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

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 mu m 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.