Numerical Study on Hydraulic Fracture Height-Propagation Behavior under the Composite Layer Effect of Continental Interbedded Shale Oil Reservoir

Summary Continental interbedded shale oil reservoirs generally exhibit obvious composite layer effect, which is characterized by multilayers of various lithologies, dense laminas, and lithologic interfaces. In such formations, the behavior of hydraulic fracture (HF) height growth is still unclear. In this study, we used a 3D discrete element method (DEM) based on fracturing model to explore the composite layer effect on HF height-growth behavior. The model considered innovatively the rock matrix as transversely isotropic material due to the presence of dense laminas. Model validation was achieved through laboratory-scale fracturing tests on shale specimens featuring dense laminas. Computed tomography (CT) scanning was used to observe fracture morphology before and after the experiment. The numerical results showed high consistency with experimental observations in terms of fracture initiation, propagation, and deflection at laminated interfaces, confirming the model’s reliability. Moreover, influence factors of dense laminas, interlayer stress difference, barrier layer thickness, pumping rate, and fluid viscosity were investigated in detail. Finally, the results demonstrate that as the interlayer stress difference increases or the barrier layer becomes thicker, the composite layer effect becomes more pronounced, making it increasingly difficult for an HF to penetrate these interlayer boundaries. Additionally, dense laminas intensify the containment effect of the composite layer effect on HF height propagation. As lamina permeability increases or strength decreases, the containment on HF height propagation is further intensified. Ultimately, it was also observed that interbedded shale oil reservoirs influenced under the composite layer effect, adjusting key fracturing parameters, such as the combination of pumping rate and fracturing fluid viscosity, can alter the propagation of HF height-growth behavior. Moreover, recommendations are provided for the optimal pumping rate and viscosity under different reservoir conditions. This study enhances the understanding of the HF height-propagation mechanism in interbedded shale oil reservoir and also offers new insights for controlling HF height in practical fracturing operations.