Characteristics of present-day in-situ stress field and the Permian coal reservoir permeability in the eastern Yunnan and western Guizhou regions

The present-day in-situ stress state (magnitude and orientation) and coal reservoir permeability are extremely important factors for the coalbed methane (CBM) exploration and development. In the present study, methods including focal mechanism solution, hydraulic fracturing were adopted to analyze the present-day in-situ stress state in the eastern Yunnan and western Guizhou regions. The correlation between the present-day in-situ stress magnitude and burial depth was established through statistical analysis. Furthermore, the variation characteristics of Permian coal reservoir permeability in the eastern Yunnan and western Guizhou regions, and the controlling factors were analyzed. The results indicated that the orientation of horizontal maximum principal stress (Sh,max) was in a dominant NWW-SEE-trending. In the study area, the magnitudes of Sh,max, Sh,min and Sv were 2.67-29.84 MPa (averaged 14.10 MPa), 1.85-23.76 MPa (averaged 10.36 MPa), and 0.86-31.93 MPa (averaged 14.49 MPa), respectively. The in-situ stress magnitudes became larger with increased burial depth and could be divided into several belts. The in-situ stress regime was a dominant of Sh,max>Sv>Sh,min in layers shallower than 600 m. Coal seams were under the strike-slip stress regime. Whereas it was a dominant normal stress regime (Sv>Sh,max>Sh,min) in layers deeper than 700 m. Coal seams were under the strike-slip stress regime. The vertical transformation depth of the in-situ stress regime in the eastern Yunnan and western Guizhou regions was approximately 600-700 m. Many factors together influenced the variations of Permian coal reservoir permeability in the study area. The coupling effect between natural fracture characteristics and in-situ stress field showed great controls on the Permian coal reservoir permeability in the eastern Yunnan and western Guizhou regions.