Permeability enhancement mechanism of sand-carrying hydraulic fracturing in deep mining: A case study of uncovering coal in cross-cut

Abstract

Coal and gas outburst disasters are prone to occur within rock cross-cut coal uncovering in deep underground coal mines. To reduce the risk of coal and gas burst under rock cross-cut coal uncovering, a technique of permeability enhancement using hydraulic fracturing sand-carrying is proposed. Field test was conducted using HF sand-carrying approach, and the effect of permeability enhancement was investigated. The results show that the evolution of cracks has been subjected three stages: energy slowly increasing and crack initiation, damage localization of coal seam and gradually failure, and crack instability expansion and formation of a complex fracture network system. In addition, it is found that “water-sand” type injection of high-pressure fluid carried sand into HF cracks, which creates a support force on crack surface and prevents crack closure. A fully developed fracture network system with high flow conductivity capacity is created and thus increases gas permeability largely. Furthermore, a clearly increasing trend associated with the HF sand-carrying is found for the treated coal seam area. The permeability coefficient is about 21.5-30.5 times higher than that from the raw coal seam. A good prevention effect of coal and gas outburst is achieved using HF sand-carrying method.