Deep-hole directional fracturing of thick hard roof for rockburst prevention

Abstract

Thick hard roof in coal mines is usually a significant factor that induces dynamic disasters, such as rockburst. This study introduces a new technology called directional hydraulic fracturing characterized by cutting out an initial groove in the borehole and then injecting high pressure liquid to break the rock. The abutment pressure on the groove tip and fracture criterion is worked out based on the fracture mechanics taking fluid seepage into consideration. Computational simulations revealed that the vertical compressive stress changed to tension immediately after high pressure liquid injected into the fracturing hole, the concentration factor up to 5 that can easily rupture the roof and reduce the rockburst hazard at the same time. The seamless steel tubes are used instead of high pressure hose and conveyed into fracturing holes by geological drill to the designed locations, so as to break through the depth limitation and make the whole process automated. In situ applications at two longwall faces of LW6305 and LW5307 show that the depth can easily reach to 20m and the fracture radius more than 13m within half an hour, the efficiency and security are greatly improved. We can determine whether the roof is split by observing the pressure changes. The pressure of liquid during fracturing process can be divided into three stages: dramatically ascending, descending and stable, corresponding to crack initiation, propagation and dissemination, respectively. Drilling bits method and microseismic system validate prevention effects of this technique notably so that lead to a foundation for large scale popularization and application in China coal mine.