Fracture propagation and permeability enhancement mechanisms induced by near-roof hydraulic fracturing in soft coal seams

Soft and low-permeability coal seams are prone to borehole collapse, unstable gas-extraction pathways, and a restricted stimulation range when conventional in-seam fracturing is applied. To overcome these limitations, this study proposes a near-roof hydraulic-fracturing strategy. Coal–immediate-roof assemblages from Heyuan Coal Mine were tested using a large-scale true-triaxial hydraulic-fracturing system under different triaxial in-situ stress states and injection rates. Fracture initiation, propagation, and failure modes were identified by jointly analyzing injection pressure, acoustic-emission (AE) hit counts, cumulative AE energy, and RA–AF parameters. A hydro-mechanically coupled model containing pore-pressure cohesive elements was then established, and the inferred mechanism was evaluated against field observations from the HYL1 surface L-shaped well. Increasing the in-situ stress state from 3/4/6 MPa to 9/12/18 MPa raised the breakdown pressure from 5.15 to 12.23 MPa and increased the contribution of mixed tensile–shear failure. Increasing the laboratory injection rate from 0.1 to 0.4 mL/s raised the breakdown pressure from 4.30 to 6.79 MPa, whereas the stable propagation pressure remained within 3.91–4.94 MPa. The minimum principal stress controlled the dominant fracture plane, whereas the intermediate principal stress and stress difference regulated the width and intensity of the damaged zone. After field fracturing, the mixed-gas drainage rate remained at 630–690 m 3 /d and the methane concentration exceeded 82%. These drainage responses indicate sustained fracture connectivity and improved gas-migration conditions, although permeability was not measured directly. The results support a coupled mechanism involving pore-pressure accumulation, effective-stress reduction, roof damage and pressure relief, coal–roof fracture connectivity, and improved gas migration.