Large-Scale Mine Experimental Study on the Crack Extension Law of Deep-Seated Coal Rock

Deep-seated coalbed methane (CBM) resources in the Daniudi Gas Field of the Ordos Basin are abundant; however, conventional laboratory-scale hydraulic fracturing experiments are unable to realistically reproduce fracture propagation behavior due to pronounced reservoir heterogeneity and the complex development of bedding and cleat structures. In this study, a self-developed 10,000-ton true triaxial hydraulic fracturing simulation platform was employed to conduct mine-scale experiments using large 2 m × 2 m × 1 m No. 8 coal-rock outcrop specimens. A full-scale steel-casing wellbore and an industrial fracturing fluid system were incorporated to replicate field conditions. Experiments were performed under varying pumping rates (0.2–0.4 m3/min) and fracturing fluid viscosities (10–50 mPa·s). The results indicate that post-failure fractures in deep coal formations primarily develop into complex fracture zones extending vertically from the wellbore. Their morphology is strongly governed by bedding planes and cleats, producing tortuous, banded, and mesh-like patterns. When the fracturing fluid viscosity is maintained between 18 and 27 mPa·s, longitudinal fracture diversion along the wellbore is effectively suppressed, while the increased static pressure promotes the activation of natural fractures. Increasing the pumping rate to 0.4 m3/min markedly enhances the stimulated reservoir volume (SRV), with an increase of approximately 1354%, and significantly increases fracture branch density. However, higher viscosities (>27 mPa·s), despite promoting fracture complexity, reduce proppant transport efficiency due to increased in-fracture tortuosity. This study quantitatively characterizes the coupled responses of fracture volume fraction, branch density, and fracture-surface roughness, and elucidates the interplay between displacement and viscosity in governing fracture network evolution. The findings provide an important experimental foundation for optimizing hydraulic fracturing parameters in the efficient development of deep-seated CBM reservoirs.