Porosity plays a key role in the physical and mechanical properties of materials, including compression and tensile strength, density, and thermal conduction or expansion. Porosity also affects the resistance of a material to chemical corrosion. The combination of these properties defines the material behavior under a given set of physical conditions and its functionality as an intermediate or final product.
If the material is to be used for catalysis or insulation, a high porosity is desired. Low porosity is needed for materials utilized because of their strength or impermeability to fluids.
In natural rocks such as sandstones or shale, pore volume is key to assessing the capacity of the rock formation as a reservoir for economically exploitable fluids such as natural gas, oil, or water. There exist a number of methods of porosity measurement in materials -some methods measure porosity in 2D and others in 3D. Each technique has its own advantages, disadvantages and limitations.
Whether using optical or SEM analysis, sample preparation is critical, requiring extreme care for porosity measurement with microscopy. It is possible to perform both methods on polished petrographic thin sections, whereas SEM can be performed on polished sections or on ion-milled surfaces. As mechanical polishing and grinding is an economic and easy preparation method, a broad range of materials prepared this way are studied.
Hence only results obtained for an ion-milled shale sample are presented here. The sample consists of a coupon ~10x10x5mm cut from the rock; a triangular area roughly 2x2x3mm near the edge was ion-milled to provide a flat and smoothsurface as shown in Figure 1a.
Figure 1. A sequence of SE images of the ion-milled shale sample acquired with an KLA Tencor8500 FE-SEM at increasing magnification (a–f). Images show pores to be primarily present in the intergranular clayey matrix (cement) and in the pyrite framboids (indicated by arrows in c and d).
Conductive carbon tape was used to make the sample adhere to an aluminium stub and the stub was mounted on the microscope sample holder. Sample imaging was done using an 8500 FE-SEM from KLA Tencor in high resolution mode with an accelerating voltage of 1kV and a working distance of ~2.7–3.0mm for high magnification and ~10.1mm for low magnification.
The KLA Tencor8500 FE-SEM is a low-voltage, compact, field-emission SEM that employs a novel electrostatic lens design. This innovative design enables high-resolution imaging of insulating samples, typically without the need for metal coating.
Results and Discussion
Figure 1a shows that observing the milled surface at low magnification indicates the shale includes several mineral phases. There are pores in a clay-like matrix, making up the intergranular cement of the rock as shown in Figure 1b. Pores are present in the framboids or pyrite spheres as shown in Figures 1c and 1d.
This heterogeneous distribution of pores among mineral phases implies that phase analysis (quantification) is needed for pore volume to be estimated correctly for the shale. Knowing the proportions of pyrite and clay in the bulk sample would allow correct calculation of pore volume of the rock from 2-D SEM images.
For instance, if the rock contains 8% pyrite and 30% clay by volume and SE image analysis gives pore volumes of 10% in clay and 5% in pyrite, then the total porosity due to these two phases is 0.1x0.3 + 0.05x0.08, which is 0.034 (or 3.4%). This calculation is possible after quantitative phase analysis of the shale rock using image analysis of the 2-D image sequence acquired for the sampled surface.
Two different pore-related features have to be considered. The first is pore size distribution. In SE images obtained with the KLA Tencor8500 and presented shown in Figures 1 and 2, two size categories of pores are seen in this sample.
Figure 2. A sequence of SE images of the ion-milled shale sample acquired with the KLA Tencor8500 FE-SEM at increasing magnification. Images show pores to have different shape and size and to be mainly present in the intergranular clayey matrix.
Micron-to-submicron pores are present in both the clay mineral phase and the pyrite framboids. However, nanopores are only present in the matrix (Figures 1c–1f, and Figure 2). The second feature, although not seen directly, is inferred from the petrology. It stems from the fact that argillaceous sediments tend to be stratified and have highly preferred orientation of layered minerals.
Stratification may cause anisotropy in both pore distribution and orientation and hence permeability of the shale formation. Hence it is suggested that shale sample imaging should be done along two perpendicular directions in order to verify anisotropy in pore orientation/distribution.
The sequence of SE images presented in Figure 3 shows that certain grains/crystals contain phase inclusions that might be mistakenly considered as pores. This confusion may occur due to poor resolution SE images obtained with other FE-SEM systems. Hence the high-resolution and excellent-contrast imaging of these grains with the powerful KLA Tencor8500 FE-SEM actually avoids EDS analysis of these inclusions when the purpose is uniquely measurement of porosity.
Figure 3. SE images of the ion-milled shale sample acquired with the KLA Tencor8500 FE-SEM. Images show the rock to have different mineral phases. Only intergranular clayey matrix (cement) and occasionally pyrite framboids have a significant amount of pores. The high-contrast grains in c through f (not determined) have inclusions that might be confused with pores in poor-resolution images obtained with other FE-SEM systems.
In this preliminary study, a KLA Tencor8500 FE-SEM was used to obtain high-contrast, high-resolution images of a shale sample. It was not required to metal-coat this insulating material. The resulting images of the milled surface show pores in two mineral phases and a wide range of size distribution and most probably preferred orientation.
The methodology followed here for porosity measurement is straightforward and image analysis allows quantitative phase analysis of the shale and then quantification of pore volume in each individual phase. Total porosity can then be readily calculated.