AVAz inversion of 4D seismic data for fracture characterization: A case study in the Barnett shale

Shale reservoirs need stimulation to be economically viable due to their low permeability. Fractures can improve shale reservoir permeability and production, and therefore, high fracture density areas are preferred. Fracture studies then become an essential aspect of unconventional reservoir characterization. Amplitude variation with offset and azimuth (AVAz) methodologies are used to determine fracture-related parameters. In this work, we utilize two different methodologies, namely, Rüger’s method and the Fourier coefficients decomposition method, to approach this problem using seismic data targeting the Barnett formation. Rüger’s method uses a linearized version of the full reflection coefficient for horizontal transverse isotropy media. On the other hand, Fourier coefficients decomposition is supported by the fact that P-wave reflectivity can be described by a Fourier series and the components of the series can be linked to different properties related to fracture parameters. We use iterative reweighted least squares and the Cauchy-Gauss method for the calculation of the weights in both methodologies to solve the inverse problem. Applied in four-dimensional seismic data from the Stocker field, we compute the variation of the AVAz response before and after the hydraulic fracturing treatment and production from the area of interest. The results clearly demonstrate how the seismic response changes with these processes and the response can be interpreted in terms of fracture attributes. Fracture density proxies such as the anisotropic gradient and the 2nd Fourier coefficient are used with fracture azimuth to estimate fracture orientation. Regional stress information is used to constrain the estimates of fracture orientation in the absence of other data. The main orientation of the fractures in the area is NE-SW. The fracture parameters and fracture orientation obtained by AVAz analysis can be used to plan well trajectories and completion, helping to increase hydrocarbon production and improving the economic viability.