Correlative multi-scale imaging of shales: a review and future perspectives

Abstract

As the fastest growing energy sector globally, shale and shale reservoirs have attracted the attention of both industry and scholars. However, the strong heterogeneity at different scales and the extremely fine-grained nature of shales makes macroscopic and microscopic characterisation highly challenging. Recent advances in imaging techniques have provided many novel characterisation opportunities of shale components and microstructures at multiple scales. Correlative imaging, where multiple techniques are combined, is playing an increasingly important role in the imaging and quantification of shale microstructures (for example, one can combine optical microscopy, SEM/TEM and X-ray radiography in 2D, or XCT and 3D-EM in 3D). Combined utilization of these techniques can characterize the heterogeneity of shale microstructures over a large range of scales, from macroscale to nanoscale (~10 0-10-9 m). Other chemical and physical measurements can be correlated to imaging techniques to provide complementary information for minerals, organic matter and pores. These imaging techniques and subsequent quantification methods are critically reviewed to provide an overview of the correlative imaging workflow. Applications of the above techniques for imaging particular features in different shales are demonstrated and key limitations and benefits summarized. Current challenges and future perspectives in shale imaging techniques and their applications are discussed.

Comparing organic-hosted and intergranular pore networks: topography and topology in grains, gaps and bubbles

“…What is more, the organic-hosted porosity frequently governing hydrocarbon flow and recovery occurs at the smallest scales, only accessible using 3D imaging techniques such as focused ion beam-scanning electron microscopy (FIB-SEM). The impact of scale in subsurface shale pore structures is complex (Ma et al 2017), and thus the scope of this study was limited to examining the structure of organic-hosted 'bubble' porosity (Loucks et al 2012;Milliken et al 2013). All multiscale imaging was performed using the Atlas 5.2 correlative imaging platform.…”

Predicting Effective Diffusion Coefficients in Mudrocks Using a Fractal Model and Small‐Angle Neutron Scattering Measurements

“…Combining small‐angle (SANS) and very small angle (VSANS) neutron scattering provides nondestructive information on the pore space of micrometer‐ to nanometer‐sized pore systems, such as in mudrocks. It has the advantage that the method requires relatively small sample volumes, in our case about 20 × 20 × 0.2 mm, which is a volume that is still considered to be representative for mudrocks when considering porosity or pore size characteristics, so a volume that is considerably larger than the representative elementary volume of mudrocks (up to lower micrometer range; Ma et al, ). A larger thickness might cause secondary scattering; a smaller thickness would lead to longer measurement times to obtain the same statistics in terms of total scatterers.…”

“…Water Resources Research micrometer range; Ma et al, 2017). A larger thickness might cause secondary scattering; a smaller thickness would lead to longer measurement times to obtain the same statistics in terms of total scatterers.…”

Fine-Scale Heterogeneity of Pyrite and Organics within Mudrocks: Scanning Electron Microscopy and Image Analysis at the Large Scale

“…Mudrocks are one of the most ubiquitous sedimentary materials, both on the Earth's surface and in terms of stratigraphic distribution [1][2][3][4], and are of great significance as seals/traps for oil and gas reservoirs, for the confinement of aquifers, for CO 2 seals, for nuclear and urban waste, for conventional and unconventional source rocks and for reservoirs, in the formation of structural foundations, and important repositories of past environmental records [5][6][7][8][9][10][11]. Although at the macro level mudrocks are visually monotonous, on closer inspection, they are a highly heterogeneous sedimentary Minerals 2020, 10, 354 2 of 16 rock group, varying widely in terms of colour, sedimentary structures, fissility, grain-size (clay and silt-sized), grain orientation, mineralogy, organic content, microfauna-macrofauna, ichofauna-fabric, porosity, permeability and diagenetic history [12][13][14][15][16]. Heterogeneity can be recognised at the micron, millimetre, centimetre and metre scales.…”