Numerical Study on Fracture Propagation in Shale Oil Reservoir with Interbedded Lithologies

The Sichuan Basin is rich in continental shale oil resources. The reservoirs of the Jurassic Lianggaoshan Formation are interbedded with shale and sandstone. The stress variation of the reservoir is large along the vertical direction, and the lithological interface between sand and shale is highly developed, resulting in a complex pattern of fracture propagation, which may suppress the fracture height and affect the fracturing effect. This paper establishes a numerical model for fracture propagation based on particle based discrete element method and three-dimensional discrete lattice theory, which can simulate the fracture propagation morphology under interbedded lithologies conditions. The simulation results are in good agreement with the results of true triaxial fracturing experiments. Several typical shale oil reservoir models with interbedded lithologies were extracted based on the actual geological conditions, and the fracture propagation under different geological parameters were studied taking the sand-interbedded shale as an example. The modeling results indicate that the smaller the interlayer stress difference, the larger the interlayer tensile strength difference, the smaller the interlayer Young’s modulus difference, and the higher the vertical-horizontal stress difference, the larger the internal friction angle of interface, the greater the interface cohesion, and the easier it is for hydraulic fracture to penetrate through the layers, both achieving adequate stimulation of shale and sandstone reservoirs. The coefficient of variation method was used to analyze the influencing factors of fracture propagation in sand-interbedded shale, and the importance of each influencing factor was clarified as follows: vertical-horizontal stress difference > interlayer stress difference > interlayer tensile stress difference > interface cohesion>interlayer Young’s modulus difference > internal friction angle of interface. The numerical study can deepen the engineers’ understanding of the factors affecting the fracture morphologies, and help them optimize the fracturing parameters of shales with interbedded lithologies.