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 and kerogen and having a complex pore microstructure. The pore microstructure is highly dependent upon the scale at which it is considered. Such a 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 the 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 locations, as sampled by FIB-SEM, within the shale samples taken for Xμ-CT. Pore volume, scale invariant surface area to volume ratios, 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 to 150 nD, broadly agreeing with experimental determinations from another author.