CO2 Gas Fracturing in High Dip Angled Coal Seams for Improved Gas Drainage Efficiency at Hashatu Coal Mine

Abstract

Effective gas drainage for the gassy coal seam is a technological measure to prevent excessive gas outing and gas related mine hazards. CO2 gas fracturing (CO2-Frac) is a new technology for coal mine gas control, and its effectiveness has been proven from many coal fields across China. CO2-Frac has been successfully implemented for gas drainage in single gently inclined coal seam, but it has not been experimented in steeply inclined multicoal seam. In this study, a pilot CO2-Frac project was carried out in Hashatu coal mine, a steeply inclined multicoal seam formation. To attain an effective gas drainage, three different types of CO2- Frac boreholes were drilled and experimented for a long-period gas drainage. One borehole type is the penetrating borehole through two seams, and other two types of in-seam boreholes include strike borehole and up-dipping borehole. With continuous field gas drainage and observation results, the gas drainage efficiency was found to be significantly improved in in-seam borehole and penetration borehole with CO2 fracturing, and the maximum methane flow rate increased 2.4 times for a given borehole length and the efficient drainage radius increased 3–4 times. In addition, the drainage effect of original drainage boreholes within the CO2-Frac circle influenced region was also significantly improved due to the borehole interference. The fractures radius of CO2-Frac in the penetrating borehole was larger than the two types of in-seam boreholes in the steeply inclined seam formation. This may be closely related to the developing property of bedding and cleat system in coal seam. This study provides both theoretical and practical evidence for effectiveness CO2-Frac in the area with steeply inclined angle and multicoal seam formations.