Improved unconventional reservoir characterization using multi-azimuth stratigraphic inversion, case study on the Fort Worth Basin.
Résumé
The development of the exploration and exploitation of unconventional hydrocarbons requires innovative skills to allow a better characterization of natural fracture networks and fracability of the rocks at depth. Within this context, we bring into focus a multi-azimuth inversion methodology to give clues about the "sweet spots" characterization through the Barnett Shales formation. To achieve our goal we used an isotropic model-based stratigraphic seismic inversion, considering a series of azimuthal sectors to build partial seismic stacks required as input of the workflow. The initial seismic survey was recorded over a surface of approximately 100 square miles to image the Barnett Shales formation, of about 150 ft thickness only and localized in a transition zone between the oil and gas window. A preliminary processing task was to define limits of azimuthal and incidence angle sectors to build several multi-azimuth post-stack and pre-stack seismic datasets. Two exploration wells with a set of log data are available in the interest area helping in the model a priori building and in the well-to-seismic calibration, both crucial steps to apply the inversion workflow. We first perform full post-stack and pre-stack usual inversion to help in finding sets of best inversion parameters, then we do the multi-azimuth post-stack and pre-stack inversion workflow using the same previous sets of parameters for each azimuthal sector. Consequently a series of acoustic impedance, P- and S-wave elastic impedance cubes are computed for each azimuthal sector. Another aspect of the study is the quantification of azimuthal variations using an ellipse fitting algorithm giving, for each bin of the interest volume, the ratio (between major and minor axes) and tilt (i.e. azimuth from the East) of the ellipse. Results showed up to 10% of anisotropy for impedances in the Barnett Shales. We go a step further computing geomechanical features like Poisson's ratio and brittleness coefficient within the Barnett Shales and we quantified their azimuthal variations. Some areas present lower Poisson's ratio and higher brittleness than other parts. It may suggest that the latter are "sweet spots" that could be more easily stimulated by induced fracturing.
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