Hydraulic fracturing characteristics of laminated shale and its enlightenment to fracture pressure

Hydraulic fracturing is a common reservoir reconstruction technology in shale gas production. Accurate prediction of hydraulic fracture break pressure is of great significance to on-site fracturing construction. In order to explore the influence mechanism of shale bedding on hydraulic fracturing, taking the black shale in Changning area of Sichuan Province as the research object, the Brazilian splitting tensile strength test of the laminated shale was carried out. The hydraulic fracturing test of shale with different bedding angles was carried out using GCTS rock mechanics testing machine. The extension morphology and the roughness characteristics of the shale fractures were characterized combined with X-ray CT and three-dimensional scanner, and the fracturing pressure in hydraulic fracturing of shale was analyzed. The results shown that, (1) The tensile strength of shale decreased firstly and then increased with the variation of bedding angle, and the obvious anisotropy was exhibited. (2) The bedding had a great influence on the shale fracture pressure, the change of fracturing pressure and tensile strength of shale hydraulic fracturing under different bedding angle shown a similar trend, with the minimum at 30° and the maximum at 90°. (3) The bedding planes had a little influence on fracture extension of hydraulic fracturing, mainly single fracture produced by the tensile failure, and roughness of the fracture surface was higher and independent of the direction of bedding. (4) The existence of the plastic zone in the fractured wellbore was determined by numerical simulation, which also explained the abnormally high fracture pressures observed in the test, and suggested that there was a of hydraulic fracturing. Shales can be completely broken only after a very short time of crack inoculation. Fracture pressure was affected by the certain characteristic length of critical fracture initiation of shale with different bedding angle, and the new model can better predict the fracturing pressure of hydraulic fracturing in shale. The research results are of great significance for understanding the mechanism of fracture initiation and extension in deep shale, and the design of hydraulic fracturing construction.