Characteristics of an in situ stress field and its control on coal fractures and coal permeability in the Gucheng block, southern Qinshui Basin, China

The stress characteristics in a lean coal reservoir in the Gucheng block, southern Qinshui Basin, are discussed in this study. Relational models among the stress, burial depth and permeability in coal reservoir No. 3 in the Shanxi Formation were built, and the relationships among the coal fractures, coal permeabilities and distribution of the modern stress were analyzed. In the Gucheng block of the southern Qinshui Basin, the distribution of modern stress and its control on coal fractures and coal permeability have been determined using geological and statistical methods applied to subsurface observations of coal rocks, log interpretations of 50 coalbed methane wells, in situ stress data from 8 coalbed methane wells, monitoring data on fractures and seismic results from 14 coalbed methane wells. The results show that the coal reservoirs have ultra-low to low permeabilities (<0.1 mD). The maximum horizontal principal stresses in the study area range from 13.23 to 29.55 MPa, with an average of 20.45 MPa; the minimum horizontal principal stresses in the study area range from 7.7 to 19.81 MPa, with an average of 13.01 MPa; and the closure pressures vary from 9.69 to 20.76 MPa, with the average value of 13.71 MPa. Additionally, the maximum and minimum horizontal principal stresses and the vertical principal stress have a positive relationship with burial depth, and the permeability derived from well tests is negatively correlated with the minimum horizontal principal stress. The maximum and minimum horizontal principal stresses decrease from the southwest to the northeast. The in situ modern stress state is situated in a transitional zone between extensional and compressional zones. The interpretation results based on the special logging and fracture monitoring results show that the modern principal stress direction is mainly NNE to NEE and that the principal stress direction is oriented from the north to the south. The modern principal direction is similar to the most common orientation of the coal fractures and is normal to the subordinate direction of the coal fractures. Finally, the pores and the fracture in the coal reservoir become smaller and poorer in the area with concentrated horizontal principal stress.