Analysis of hydraulic fracturing cracks for coal reservoirs and in-situ stress direction in Southern Qinshui Basin

The distribution characteristics of hydraulic fracturing cracks in coal reservoirs and the direction of in-situ stress are the basis for the design of coal reservoir fracturing. The paper statistically analyzes the microseismic monitoring data and geodetic potential data of 87 hydraulic fracturing cracks in 51 CBM wells constructed in the southern Qinshui Basin. The distribution characteristics of hydraulic fracturing cracks in coal reservoirs of the study area are revealed, the relationship between hydraulic fracturing cracks extension and in-situ stress is established, and the distribution direction of in-situ stress in the study area is determined. The results show that the distribution of fracturing cracks in southern Qinshui Basin is dominated by NEE. The main distribution directions are NEE, NE, and near EW, accounting for 55.12% of all distribution directions, then followed by near-SN and NWW—SEE. The length of fracturing cracks, detected by microseismic monitoring, varies from 108 to 452 m, with an average of 224.76 m. The length of fracturing cracks, tested by geopotential method, is between 40 m and 131 m, mainly concentrated between 51 m and 90 m, with an average of 73.94 m. Significant differences exist between the length of the fractures obtained by the microseismic monitoring and by the geopotential method. The length of the fractures detected by the microseismic monitoring is over twice larger than that tested by the geopotential method. According to the relationship between hydraulic fracturing cracks extension and in-situ stress,the coal reservoir fracturing cracks geometry is summarized as hori-zontal fracture,vertical fracture, and dipping fracture. During the fracturing process, the types of fractures in coal reservoir formed by hydraulic fracturing depend on the relative stress of three principal stresses, namely vertical stress, the maximum horizontal principal stress, and the minimum horizontal principal stress. The current in-situ stress direction in this area is mainly characterized by the NEE—SWW direction. The research results of this paper provide a theoretical basis for the hydraulic fracturing of CBM wells.