Evaluating the impact of fracture-induced anisotropy on reservoir rock property estimates made from seismic data

Abstract

The main application of the surface seismic method in the appraisal and production environment is in reservoir characterisation. To target high porosity and net-to-gross reservoir intervals, an accurate assessment of reservoir rock properties from surface seismic data is required prior to drilling. One of the common technologies for estimation of reservoir properties from seismic data involves the inversion of AVO attributes using well log-derived rock property trends. These rock property trends provide the vital link between seismic impedance data and subsurface geology. The standard workflow for producing these rock property trends assumes an isotropic reservoir, thus ignoring the effects of anisotropy. Therefore, rock property predictions made using this isotropic workflow will be incorrect if the reservoir is anisotropic. Reservoir anisotropy may be caused by alignment of clay particles in shales, intra-reservoir layering or reservoir fracturing. In this study, the effect of fractures on the estimation of rock properties in a porous laminated sand-shale reservoir was analysed. Both isotropic and anisotropic rock property prediction workflows were constructed and utilised to estimate rock properties from an input dataset. From these predictions, the potential error in the rock properties estimated using an isotropic workflow in a fractured sand-shale reservoir was quantified. From the study results, it was determined that ignoring fracturing anisotropy in the rock property prediction workflow produced substantial errors in estimated rock properties, in the order of ±5% in predicted porosity, and ±40% in predicted net-to-gross for a specified fluid-fill. The conception and construction of a viable anisotropic rock property prediction workflow expands the current isotropic workflow to incorporate the effects of shale anisotropy, layering anisotropy and fracturing anisotropy.