Physical simulation experiments and the effects of fracture propagation in the Lianggaoshan Formation shale

Shale gas is an unconventional natural gas resource characterized by its extensive distribution and abundant reserves, offering a stable energy supply. The Lianggaoshan Formation shale features a complex fracture network and multiscale pore structures, making its mechanical properties and fracture propagation laws critical research areas in shale gas development. This study focuses on the Lianggaoshan Formation shale, utilizing laboratory-scale physical simulation experiments to systematically analyze its physical and mechanical properties as well as the key factors influencing fracture propagation. Fracture propagation was investigated through acoustic emission testing, while hydraulic fracturing experiments were conducted to observe the processes of fracture initiation, propagation, and intersection. Additionally, the effects of horizontal stress and fracturing fluid viscosity on fractures were analyzed. (Comment: The meaning of this sentence should be carefully examined, as bedding and natural cracks can only be activated and will not dynamically propagate.) The results indicate that under low horizontal stress differentials (5 MPa), fractures propagate along bedding planes with minimal longitudinal fractures and low complexity despite an increase in shear events. As the stress difference increases (11 MPa), fractures become more complex and tend to extend in the direction of the maximum horizontal principal stress. Low-viscosity fracturing fluids (2 mPa s) enhance fracture deflection and increase complexity, whereas high-viscosity fluids (100 mPa s) promote the formation of wider main fractures with fewer branches. In contrast, low-viscosity fluids generate more branch fractures, resulting in a higher fracture network complexity. The combination of high and low viscosities enhanced the penetration ability of fractures. The occurrence of branching fractures increased, leading to the formation of a complex fracture network by supercritical CO2. However, the primary fracture was not clearly defined. Additionally, the complexity of fractures created by CO2 mixed with slickwater increased by 75% compared to that of low viscosity fracturing fluids. The findings of this study provide a scientific basis for optimizing hydraulic fracturing designs and reservoir stimulation strategies in the Lianggaoshan Formation shale gas development. Overall, these results offer significant insights into advancing unconventional reservoir development technologies.