Fracture propagation behavior in coal hydraulic fracturing and impact mitigation effectiveness evaluation

Hydraulic fracturing technology can induce fractures and delamination in the hard roof of coal mine underground, facilitating timely collapse and pressure relief. However, research on the geometric propagation patterns of fractures and their influencing factors remains insufficient. Theoretical analysis methods were employed to calculate the initiation pressure of coal rock in laboratory settings, with the results compared against experimental data and showing good agreement. Using a true triaxial hydraulic fracturing physical simulation device combined with acoustic emission characteristics, the initiation and propagation patterns of hydraulic fractures under different stress conditions and injection rates, as well as the variations in fracture propagation range, were investigated. The extended finite element method (XFEM) in ABAQUS software was used to study the influence of different stress conditions and injection rates on hydraulic fracture propagation, and the results were compared with laboratory experiments. The findings indicate that vertical stress significantly affects the initiation and propagation of hydraulic fractures. As vertical stress increases, the initiation pressure required for the specimen increases significantly, while the stability and connectivity of fracture propagation gradually improve, with the propagation direction tending to align with the vertical stress direction. Under lower vertical stress, the fracture propagation direction becomes unstable, exhibiting more dispersed morphology and a tendency to develop along the maximum horizontal principal stress direction. Acoustic emission characteristics show that under high vertical stress conditions, acoustic emission events are mainly concentrated in the main fracture area, with stable distribution of micro-fracturing activities. In contrast, under low vertical stress, acoustic emission events are more dispersed, indicating more local stress adjustments and secondary fracturing during fracture propagation. A decrease in the maximum horizontal principal stress reduces the driving force for fracture opening along its direction, and fracture initiation is subjected to stronger lateral constraints, leading to an increase in the equivalent tensile strength of the rock mass. Hydraulic fractures need to overcome greater stress barriers to initiate, resulting in a significant increase in initiation pressure and corresponding peak water pressure as the maximum horizontal principal stress decreases. The maximum horizontal principal stress also significantly influences fracture propagation direction, stability, and acoustic emission characteristics. When the maximum horizontal principal stress is high, fracture propagation direction is more stable, morphology tends to be straighter, connectivity improves, and acoustic emission events are mainly concentrated along the fracture path, indicating more concentrated stress release during propagation. Conversely, when the maximum horizontal principal stress is low, fracture propagation becomes unstable, morphology more complex, and acoustic emission events more dispersed, indicating unstable fracture propagation. An increase in injection rate significantly raises the initiation pressure of the specimen. Under low injection rates, fracturing fluid accumulates slowly in the borehole, allowing water molecules to infiltrate micro-fractures and pores of the rock mass, gradually weakening local rock strength and promoting stable fracture initiation. In contrast, high injection rates cause water pressure to concentrate rapidly within a short time, leading to swift stress concentration at the fracture tip, which easily exceeds the tensile strength of the rock, requiring higher water pressure to achieve fracture. Simultaneously, the intense hydraulic action induced by high-speed injection increases the instability during fracture initiation, making the fracture path more susceptible to disturbances and resulting in more complex propagation forms. Injection rate significantly affects the propagation range of hydraulic fractures, fracture complexity, and the distribution characteristics of acoustic emission location points. Under low injection rates, fracture propagation is stable, the fracture path is clear, and micro-fracturing activities are limited. At medium injection rates, fluid dynamic effects cause local fracture deflections, with acoustic emission points concentrated but showing high-density clustering in local areas, indicating influence from local stress disturbances. High injection rates lead to highly unstable fracture propagation, complex fracture networks, enhanced micro-fracturing activities, and intense energy release. This demonstrates that increasing the injection rate reduces the stability of hydraulic fracture propagation, relatively decreases the propagation range, intensifies local fracturing, and results in complex fracture structures and energy dissipation. Industrial-scale hydraulic fracturing tests were conducted, validating the fracture propagation patterns and mitigating the risk of rockburst in coal mining faces. The research results can guide segmented hydraulic fracturing operations in coal mines with similar mining geological conditions and enrich the theoretical foundation of hydraulic fracturing.