Investigation on damage evolution law of anisotropic shale at different hydraulic pressures

Abstract

Shale gas is a significant energy worldwide, and hydraulic fracturing is an essential method for shale gas extraction. To understand the effect of drilling direction and hydraulic pressure on shale failure property in hydraulic fracturing, Brazilian splitting and acoustic emission tests are applied to shales with 5 layer orientations (θ = 0°, 30°, 45° 60° and 90°) and 4 hydraulic pressures (p = 0, 5, 10 and 15 MPa). The results show that with the p increase, the tensile strength, absorbed energy and crack deviation distance of shale decrease by 19.32–42.03%, 12.13–49.21% and 12.23–42.44%, respectively. The high hydraulic pressure can weaken the shale mechanical strength, then hydraulic cracks form more easily. When θ = 60°, the layer shear slip is the most aggravated, average tensile strength of shale is also the smallest, only 6.14 MPa. The stability of shale AE energy can be characterized by avalanche events. The larger the power exponent s′ of avalanche event, the low AE energy released more from shale, which means the more secondary crack propagation. The shale with θ = 60° and p = 15 MPa has the maximum s′ value of 1.835. To ensure the propagability and stability of hydraulic fracture at the same time, p = 10 MPa and θ = 45°, were selected as the reference technical conditions for hydraulic fracturing.