The experimental study of shale laminae influence on the mechanical properties and brittle failure of shale oil reservoirs

The laminae in shale oil reservoirs are relatively well-developed, exhibiting strong microscopic heterogeneity. The fabric, lithology, and mechanical properties of the shale change rapidly along the vertical direction. In order to investigate the influence of laminae on the mechanical properties and brittle failure of shale that is of great significance for the efficient development of shale oil, the compression experiments were conducted on the laminated shale and the layered shale of the Ganchaigou Formation. The results showed that the rock fabric has a greater impact than lithology on the mechanical properties of shale. The compressive strength, cohesive force, and internal friction angle of the laminated shale are less than those of the layered shale. Compared to the layered shale, the laminated shale has more developed natural micro-cracks with higher anisotropy and higher stress sensitivity for wave velocity and permeability. Cracks in laminated shale tend to propagate along the laminae, with smaller crack aperture and crack volume caused by the sliding of interfaces and frictional slips of natural cracks. As the laminated shale after failure has a large number of cracks with smaller crack aperture, the specific surface area of cracks is larger which will be advantageous for oil recovery. The stress at dominate dilatation of the laminated shale are very close to the compressive strength, indicating that the laminated shale are more brittle. Although the laminated shale exhibits strong micro-heterogeneity, the weak plane effect is not significant, and its mechanical properties are relatively homogeneous at a macro level. It is recommended to mainly use a “gel+slippery water” composite stimulation strategies for the hydraulic fracturing of the laminated shale. First, high viscosity gel is used to support the micro-cracks in the laminated shale, and sand is carried to increase the effectiveness of the cracks. Then, slippery water is used to increase the complexity of the fracture networks.