Well Orientation Effects on Hydraulic Fracturing in Tight Sandstone: A True Triaxial Experimental Study

To clarify the controlling effects of the well deviation angle and azimuth angle on the breakdown pressure, propagation morphology, and fracture-network complexity of hydraulic fractures in tight sandstone, true triaxial hydraulic fracturing physical simulations were systematically conducted on Chang 7 Member sandstone from Yanchuan County, Ordos Basin, under different well deviation and azimuth angles. By combining injection-pressure monitoring, surface fracture-morphology observation, and three-dimensional laser scanning, the breakdown pressure, propagation path, surface roughness, fractal dimension, and overall complexity of the fractures were quantitatively analyzed. The results show that, at an azimuth angle of 90°, the breakdown pressure of the sandstone generally decreases as the well deviation angle increases from 0° to 90°, dropping from 19.125 MPa to 13.569 MPa, indicating that horizontal wells are easier to fracture. At a well deviation of 60°, the fracture is more prone to deflect and communicate with natural weak planes, yielding the highest overall complexity (f = 1.629). For horizontal wells under normal-faulting stress, the breakdown pressure decreases as the azimuth angle increases; the lowest breakdown pressure (12.933 MPa) is obtained when the wellbore is drilled along the maximum horizontal principal stress (σH), and the highest (18.310 MPa) when parallel to the minimum horizontal principal stress (σh). When the azimuth angle is 30°, both the fracture-surface roughness (Sa = 2.037 mm, Sq = 2.691 mm) and the overall complexity (f = 1.679) reach their maxima, which is most favorable for forming tortuous, rough, and complex fracture networks. The fractal dimension of the fractures varies little across the tested conditions (2.0339–2.1379), indicating that it is mainly controlled by the intrinsic heterogeneity of the rock. The research results can provide an experimental basis for the optimization of horizontal-well trajectories and fracturing-parameter design in tight sandstone reservoirs.