Mechanical Mechanism of Longitudinal Fracture Extension in Multilithologic Continental Shale Reservoirs

The laminated structure and lithological alternation characteristics of continental shale restrict the height propagation of hydraulic fractures, thereby increasing the challenges in fracturing design. Based on scratch tests, indentation experiments and seepage–stress coupling model, this study systematically characterizes fracture height propagation patterns in monolithologic reservoir formations. The results demonstrate that argillaceous shale tends to develop short and wide fractures, whereas felsic shale predominantly generates long and narrow fractures. Within the multilithology fracture propagation framework, we establish a fracture height growth coefficient through analytic hierarchy process by integrating three key mechanical parameters: brittleness index, tensile strength, and fracture toughness. The cross-layer propagation capability of fractures is characterized using a cross-lithology gradient cumulative value. A negative correlation is observed between the fracture height growth coefficient and the achieved fracture height. As the cross-lithology gradient cumulative value increases, fracture height exhibits a decreasing trend. Consequently, the fracturing initiation point should preferentially be selected at locations minimizing the vertical cross-lithology gradient cumulative value. This study provides a theoretical basis for optimizing fracturing locations. By enhancing fracture height growth, it improves vertical reservoir utilization efficiency and contributes to the efficient development of shale oil resources.