Experimental Study on Factors Influencing the Propagation of Hydraulic Fractures in Shale Reservoirs with Developed Natural Weak Planes

Hydraulic fracturing is a key technology to build productivity in shale reservoirs; however, the evolution mechanism of fractures is extremely complex, especially in reservoirs with natural weak-planes development. There is an urgent need to conduct systematic research on the influence of natural weak planes on the vertical propagation of hydraulic fractures. This article takes the deep shale gas block of Luzhou in Southern Sichuan as the research basis and conducts different conditions of true triaxial large-scale hydraulic fracturing physical simulation experiments as well as the characteristics of natural weak-plane reservoir development and reservoir geological characteristics. This study clarifies the interaction mechanism between hydraulic fractures and natural weak planes and identifies the influence of parameters such as vertical stress difference, natural fracture strength, and approach angle on the propagation path of hydraulic fractures in reservoirs with developed natural weak planes, which help us gain a deeper insight into the interaction mechanism between fracture and weak plane. This study indicates that the widely developed natural weak planes in shale reservoirs significantly affect the initiation, propagation, and final distribution of hydraulic fractures. Based on pressure response characteristics, the fracture initiation types can be categorized into two scenarios: initiation along the direction of the maximum principal stress and initiation along natural weak planes. The propagation modes of fractures can be divided into three types: propagation perpendicular to natural weak planes, propagation parallel to natural weak planes, and multi-fracture propagation. The post-pressure fracture distribution patterns can be classified into four types: through-going fractures, T-shaped fractures, compound fractures, and complex fracture networks. The absence of developed natural weak planes, high vertical stress differences, high natural weak-plane cementation strength, and large intersection angles are favorable conditions for the vertical propagation of hydraulic fractures. The research findings enrich the fundamental theory of vertical propagation of hydraulic fractures in shale reservoirs with developed natural weak planes and provide a scientific basis for the formulation and optimization of stimulation schemes for deep shale reservoirs, contributing to better stimulation effects in the Southern Sichuan shale gas block.