FIB/SEM and SEM/EDX: a New Dawn for the SEM in the Core Lab?
- H.J. Lemmens (FEI) | A.R. Butcher (FEI) | P.W.S.K. Botha (FEI)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2011
- Document Type
- Journal Paper
- 452 - 456
- 2011. Society of Petrophysics and Well Log Analysts
- 2 in the last 30 days
- 364 since 2007
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Increased interest in shale and tight gas reservoir characterization has led to the adoption of FIB/SEM technology as the next step in resolution to visualize the pore network. The high resolution of a Scanning Electron Microscope (SEM) combined with the precise cutting capability of a Focused Ion Beam (FIB) enables 3D reconstructions with resolution of a few nanometers. The FIB is capable of removing a controlled amount of material to create a subsequent 2D section parallel and aligned with the previous one, with inter-section spacing of the order of 10 nm, and having resolution of a few nanometers in the section plane. In this way, after careful combination of the subsequent slices, a 3D model with nanometers resolution in the XY direction and 10 nm in the z-direction is obtained. Examples of shale reservoir rock reconstructions are discussed in this paper.
Curtaining is the most common artifact caused by het-erogeneities in the material under study. Shale samples are especially prone to this with the combination of porosity, organic phases, clay particles and pyrite inclusions all having a different milling rate with respect to the ion beam. Curtaining creates vertical lines in the images with a grey level that can be comparable to another, real phase. Improper segmentation can classify curtaining lines as real phases, creating non-existing pore throats and leading to an overestimation of permeability.
Another emerging application of the SEM is automated mineral identification and textural mapping of cores and cuttings. The electron beam of the SEM not only generates secondary (SEI) and backscatter (BSE) electrons that create the familiar high-resolution black and white photomicrographs, but also secondary X-rays that are characteristic of the phase under the beam. By taking the energy dispersive X-ray (EDX) spectrum, comparing it with a library of phases, and combining this with the BSE and SEI signals, it is possible to create false-colored digital phase and texture maps of cores. This allows cores to be quantitatively lithotyped, especially in terms of how detrital mineral grains, authigenic overgrowths, and pore structures are associated. This in turn leads to a better understanding of pore-matrix interactions, including pore surfaces. Examples of mineral maps generated by automated SEM/EDX analysis of cores and cuttings are presented to illustrate how these data can be integrated with FIB/SEM data to provide a holistic petrographic approach.
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