Fracture propagation and permeability enhancement mechanisms induced by near-roof hydraulic fracturing in soft coal seams
- 1School of Mining Engineering, Guizhou University, Guiyang, Guizhou, China
- 2Guizhou Provincial Laboratory of Intelligent Development and Efficient Utilization of Energy, Guiyang, Guizhou, China
- 3State Key Laboratory of Digital and Intelligent Technology for Unmanned Coal Mining, Anhui University of Science and Technology, Huainan, Anhui, China
Abstract
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 m3/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.
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BibTeX
@article{zhu2026fracture,
title = {{Fracture propagation and permeability enhancement mechanisms induced by near-roof hydraulic fracturing in soft coal seams}},
author = {Yunheng Zhu and Xionggang Xie and Shujin Zhang and Peng Zhang and Huaiqian Liu and Deheng Jia},
journal = {PLOS One},
year = {2026},
month = oct,
doi = {10.1371/journal.pone.0359936},
url = {https://doi.org/10.1371/journal.pone.0359936},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Fracture propagation and permeability enhancement mechanisms induced by near-roof hydraulic fracturing in soft coal seams
AU - Yunheng Zhu
AU - Xionggang Xie
AU - Shujin Zhang
AU - Peng Zhang
AU - Huaiqian Liu
AU - Deheng Jia
PY - 2026
DA - 2026/10/05
JO - PLOS One
DO - 10.1371/journal.pone.0359936
UR - https://doi.org/10.1371/journal.pone.0359936
AB - 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 m3/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.
SN - 1932-6203
ER -