Direct inversion for sensitive elastic parameters of deep reservoirs

Abstract

The deep reservoir is usually a type of tight reservoir with high pressure, high stress, low permeability and low porosity. The elastic parameters including Poisson’s ratio and Young’s modulus are important sensitive parameters to the tight reservoir, and the Gassmann fluid term is frequently used in the field of fluid identification as a highly sensitive fluid factor. Such parameters can be obtained by the common prestack seismic inversion method, but not directly. It must first invert for other elastic parameters and then convert them into the Poisson’s ratio, Young’s modulus and Gassmann fluid term by some formula. The errors will be accumulated in the conversion step, and the inversion results will have a large deviation. We propose a one-step inversion method to solve this problem. Firstly, a new form of P-wave reflection coefficient equation in terms of Poisson’s ratio, Young’s modulus and Gassmann fluid term is derived which can directly establish the functional relationship between the P-wave reflection coefficient and these elastic parameters. Considering seismic data of deep reservoir generally have a lower signal-to-noise ratio (S/N) and the partial angle stack gather has a higher S/N than single angle gather, we then derive a stack impedance equation which is suitable for the partial angle stack gather. By using three stacked impedance inversion data with different angle stack ranges, we can directly get the Poisson’s ratio, Young’s modulus and Gassmann fluid term simultaneously. Model and real data tests both prove that the one-step direct inversion method can reduce the cumulative errors effectively and has higher inversion accuracy.