Shale is a complex rock composed of a complex mixture of matrix minerals, kerogen and having a complex pore microstructure. The pore microstructure is highly-dependent upon the scale at which it is considered. Such microstructure is important for the assessment of the potential of gas shales based on the connectivity and pores at each scale, and the ability of the rock to be hydraulically fractured. In this work the three dimensional (3D) structure of Bowland shale has been investigated at both microscopic and nanoscopic scales on the same sample for the first time using (i) a combination of serial sectioning, using focused ion beam (FIB) milling and scanning electron microscopy (SEM), and (ii) X-ray micro-computed tomography (Xµ-CT). The reconstructed matrix, kerogen and pore space volumes from each approach showed significant scale-dependent differences in microstructure. The shale samples displayed a high kerogen content with high connectivity. Porosity in the shale rock sample was observed to be prevalent in either the inorganic matrix, the kerogen, or both. Furthermore, the porosity from the reconstructed shale volumes was found to vary with location, as sampled by FIB-SEM, within the shale samples taken for Xµ-CT. Pore volume, scale invariant surface area to volume ratio and two orthogonal pore aspect ratio distributions were extracted from the reconstructed image data by 3D image analysis. These data show that voids within the rock are oblate at all scales. However, the smaller pores visible by FIB-SEM present higher scale invariant surface area to volume ratios, indicating that they are more likely to interlink the larger pores visible by Xµ-CT and form a small scale but highly connected pore network for fluid flow. Permeabilities have been calculated from both the FIB-SEM and Xµ-CT images and fall in the range 2.98 nD to 150 nD, broadly agreeing with experimental determinations from another author.

This is a repository copy of Ultrahigh-Resolution 3D Imaging for Quantifying the Pore Nanostructure of Shale and Predicting Gas Transport.

<p> </p><p>Cost-effective and environmentally sensitive shale gas production requires detailed knowledge of the petrophysical characteristics of the shale from which the gas is extracted. Parameters such as the kerogen fraction, pore size distributions, porosity, permeability, the frackability of the rock and the degree to which natural fracturing already occurs are required in order to be able to estimate potential gas reserves and how easily it can be extracted. Innovative imaging techniques, including Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Nanoscale X-Ray Tomography (nano-CT), can be used to characterise the microstructural properties of shale. Here we report using FIB-SEM serial sectioning and nano-CT on approximately cubic samples of side length about 25 µm. The resolution of the FIB-SEM scanning is approximately 20 nm, while that of the nano-CT is about 50 nm, providing between 125 and 1953 million voxels per scan. These ultra-high resolution techniques have been shown to be effective methods for the analysis and imaging of shale microstructure. Each technique can provide data over a different and separate range of scales, and with different resolutions. Results analysed so far indicate that pores which seem to be unconnected when imaged on a micrometre scale by micro-CT scanning, are connected by thin pathways when imaged at these higher resolutions. This nano-scale connectivity is responsible for the small but non-zero permeability of gas shales to gas flow, which is typically measured in the range 5 nD – 200 nD. The volume, size, aspect ratios, surface area to volume ratio and orientations have all been calculated from the scanned data as a function of scale. These data indicate an extremely complex, heterogeneous, anisotropic and multimodal pore nanostructure and microstructure for the shales, with structure at all scales contributing to both gas storage and gas flow. Further work analysing the connectivity of the microscale and nanoscale pore spaces within the rock is underway. We believe that the combination of nano-CT with FIB-SEM on the same sample has the potential for providing an enhanced understanding of shale microstructure, which is necessary for modelling elastic behaviour, gas storage, gas desorption and gas flow in gas shales.</p><p><strong>Keywords:</strong> Gas shale, FIB-SEM, nano-CT, porosity, permeability, Kerogen, pore volume, size distribution, pore aspect ratio and surface area to pore volume.</p>

The ability to transport nanoparticles through porous media has interesting engineering applications, notably in reservoir capacity exploration and soil remediation. A series of core-flooding experiments were conducted for quantitative analysis of functionalized TiO2 nanoparticles transport through various porous media including calcite, dolomite, silica, and limestone rocks. The adsorption of surfactants on the rock surface and nanoparticle retention in pore walls were evaluated by chemical oxygen demand (COD) and UV–Vis spectroscopy. By applying TiO2 nanoparticles, 49.3 and 68.0 wt.% of surfactant adsorption reduction were observed in pore walls of dolomite and silica rock, respectively. Not surprisingly, the value of nanoparticle deposition for dolomite and silica rocks was near zero, implying that surfactant adsorption is proportional to nanoparticle deposition. On the other hand, surfactant adsorption was increased for other types of rock in presence of nanoparticles. 5.5, 13.5, and 22.4 wt.% of nanoparticle deposition was estimated for calcite, black and red limestone, respectively. By making a connection between physicochemical rock properties and nanoparticle deposition rates, we concluded that the surface roughness of rock has a significant influence on mechanical trapping and deposition of nanoparticles in pore-throats.

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