlights
Methane disasters are the most threatening to safety in underground mining [1,2]
The original high methane pressure in coal seams might lead to a coal and gas outburst
Low permeability restricts the extraction of methane in coal seams
Summary
Methane disasters are the most threatening to safety in underground mining [1,2]. The original high methane pressure in coal seams might lead to a coal and gas outburst. Coal seam methane is a clean energy resource. The extraction of coal seam methane can prevent methane disasters and can satisfy the growing demand for clean energy. The most minable coal seams in China have low or ultralow permeability, rendering it impossible to achieve the same permeability as in other countries (such as the United States and Australia) [3,4]. Low permeability restricts the extraction of methane in coal seams
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