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<p>Perforation fracturing in coal seam surrounding rock is an effective method for coalbed methane extraction. To investigate the expansion mechanism of hydraulic fractures and the key parameters for enhancing CBM, we designed an experimental model composed of coal–sandstone interlayers and conducted true triaxial fracturing. The key factors such as crustal stress, perforation, and coal-interface strength were considered. Utilizing acoustic emission and computed tomography techniques, we characterized the fracture morphology. The results reveal four distinct interaction modes between fractures and the interface: A—direct penetration; B—crack orientation + penetration; C—crack orientation + deflection; D—crack orientation + deflection + penetration. The interface strength significantly influences fractures propagation behavior; high interface strength favor direct penetration of fractures but leads to simpler fracture distribution patterns. Conversely, low strength increases the likelihood of fracture reorientation. Additionally, the study explores the influence of perforation direction <i>θ</i> under varying stress differentials Δ<i>σ</i> on fractures initiation. When Δ<i>σ</i> &lt; 4 MPa and <i>θ</i> &lt; 60°, fractures initiate at non-dominant planes, allowing them to overcome the limitations imposed by the maximum principal stress before entering coal. This process is typically accompanied by elevated initial pressure, which can be alleviated by positioning perforations closer to the interface or directly into the coal. Furthermore, it is found that after entering the coal seam along non-dominant planes, the proportion of shear cracks increases significantly, the maximum ratio can reach 5.3 times that of dominant plane. It provides a new and valuable insight for monitoring fracture propagation while adopting underground microseismic technique. These conclusions provide qualitative guidance for enhancing hydraulic fracturing design in coal seam roof and floor applications.</p>

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Experimental Study on Factors Influencing Interaction Behaviors of Hydraulic Fractures and Coal–Rock Interface: Stress, Interface and Perforation

  • 期刊论文

  • 发表日期：

    2025年2月21日

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  • Changzheng Lu,
  • Zhaolong Ge,
  • Zhe Zhou,
  • Quangui Li,
  • Jianming Shangguan,
  • Shan Huang,
  • Qinglin Deng

摘要

Perforation fracturing in coal seam surrounding rock is an effective method for coalbed methane extraction. To investigate the expansion mechanism of hydraulic fractures and the key parameters for enhancing CBM, we designed an experimental model composed of coal–sandstone interlayers and conducted true triaxial fracturing. The key factors such as crustal stress, perforation, and coal-interface strength were considered. Utilizing acoustic emission and computed tomography techniques, we characterized the fracture morphology. The results reveal four distinct interaction modes between fractures and the interface: A—direct penetration; B—crack orientation + penetration; C—crack orientation + deflection; D—crack orientation + deflection + penetration. The interface strength significantly influences fractures propagation behavior; high interface strength favor direct penetration of fractures but leads to simpler fracture distribution patterns. Conversely, low strength increases the likelihood of fracture reorientation. Additionally, the study explores the influence of perforation direction θ under varying stress differentials Δσ on fractures initiation. When Δσ < 4 MPa and θ < 60°, fractures initiate at non-dominant planes, allowing them to overcome the limitations imposed by the maximum principal stress before entering coal. This process is typically accompanied by elevated initial pressure, which can be alleviated by positioning perforations closer to the interface or directly into the coal. Furthermore, it is found that after entering the coal seam along non-dominant planes, the proportion of shear cracks increases significantly, the maximum ratio can reach 5.3 times that of dominant plane. It provides a new and valuable insight for monitoring fracture propagation while adopting underground microseismic technique. These conclusions provide qualitative guidance for enhancing hydraulic fracturing design in coal seam roof and floor applications.