Microstructure and Pores

With the development of imaging technology, tools to quantitatively describe pore structure, morphology, and connectivity have been widely applied on low permeable rocks; however, it is still questionable to what extent this information can be used to predict permeability. Applicability and comparability of different techniques are discussed here for the Middle Ordovician Cobourg limestone (Canada), a rock dominated by calcite grains of variable sizes (µm–cm) and heterogeneously distributed quartz, dolomite, pyrite, and meshy clay minerals. Absolute porosities determined by helium pycnometry (HP) in literature are approximately 1.6% (±0.9%), and gas permeabilities range from 10−20 to 10−19 m2. Porosities obtained from BIB-SEM are much smaller compared to those from HP (16–69% of HP). Pores found in clays are smaller, slit-shaped, and more densely spaced when compared to those in calcite minerals. Connectivity between pores could not be resolved with 3D micro-CT or FIB-SEM reconstructions, which have a resolution limit of 8 µm and 10 nm, respectively. However, assuming the pores to be connected, laboratory-derived permeability data could be fitted using a simple capillary bundle model, including information about the visible pore size distributions obtained from BIB-SEM images and a tortuosity range of 8 to 15.

<strong class="journal-contentHeaderColor">Abstract.</strong> A microphysics-based understanding of mechanical and hydraulic processes in clay shales is required for developing advanced constitutive models, which can be extrapolated to long-term deformation. Although many geomechanical tests have been performed to characterise the bulk mechanical, hydro-mechanical, and failure behaviour of Opalinus Clay, important questions remain about micromechanisms: how do microstructural evolution and deformation mechanisms control the complex rheology? What is the in situ microstructural shear evolution, and can it be mimicked in the laboratory? In this contribution, scanning electron microscopy (SEM) was used to image microstructures in an Opalinus Clay sample deformed in an unconsolidated–undrained triaxial compression test at 4 MPa confining stress followed by argon broad ion beam (BIB) polishing. Axial load was applied (sub-)perpendicular to bedding until the sample failed. The test was terminated at an axial strain of 1.35 %. Volumetric strain measurements showed bulk compaction throughout the compression test. Observations on the centimetre to micrometre scale showed that the samples exhibited shear failure and that deformation localised by forming a network of micrometre-wide fractures, which are oriented with angles of 50<span class="inline-formula">^∘</span> with respect to horizontal. In BIB–SEM at the grain scale, macroscale fractures are shown to be incipient shear bands, which show dilatant intergranular and intragranular microfracturing, granular flow, bending of phyllosilicate grains, and pore collapse in fossils. Outside these zones, no deformation microstructures were observed, indicating only localised permanent deformation. Thus, micromechanisms of deformation appear to be controlled by both brittle and ductile processes along preferred deformation bands. Anastomosing networks of fractures develop into the main deformation bands with widths up to tens of micrometres along which the sample fails. Microstructural observations and the stress–strain behaviour were integrated into a deformation model with three different stages of damage accumulation representative for the deformation of the compressed Opalinus Clay sample. Results on the microscale explain how the sample locally dilates, while bulk measurement shows compaction, with an inferred major effect on permeability by an increase in hydraulic conductivity within the deformation band. Comparison with the microstructure of highly strained Opalinus Clay in fault zones shows partial similarity and suggests that during long-term deformation additional solution–precipitation processes operate.

Summary Type II-S kerogen source rock plugs with high TOC contents (10 to 20 wt.%) were investigated in their initial immature state as well as in different thermal maturities induced by hydrous pyrolysis under both unstressed and stressed conditions in order to investigate hydrocarbon generation and primary migration. Applied methods comprise organic geochemical investigations to assess the state of kerogen conversion as well as visual techniques such as reflected light microscopy and BIB-SEM to investigate the distribution and evolution of kerogen, solid bitumen and pore space within these rocks. Our first results indicate that primary migration within such high TOC rocks occurs along an interconected porenetwork that evolves upon kerogen conversion rather than due to the buildup of overpressure and microfracturing.

Abstract Direct assessment of porosity and permeability in unconventional reservoirs such as shales and tight gas sands using imaging technologies is a challenge because of small pore throats, a broad pore size range, and sample heterogeneities with respect to mineralogy and grain size. This contribution presents the outcome of a range of Liquid Metal Injection (LMI) experiments in which we injected various fusible alloys into unconventional reservoir type of rocks, followed by Broad Ion Beam (BIB) polishing and SEM imaging to visualize and to better understand the pore connectivity and transport mechanisms in these tight rocks.In a series of experiments liquid metal was injected into a variety of organic-rich shales and other tight rocks at elevated temperature (approximately 90 °C) by increasing the pressure stepwise, up to a maximum of 500 MPa. Once the desired pressure was reached, the experiment was completed by turning off the heating and letting the samples cool down while maintaining the pressure. After solidification of the alloy, the sample was argon-ion polished to create a smooth surface for SEM imaging. Imaging sister samples on which we applied different intrusion pressures enables insight into how these tight rocks are progressively filled with the alloy.Firstly, injection of a non-wetting alloy followed by BIB surface polishing and SEM imaging allows comparison with Mercury Intrusion Porosimetry and visualizing which parts of the pore space are filled during intrusion of a liquid metal. SEM enables identifying alloy-filled pores below ten nanometers in size. High-resolution SEM images provide insight in preferred transport pathways and the variability of pore throat sizes. The experiments show that the non-wetting alloy does not intrude into samples in a uniform way. The relatively lowest porous sample investigated displayed the intrusion of alloy only into fractures, interpreted as induced micro-fractures, while others showed alloy within parts of the in-situ pore space. In a Posidonia Shale sample, intruded and non-intruded layers exist within the same sample, while the layers show no significant difference in mineral fabric at the large scale. At higher resolution, porous calcite fossils and framboidal pyrites were mostly filled with the alloys as was the clay-rich matrix. Injection of a sister sample with a more wetting alloy showed uniform intrusion of the alloy throughout the sample in the in-situ pore space and micro-fractures. The segmented SEM images allow quantification of the connected porosity. The question, which of the identified micro-fractures are induced or not, is subject of current research.Recent advances in LMI in combination with BIB milling technologies and SEM facilitate imaging connected porosity, both induced and in-situ, down to the nanometer scale in representative 2D cross-sections. Hence, LMI-BIB-SEM is a relatively easy method to verify whether unconventional reservoir samples contain good poro-perm and fracability properties.

Sensors UpdateVolume 4, Issue 1 p. 167-193 Sensor Applications CMOS Imaging Sensors for Space Applications Werner Ogiers, Werner Ogiers I.M.E.C.; Division MAP; Kapeldreef 75; 3001 Leuven; BelgiumSearch for more papers by this author Werner Ogiers, Werner Ogiers I.M.E.C.; Division MAP; Kapeldreef 75; 3001 Leuven; BelgiumSearch for more papers by this author First published: 12 September 2001 https://doi.org/10.1002/1616-8984(199808)4:1<167::AID-SEUP167>3.0.CO;2-MAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume4, Issue1August 1998Pages 167-193 RelatedInformation