Abstract
Quantitative inversion of fracture weakness plays an important role in fracture prediction. Considering reservoirs with a set of vertical fractures as horizontal transversely isotropic media, the logarithmic normalized azimuthal elastic impedance (EI) is rewritten in terms of Fourier coefficients (FCs), the 90° ambiguity in the azimuth estimation of the symmetry axis is resolved by judging the sign of the second FC, and we choose the FCs with the highest sensitivity to fracture weakness and present a feasible inversion workflow for fracture weakness, which involves: (1) the inversion for azimuthal EI datasets from observed azimuthal angle gathers; (2) the prediction for the second FCs and azimuth of the symmetry axis from the estimated azimuthal EI datasets; and (3) the estimation of fracture weakness combining the extracted second FCs and azimuth of the symmetry axis iteratively, which is constrained utilizing the Cauchy sparse regularization and the low-frequency regularization in a Bayesian framework. Tests on synthetic and field data demonstrate that the 90° ambiguity in the azimuth estimation of the symmetry axis has been removed, and reliable fracture weakness can be obtained when the estimated azimuth of the symmetry axis deviates less than 30°, which can guide the prediction of fractured reservoirs.
Highlights
Subsurface fractures contribute to providing pathways for fluid flow and increasing the permeability of reservoirs
The seismic fracture prediction can be classified into two basic types: One is the seismic fracture qualitative prediction, which is mainly based on the discontinuity of fractures and uses the prestack seismic diffraction wave imaging approach or poststack geometric seismic attributes to qualitatively describe macroscale fractures, such as faults
The other is the seismic fracture quantitative prediction, which is mainly based on seismic azimuthal anisotropy to predict mesoscale fractures, including obtaining fracture density and direction by ellipse fitting using azimuthal anisotropy attributes and quantitatively inverting fracture parameters using azimuthal prestack seismic data (Liu et al 2015; Chen et al 2016; Liu et al 2018; Yuan et al 2019)
Summary
Subsurface fractures contribute to providing pathways for fluid flow and increasing the permeability of reservoirs. Based on the rock physics model, Chen et al (2014a) proposed the AVAZ inversion method to estimate elastic and fracture weakness parameters. One tentative approach to reduce the number of unknown parameters is to implement azimuthal seismic amplitude difference inversion for fracture weakness estimation (Chen et al 2017a; Pan et al 2017; Xue et al 2017). Our study is the extension of Downton and Roure (2015) research, which stabilizes the inversion process by treating the fracture weakness separately from the elastic parameters using FCs. Different from traditional fracture weakness inversion using azimuthal reflection amplitude or azimuthal EI or azimuthal reflection amplitude FCs, in this paper, a novel azimuthal EI-based FC variation with angle inversion method is proposed. The influence of the azimuth of the symmetry axis on fracture weakness inversion is analyzed, and the proposed approach is demonstrated on both synthetic and real data
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