Elastic parameters predicting and its influencing factors analyzing in orthotropic shale formation

In view of the problem that the limited wave velocity in logging measurements makes it difficult to accurately describe the elastic characteristics of orthotropic (ORT) shale formations, this paper first applies rock elastic theories such as the Self-Consistent Model (SCA) and the Differential Effective Medium model (DEM) to construct a background medium with vertically transverse isotropic (VTI) elastic characteristics. On this basis, by introducing the Schoenberg linear slip model, an anisotropic rock physics model capable of describing the ORT elastic characteristics of shale is established. Finally, practical application is carried out in Well x1. The research results show that obtaining the stiffness matrix of the shale background medium with VTI properties and avoiding the influence of fluids in vertical fractures on the P-wave velocity are the keys to the rock physics modeling of ORT shale. Through the inversion of the ORT rock physics model, it is obtained that the fracture weakness in the Longmaxi Formation shows a relationship of △V>△N>△H. The compressive stiffness coefficients satisfy c11>c22>c33, and the shear stiffness coefficients satisfy c66>c44>c55. In the same Tsvankin plane, the anisotropy coefficients of P-waves and S-waves usually vary in magnitude. An increase in shale content will increase the anisotropy of P-wave and S-wave velocities, decrease Young's modulus, and increase Poisson's ratios ν12, ν13, ν21, ν23. Through this study, basic data can be provided for subsequent in-situ stress modeling of orthotropic shale, wellbore stability analysis, and optimized design of fracturing stages.