Amplitude Variation With Angle of Incidence and Azimuth Inversion for Pore Pressure and Horizontal Stresses in Shale Gas Reservoirs

Accurate characterization of fracture attributes and in-situ stress fields is critical for optimizing horizontal well placement and hydraulic fracturing strategies in fractured shale gas reservoirs. This study introduces a novel model-constrained damped least-squares amplitude variation with angle of incidence and azimuth (AVAZ) inversion method to predict pore pressure and horizontal stresses. By integrating generalized Hooke’s law with Schoenberg’s linear-slip model, horizontal stress equations are derived for horizontally transversely isotropic (HTI) media. Pore pressure estimation is established through a vertical stress sensitivity parameter linked to porosity via a critical porosity model. A saturated stiffness matrix incorporating this parameter and fracture weakness parameters is developed. The P-wave reflection coefficient is formulated using a coupled scattering function and perturbed stiffness matrix, integrating five key parameters: fluid bulk modulus, porosity-dependent vertical stress sensitivity, rock P-wave modulus, fluid modulus-density product, and fracture density. A multi-parameter inversion algorithm is proposed to simultaneously estimate pore pressure, maximum, and minimum horizontal stresses. Validation with field data demonstrates that this method provides reliable estimates of pore pressure and horizontal stresses.