Detailed in Situ Stress Characterization Combining Well Logging and Improved Magnitude Calculation Model of Nos. 8+9 Coal Seam in Northeastern Ordos Basin

The characteristics of in situ stress directly influence reservoir modification and gas production performance, and accurate evaluation of stress distribution can help improve coalbed methane production and development efficiency. Taking the deep carboniferous Nos. 8+9 coal seam in the northeastern Ordos Basin as an example, different in situ stress calculation models were evaluated with comparison to injection/falloff tests based on logging data and coal mechanical parameters. Then, the hydraulic fracture extension mechanism under different in situ stress conditions was simulated. The results show that the comprehensive use of microresistivity imaging logging and dipole array acoustic logging reveals that the main maximum horizontal principal stress direction of the Nos. 8+9 coal seam is E–W, with some area affected by faults showing NE–SW and SEE–NWW. Comparing the commonly used acoustic logging model, Huang's model, the combined spring model, and Ge's model, it was found that the combined spring model is more applicable. This model was optimized by adding a gas saturation parameter, decreasing the relative error of stress magnitude from 7.80 to 3.62% compared to the injection/falloff measurement results. The stress field shows vertical stress > maximum horizontal principal stress > minimum horizontal principal stress. The simulation indicates that the higher the horizontal stress difference, the smaller the fracture initiation pressure and width. The larger the injection angle, the higher the fracture initiation pressure and the smaller fracture width. The above results reveal the in situ stress distribution characteristics of the Nos. 8+9 coal seam in the northeastern Ordos Basin, providing data support for the subsequent deployment of wells and reservoir fracturing as well as guidance for deep coal seams in other basins.