Effect of Wellbore Orientation and Axial Stress on Hydraulic Fracture Initiation and Propagation in Lushan Shale with Inclined Bedding Planes

Shales with inclined beddings are frequently encountered in formation reservoirs as burial depth increases. However, limited research has focused on the influence of wellbore orientation and stress magnitudes when hydraulically fracturing inclined shale reservoirs. To close this research gap, a series of hydraulic fracturing experiments were conducted to investigate the effects of different wellbore orientation angles (α = 0–90°) and axial stress magnitudes (σ_v = 0–20 MPa) on hydraulic fracture initiation and propagation in inclined bedding shales. After fracturing, a 3D laser scanner is employed to characterize the fracture surface roughness, and three independent parameters (SD, θ_s, R_s) are used to quantitatively evaluate the roughness of the fracture surfaces. The results show that increasing the wellbore orientation angles (α) results in a first decreasing and then increasing trend of the fracture initiation pressure (P_i), breakdown pressure (P_b) and roughness of fracture surfaces. The wellbore orientation angle (α) of 45° is beneficial to reducing the P_i, P_b and roughness of the fracture surface. However, relatively low critical fluid pressures will facilitate the reduction of surface roughness under the impact of wellbore orientation. Increasing axial stress in inclined bedding shale reduces the fracture initiation pressure (P_i) and breakdown pressure (P_b) while enhancing the roughness of the fracture surfaces. In field fracturing, adjusting the wellbore inclined to the bedding (45° is preferred) and in the burial depth with a stress difference greater than 10 MPa benefit the improvement of hydraulic fracturing performance. These findings offer an essential reference for designing hydraulic fracturing in shale reservoirs.