Study on the Effect of Fluid Viscosity and Injection Rate on the Geometry of Hydraulic Fractures Penetrating Through Laminae Planes and the Proppant Distribution in Deep Shale Oil Reservoirs

To study the geometry of hydraulic fractures and the proppant distribution under different injection rates and fluid viscosity, small-scale true triaxial sand fracturing experiments were conducted on core samples of the Fengcheng Formation shale oil reservoir in Mahu Sag, Junggar Basin. Then, based on the proposed elastic–plastic shear constitutive model of laminae planes, a three-dimensional fracture propagation numerical model considering proppant distribution was established using the discrete element numerical simulation method. The experimental results show that penetrating propagation is not easy to occur. However, the laminae planes are more likely to be activated, forming an artificial fracture morphology of “multi-planes opened and unilateral penetrated” with small fracture aperture. In contrast, under high injection rates, hydraulic fractures tend to “step-like” penetrate through the laminae planes in morphologies of “single-plane opened and penetrated” or “multi-planes opened and penetrated” with larger fracture aperture. Under the condition of high fracturing fluid viscosity, the hydraulic fractures tend to penetrate through the laminae planes vertically, forming a morphology of “vertical penetration without opening plane”. The laminar fracture propagation is limited. High viscosity is beneficial for maintaining a high propagation pressure. The simulation results show that length and height extension of the primary fracture are hindered by the laminae planes at a low injection rate. The laminar fracture area is dominant, and the fracture aperture is small. Most of the proppant is distributed in the primary fracture, and the proppant accumulation near the wellbore makes it easy to form a sand plug. A high injection rate and a high viscosity are beneficial for penetrating propagation through the laminae planes, as well as increasing the fracture aperture and the propped fracture area. For Mahu deep shale oil, the fracturing strategy of fracture initiation with high viscosity, fracture extension with medium viscosity, and penetrating laminae planes with high viscosity under a high injection rate was proposed.