Experimental study on the influencing factors of hydraulic fracture initiation from prefabricated crack tips

In this work, large-scale true triaxial tests were conducted with sandstone specimens to investigate several factors that determine whether hydraulic fractures can propagate from the tips of axial prefabricated cracks (pre-cracks). Specifically, we investigated the influence of pre-crack inclination angles on hydraulic fracture initiation locations and how the critical pre-crack angle varies with different confining stresses, fluid viscosities, and leak-off effects. The experimental results show that (1) the critical pre-crack angle declines with an increase in the horizontal stress difference. When the viscosity of the fracturing fluid is maintained at 70 mPa.s, as the horizontal stress difference increases from 2 MPa to 3 MPa to 4 MPa, the critical pre-crack angle decreases from 90 degrees to 75 degrees to 70 degrees. (2) The critical pre-crack angle increases with an increase in the fracturing fluid viscosity. With a 3 MPa horizontal stress difference, the critical precrack angle increases from 60 degrees to 70 degrees to 75 degrees as the viscosity of the fracturing fluid increases from 1 mPa.s to 40 mPa.s to 70 mPa.s. Under the condition that the horizontal stress difference is set to 4 MPa, as the fracturing fluid viscosity increases from 1 mPa.s to 40 mPa.s to 70 mPa.s, the critical pre-crack angle increases from 50 degrees to 65 degrees to 70 degrees. (3) After waterproof treatment of the boreholes and pre-cracks, the critical pre-crack angle increases from 75 degrees to 80 degrees to 85 degrees as the viscosity of the fracturing fluid increases from 1 mPa.s to 40 mPa.s to 70 mPa.s under the condition that the horizontal stress difference is fixed at 4 MPa.