The connectivity of pore space in mudstones: insights from high‐pressure Wood's metal injection, BIB‐SEM imaging, and mercury intrusion porosimetry

Abstract

Study of the pore space in mudstones by mercury intrusion porosimetry is a common but indirect technique and it is not clear which part of the pore space is actually filled with mercury. We studied samples from the Opalinus Clay, Boom Clay, Haynesville Shale, and Bossier Shale Formations using Wood's metal injection at 316 MPa, followed by novel ion beam polishing and high-resolution scanning electron microscopy. This method allowed us to analyze at high resolution which parts of a rock are intruded by the liquid alloy at mm to cm scale. Results from the Opalinus Clay and Haynesville Shale show Wood's Metal in cracks, but the majority of the pore space is not filled although mercury intrusion data suggests that this is the case. In the silt-rich Boom Clay sample, the majority of the pore space was filled Wood's metal, with unfilled islands of smaller pores. Bossier Shale shows heterogeneous impregnation with local filling of pores as small as 10 nm. We infer that mercury intrusion data from these samples is partly due to crack filling and compression of the sample. This compaction is caused by effective stress developed by mercury pressure and capillary resistance; it can close small pore throats, prevent injection of the liquid metal, and indicate an apparent porosity. Our results suggest that many published MIP data on mudstones could contain serious artifacts and reliable metal intrusion porosimetry requires a demonstration that the metal has entered the pores, for example by Wood's metal injection, broad ion beam polishing, and scanning electron microscopy.

Investigation of Microstructures in Naturally and Experimentally Deformed Reference Clay Rocks Using Innovative Methods in Scanning Electron Microscopy

“…In comparison with the FIBcryo-SEM method, the BIB-cryo-SEM method allows investigation of much larger representative cross-sections (in the range of mm within the SEM (Figure 3c,d) was also used to estimate pore-scale damage after drying . (Hildenbrand and Urai, 2003;Klaver et al, 2015b) with comparable values of surface tension of the fluid (g ¼ 0.420 N/m) and wetting angle (w ¼ 1408) (Abell et al, 1999;Hildenbrand and Urai, 2003). The WMI is accomplished at a temperature slightly above the melting point of WM (708C), which fills the pores and then solidifies upon cooling to room temperature.…”

“…The SEM imaging of pore structures on ion beam-prepared surfaces gives both qualitative and quantitative insights into porosity within 2D cross-sections (Desbois et al, 2009;Hemes et al, 2013Hemes et al, , 2015Houben et al, 2013Houben et al, , 2014aHouben et al, , 2014bKlaver et al, 2012Klaver et al, , 2015aKlaver et al, , 2015b. Segmentation of porosity combined with segmentation of mineralogy provides porosity-related mineralogy maps and results in the concept of "elementary building blocks" (Houben et al, 2014a;Hemes et al, 2015, after Desbois et al, 2011a.…”

“…The cross-sections produced by FIB and BIB are of such a high quality (the remaining topography is in the range of 5 nm in height; Klaver et al, 2012) that they allow direct, unambiguous investigation of the pores at the resolution (,10 nm) of state-of-the-art SEM. The direct detection of porosity within a truly planar 2D cross-section enables the statistical and stereological description of the porosity (Desbois et al, 2009;Hemes et al, 2013Hemes et al, , 2015Houben et al, 2013Houben et al, , 2014aHouben et al, , 2014bKlaver et al, 2012Klaver et al, , 2015aKlaver et al, , 2015b. On one hand, BIB cross-sectioning produces large representative areas up to 2 mm 2 for quantitative porosity analyses, but is restricted to 2D (Figure 1c -e).…”

“…The emergence of ion-beam milling tools (i.e. BIB and FIB) has improved microstructural investigations in fine-grained geomaterials, providing unprecedented clarity of microstructures at below the micrometer scale (Loucks et al, 2009;Curtis et al, 2010;Desbois et al, 2009Desbois et al, , 2011bHeath et al, 2011;Hemes et al, 2013Hemes et al, , 2015Houben et al, 2013Houben et al, , 2014aHouben et al, , 2014bKeller et al, 2011Keller et al, , 2013aKeller et al, , 2013bKlaver et al, 2012Klaver et al, , 2015aKlaver et al, , 2015b. Cryogenic methods combined with FIB, BIB, and SEM has delivered, for the first time, direct imaging of pores filled with original pore fluids (Desbois et al, 2008(Desbois et al, , 2009.…”

“…Cryogenic methods combined with FIB, BIB, and SEM has delivered, for the first time, direct imaging of pores filled with original pore fluids (Desbois et al, 2008(Desbois et al, , 2009. Injection of Wood's metal (WM), as an analog of MIP, but solid at room temperature, combined with BIB has been developed to visualize effective porosity decorated with WM at a given pore-throat entry pressure (Abell et al, 1999;Hildenbrand and Urai, 2003;Lloyd et al, 2009;Klaver et al, 2015b). The innovative methods mentioned above have thus allowed, for the first time, investigation of microstructures in natural samples and development of a new integrated approach including deformation mechanism-based microstructures.…”