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 reserve estimation and improves understanding of flow behavior including potential geological CO2 storage. Multiscale imaging techniques are applied to characterize pore and micro-fracture structure of a Barnett shale sample including connectivity and heterogeneity. Our tools include X-ray computed tomography (CT) to image the Krypton-accessible porosity of cm-sized shale cores, transmission X-Ray microscopy (TXM) to image micron-sized shale samples, and high-resolution scanning electron microscopy (SEM) of pore, fracture, and textural features of the same core. Registration of 190 μm-resolution CT images with μm to nm resolution TXM and SEM images provides 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 micro- and nano-scale porosity. Fractures are likely both natural and induced. For the samples studied, pore accessibility determined by CT imaging corresponds with open microfractures that cross calcite-filled fractures and adjacent shale matrix. Such observations are made using corresponding micro- to nano-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 10 to 100's of nm) determined by 3D nanocharacterization using TXM and FIB 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 the its susceptibility to reopening and fracturing.
Imaging-Based Characterization of Calcite-Filled Fractures and Porosity in Shales
Vega, Bolivia , Ross, Cynthia M., and Anthony R. Kovscek. "Imaging-Based Characterization of Calcite-Filled Fractures and Porosity in Shales." Paper presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Denver, Colorado, USA, August 2014. doi: https://doi.org/10.15530/URTEC-2014-1922521
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