Study on the Calculation Method of In-Situ Stress of the Sand-Shale Interbedded Reservoir in the Lianggaoshan Formation of the Pingchang Block in the Sichuan Basin

In-situ stress is the key data for borehole stability analysis, determination of engineering sweet spots, stage and cluster division schemes, and fracturing operation parameters. The magnitude of the three-directional stresses controls the propagation direction and morphology of artificial fractures. Therefore, accurate prediction of in-situ stress is crucial for reducing the risk of wellbore instability during drilling and ensuring effective reservoir stimulation during fracturing operations. The Lianggaoshan Formation reservoir is a sand-shale interbedded reservoir with strong heterogeneity and significant variations in anisotropy, which poses great challenges to in-situ stress prediction. Traditional isotropic-based in-situ stress calculation methods are no longer applicable. Based on the analysis of reservoir anisotropy characteristics, this study analyzes the conversion relationship between the two-way shear wave transit time differences of the target layer in the pilot hole and the corresponding horizontal well, achieving the derivation of the vertical P-wave in the horizontal well and the horizontal P-wave and S-wave in the vertical well. By using acoustic velocity experimental data, calculation methods for C12 and C13 were established, thereby enabling the computation of the six independent stiffness coefficients in the VTI stiffness matrix for both vertical and horizontal wells. Through calibration with triaxial rock mechanics experimental data, a static elastic parameter calculation model was constructed. The anisotropic in-situ stress model was comprehensively calibrated by combining closure pressure from mini-fracturing tests with differential strain in-situ stress experimental data. Error analysis shows that the anisotropic in-situ stress model has high accuracy, with an average relative error of 4.82% for the minimum horizontal principal stress and 2.16% for the maximum horizontal principal stress, meeting the requirements of geomechanical evaluation.