The roof composed of hard and thick strata cannot cave in time with advance of working face in underground coal mining, which leads to extensive overhanging or unsupported areas in the gob and strong loading on supports in working face and entries. A successful technique for hard roof control is recognized as directional hydraulic fracturing through creating directional fractures in roof to weaken its integrity. A comprehensive investigation into stress distribution around borehole and fracture propagation patterns was carried out in this paper through theoretical analysis, numerical modeling and field test. Finite element code ABAQUS was employed for the analysis of stress distribution at transverse notching tip and effect of transverse notching on fracturing initiation condition and orientations, which was verified further by the field test. Transverse notching plays an important role on borehole sealing, fracturing propagation in large scope and fracturing at freewill section in boreholes.
Hydraulic fracturing, the propagation of fractures in a rock layer by a pressurized fluid, is a technique widely used to release petroleum, natural gas (including shale gas, tight gas and coal seam gas) or other substances for extraction (Charlez, 1997). Its applications have extended to a wide range of areas related to hard rocks of coal mine including hard roof rockburst control and acceleration of massive hard roof caving in longwall gob. However, the growth of fractures and, ultimately, the treatment effectiveness depend on the evolution of the fracture path in near-borehole region. Thus, transverse notching, perpendicular to borehole axis, is frequently created on the internal walls of boreholes before fracturing to improve the fracturing treatment, and then inflatable straddle packers, generally, are employed for hole sealing in the selected section where the fracturing-fluid is pumped at a high pressure. The procedure is commonly known as directional hydraulic fracturing as shown in Figure 1.
It's clear that the fracturing pressure first balances the reservoir pressure or pore pressure, then overcomes the compressive borehole stress, and tensile stress concentration will appear preferentially at the notching tip as pumping continues. Fractures are initiated along the tip as concentration stress reaches the tensile failure stress of the rock medium.