Rock Imaging Reveals Flow Anisotropy in Mega-Porous Carbonates

Dissolution secondary porosity represented in the core as vugs within carbonates, represents enhanced fluid capacity and filtration potential. This paper outlines the results of the reservoir characterization study on such a pore system from one of the Jurassic formations in Saudi Arabia. Refinement of permeability values were investigated by employing whole-core (WC) computed tomography (CT), 3D X-ray microscopy or micro-computed tomography (micro-CT) and digital rock analysis computational workflows. Studied limestones contained diagenetic dissolution fabrics displaying comparatively higher porosities in oolitic, mixed skeletal peloidal, and fragmented cladocoropsis facies. Sample selection was based on the screening of associated WC images and core viewing. Associated criteria were focused on the availability of plugs measured properties, and where available WC laboratory petrophysical properties. The digital rock analysis workflow included: 1) scanning of the WC samples by CT at voxel size of 200 μm; 2) micro-CT at a voxel size of 30 μm; 3) high-resolution imaging at a voxel size of 35 μm; 4) segmentation of the tomograms to identify pores and grain fabrics and 5) building the integrated pore network models and calculation of petrophysical properties. Digital analysis of air permeability of both macro- and microporous pore space was achieved through the Stokes equation in three orthogonal directions illustrating flow heterogeneity. Twelve whole-core sections were imaged with high resolution micro-CT as they represented the most vuggy intervals seen during the core screening. This allowed an adequate section of the WC to be imaged and represented a much larger sample size than the corresponding plug set. Vugs to matrix hydraulic connectivity is a feature of the pore system that could only be extracted from micro-CT images. Permeability simulation results yielded new porosity/permeability trends and permeability anisotropy that were further utilized to update 3D geological models. This study was able to supply evidence of how the flow anisotropy could be resolved using micro-CT for high-end non-matrix porosity carbonates and facilitated the understanding of petrophysical differences between plug and WC components, as well as the matrix and non-matrix components. The workflow used, not only predicted rock properties, but also the spatial distribution of the rock features and pore network assemblages causing these properties. Due to the well-known issues with conventional laboratory measurements on vugular WC, this study demonstrates that reliable porosity and permeability values can be obtained using digital analysis such as micro-CT or CT-scanning on WC at the finest resolution possible. The data infer that digital rock analysis is the only method that yields flow anisotropy in the aforementioned vugular rocks. The results improve the understanding of storage capacity and recovery mechanisms of the studied formation.