High-precision inversion method for two-phase complex fracture medium

Fractures serve as the channel of oil and gas storage and migration, and they are factors that induce mine disasters. Therefore, a detailed characterization of fractures contributes to resource security and effective development. Traditional fracture medium inversion usually assumes that the fracture is horizontal or vertical, which is not consistent with the natural morphology of the fracture. Using approximate solutions to solve the weak seismic response of the fracture is prone to errors, and the single-phase medium hypothesis is not in line with the geological conditions of reservoirs containing gas or water. To address these issues, an exact Zoeppritz inversion method based on linear slip theory was proposed for two-phase complex fractured media. The fluid replacement and the calculation method of elastic parameters in two-phase media were introduced. The compliance parameter, which could describe the fracture characteristics, was input into the Zoeppritz equation, and the reflection and transmission coefficients of the two-phase fractured media at any angle were derived. Based on the linear slip theory, the seismic response characteristics caused by the fracture and impedance interface of two-phase media were analyzed, and the problem of seismic wave energy distribution at the fracture interface was explained. In view of the energy difference of seismic reflection wave between fracture and impedance interface, a comprehensive multi-scale inversion flow of exact and approximate solutions was proposed. The test results of synthetic and field data show that the proposed inversion method can simultaneously obtain high-precision parameters of P- and S-wave velocities, density, and normal and tangential compliances and predict fracture attributes on the basis of fractures’ spatial distribution. The method has higher computational efficiency and accuracy than the traditional inversion method and can provide effective technical support for oil and gas exploration and development, as well as for the detection of CO₂ geological sequestration.