Distribution rule of the in situ stress state and its influence on the permeability of a coal reservoir in the southern Qinshui Basin, China

The recent development of the coalbed methane (CBM) industry has a significant role in advancing hydraulic fracturing theory and technology. However, further development requires a better understanding of how fractures influence reservoir permeability. In situ stress data from 54 CBM wells in the southern Qinshui Basin, China, were obtained by the injection/falloff test method to analyse the effect of in situ stress on the permeability of the CBM reservoir. The types of in situ stress states were classified, and the coal reservoir permeability under different in situ stress states was analysed. The results indicate that the maximum horizontal principal stress ( σ H ), minimum horizontal principal stress ( σ h ) and vertical principal stress ( σ v ) all have positive linear relationships with the coal seam burial depth. Three in situ stress states were observed from the shallow to deep regions of the CBM reservoir in the study area: σ H > σ h > σ v , σ H > σ v > σ h and σ v > σ H > σ h , which account for 9, 76 and 15% of the test wells, respectively. Coal reservoir permeability decreases with increasing horizontal principal stress, whereas it first decreases with increasing σ v , then increases and finally decreases. The variation in permeability with σ v is due to the conversion of the in situ stress states. Coal reservoir permeability has obvious differences under different in situ stress states. The permeability is the largest when σ v > σ H > σ h , followed by σ H > σ h > σ v and smallest when σ H > σ v > σ h . The permeability differences are caused by the fracture propagation shape of the rock strata under different in situ stress states.