In Situ Stress Control on Coal Reservoir Permeability and Hydraulic Fracturing Fracture Propagation: A Case Study in Yangquan Mining Area, Northeastern Qinshui Basin, China

Permeability is the key factor controlling coalbed methane (CBM) production, and hydraulic fracturing is an essential technique for permeability enhancement in low-permeability coal reservoirs. In situ stress plays a dominant role in both coal reservoir permeability evolution and hydraulic fracture propagation. By taking the Yangquan Mining Area in the northeastern Qinshui Basin, China, as the study area, this study integrated injection/falloff well testing and microseismic monitoring data to investigate the controlling effect of in situ stress on coal reservoir permeability and hydraulic fracture geometry. The results show that the in situ stress and reservoir pressure generally increase with burial depth. The measured maximum and minimum horizontal principal stresses ranged from 4.04 to 23.41 MPa and 3.72 to 13.24 MPa, with average values of 14.54 MPa and 8.64 MPa, respectively, while the vertical principal stress ranged from 6.50 to 19.87 MPa, averaging 13.31 MPa. The reservoir pressure ranged from 0.55 to 6.09 MPa, with an average value of 2.32 MPa. The lateral stress coefficient (λ) ranged from 0.45 to 1.68 (average of 0.89), exhibiting a pattern of dispersion in shallow strata and convergence in deeper strata. The regional stress regime is dominated by strike-slip faulting, with approximately 60.5% of the deeper measurement points characterized by σ_H > σ_v > σ_h. The coal seam permeability ranged from 0.03 to 15.52 mD, with an average value of 1.62 mD, and decreased exponentially with increasing effective stress. The controlling effect of the coal seam thickness on the permeability exhibited distinct segmented characteristics. The microseismic monitoring results from 23 hydraulic fracturing treatments indicate that approximately 74% of the fractures trended NE5–NE55°, consistent with the regional maximum horizontal principal stress direction. The fracture lengths ranged from 124.9 to 447.7 m, with an average of 344.7 m, while the fracture heights ranged from 12.9 to 79.09 m. All the induced fractures were predominantly vertical. A greater horizontal principal stress difference was associated with a lower permeability and promoted the formation of longer and more planar hydraulic fractures. These results demonstrate that the regional in situ stress field exerts significant control on both the coal reservoir permeability and hydraulic fracture morphology, providing important guidance for stress-based optimization of CBM development and hydraulic fracturing design.