Micro- and Nano-Scale Pore Structure in Gas Shale Using Xμ-CT and FIB-SEM Techniques

Abstract

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.

Integration of Multiscale Imaging of Nanoscale Pore Microstructures in Gas Shales

“…Therefore, multi-scale imaging techniques are required to characterize the shale microstructure. This paper extends our previous work [17] by using the same sample volume to characterize the 3D microstructure using the nano-CT method in order to bridge the gap in information between previously obtained FIB-SEM data, which has a maximum pore size and volume resolution of about 20 nm and 0.008 m 3 , respectively, and micro-CT, which has a minimum pore size and volume resolution of about 930 nm and 0.804 m 3 , again respectively [17]. We then compare the combined multi-CT dataset with allied measurements made using the MIP and nitrogen absorption techniques to (i) examine the extent to which each measurement provides a consistent description of the nanostructure and microstructure of the shale, and (ii) to arrive at a full multi-scale quantitative characterisation of the complex pore system and microstructure of Bowland gas reservoir shales.…”

Disinfection Treatments of Disposable Respirators Influencing the Bactericidal/Bacteria Removal Efficiency, Filtration Performance, and Structural Integrity

“…For a non-destructive 3D visualizations of an internal structure of materials, X-ray computed tomography (Xµ-CT) was performed using Zeiss X-Radia 510 Versa (Zeiss, Oberkochen, Germany) ( Figure S1, see Supplementary Materials ) [ 38 ]. The X-ray source was operated at a voltage of 60 kV with a power of 5.0 W. The fields of view used were 800, 1200, 2000 µm, and the corresponding pixel sizes were 0.8, 1.2, 2.0 µm, respectively.…”

Ultrahigh-Resolution 3D Imaging for Quantifying the Pore Nanostructure of Shale and Predicting Gas Transport

“…Aspect ratios are measures of the shape of a pore by comparing two characteristic dimensions, often the largest to the smallest, such as the length of a crack to its aperture [27].…”

“…Consequently, , we have introduced a parameter in a previous paper [27], which we name the Scale-Invariant Surface Area to Volume Ratio, presented by , to avoid any inconsistencies, and it is defined as…”