EFFECT OF BEDDING ORIENTATION ON HYDRAULIC FRACTURE PROPAGATION IN SHALE UNDER UNIAXIAL COMPRESSION

Hydraulic fracturing is a well-stimulation technique and has been used as one of the key methods of extracting unconventional oil and unconventional gas resources. Uniaxial hydraulic fracturing experiments are carried out in order to study the effect of shale bedding plane orientation with respect to loading direction on breakdown pressure and hydraulic fracture propagation. Fracturing is completed on cylindrical samples containing a single blind axial borehole under simple uniaxial conditions with axial stress of 10MPa. The results show that the effect of bedding plane orientation with respect to principal stress on the breakdown pressure of shale is obvious. Under the same applied stress conditions, 90° sample returns the highest breakdown pressure, followed by 0° sample, while 30° sample returns the lowest. As the result, the breakdown pressure curve shows a U-like shape. In terms of fracture morphology, there are two types of fracturing patterns founded in samples. One type is the cracks extending along the loading direction and the other is the crack extending along the loading direction at first and then turning to the bedding plane direction. Fracture propagation by hydraulic fracturing is induced by both shear and tensile stresses, which means the failure modes are tensile splitting, sliding failure or both. The experiment result has a significant impact on understanding mechanism of fracture extension and provides references for fracture optimization design in exploiting shale gas.