Imaging-Based Characterization of Calcite-Filled Fractures and Porosity in Shales

Summary

Gas- and liquid-rich shales exhibit structural and compositional features across a broad range of length scales from meters to nanometers. This laboratory characterization effort of a shale sample aids hydrocarbon resource and reserves estimation, and improves understanding of flow behavior including potential geological carbon dioxide storage. Multiscale laboratory-imaging techniques were applied to characterize pore and microfracture structure of a Barnett Shale sample including connectivity and heterogeneity. X-ray computed tomography (CT) illuminated the krypton (Kr)-accessible porosity of centimeter-sized shale cores. Transmission X-ray microscopy (TXM) imaged micrometer-sized shale samples, and high-resolution scanning electron microscopy (SEM) revealed pore, fracture, and textural features. Registration of 190 μm resolution CT images with micrometer to nanometer resolution TXM and SEM images improved physical understanding of transport through organic-rich shale. Results focus on calcite-filled fractures and the calcite/shale-matrix interface as well as the distribution of micrometer- and nanometer-scale porosity. Fractures are likely both natural and induced. For the sample studied, pore accessibility determined by CT imaging corresponds with open microfractures that cross calcite-filled fractures and adjacent shale matrix. Such observations are made with corresponding micrometer- to nanometer-scale SEM images as well as compositional data. Taken together, these data indicate that calcite-filled fractures in this core act as a barrier to flow parallel to bedding except where breached by numerous open fractures. In contrast, these filled fractures enhance vertical flow, that is, flow between laminations. A region containing porosity and organic matter (with dimensions of tens to hundreds of nanometers) determined by 3D nanocharacterization with TXM and focused ion beam/SEM at the filled-fracture/shale-matrix interface facilitates this observed gas transport along the wall of the fracture fill. Areas adjacent to calcite-filled fractures and carbonaceous laminations within the shale matrix of the study sample are most readily accessed by Kr and may therefore be more readily produced than comparatively clay-rich laminations. The numerous open fractures and sheet pores within the calcite fracture fill, as well as the inherent weakness of the porosity and organic matter at the fracture-fill/shale-matrix interface, are indicative of its susceptibility to reopening and fracturing.