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Rock porosity determination by combination of X-ray computerized tomography with mercury porosimetry
(1997)
Abstract First results are described of coupling of mercury porosimetry with X-ray computerized tomography (CT) as a new combination technique for rock porosity studies. This technique is suitable for rock samples with a pronounced mercury intrusion-extrusion hysteresis and includes CT measurements before and after mercury intrusion. The entrapped portion of mercury, when the pressure after the intrusion into the rock sample is reduced to 0.1 MPa, serves as a contrast agent in the porous network to localize spatial distribution of rock porosity by CT. The results obtained show that the mercury intrusion and therefore the porosity were quite different for the separate mineral phases. Therefore the combination of mercury porosimetry and computer tomography can give 3-D data on mineral-specific porosity distributions with additional pore size information. In contrast to mercury porosimetry as a single method, results of the combination technique with CT represent a direct visualization of porosity variation and do not depend on any special pore network model.
The application of two sets of methods for the characterization of rock pore systems is reported. Both are discussed in the context of other techniques described in the literature. (i) Impregnation with labelled polymethylmethacrylate (PMMA) and autoradiography or direct measurement of the tracer activity allows the investigation of the pattern of the spatial porosity distribution and quantitative measurement of mineral-specific, local porosities, porosity gradients as well as an assessment of hydraulic and diffusive transport properties. (ii) Quantitative information on surface areas within certain pore size ranges can be achieved by combination of mercury intrusion porosimetry with X-ray absorption computertomography. As examples of applications of these techniques, the effects of weathering, alteration, mechanical stress and large diameter coring on the pore network of rocks have been studied.
Die Struktur eines Materials bestimmt wesentlich dessen Gebrauchsverhalten. Diese einfache Weisheit ist Motivation für die ständige Weiterentwicklung des Untersuchungsinstrumentariums zur Gefügekennzeichnung von Baustoffen. Nachdem die Gefügeaufklärung insbesondere bei mineralischen Baustoffen bereits integrierter Bestandteil der Baustoffcharakterisierung ist, erhöht insbesondere die zunehmende Spezialisierung und Weiterentwicklung der Asphaltbaustoffe zusammen mit der Forderung nach gebrauchsgerechtem Leistungsnachweis den Bedarf an methodischer Entwicklung bei der Strukturkennzeichnung. Besondere Möglichkeiten zur tiefenaufgelösten und zerstörungsfreien räumlichen Visualisierung des Gefüges, der Ableitung strukturbeschreibender Kennwerte aber auch für eine anschließende Diskretisierung der inneren Struktur mittels finiter Elemente bietet das Verfahren der Computertomographie mit Röntgenstrahlen. Mit Hilfe verschiedener Anwendungen soll auf die Möglichkeiten dieser Methodik aufmerksam gemacht werden.
Algorithms were developed to approximate the 3-dimensional internal packing density distributions of fibrous filter media at different levels of detail and complexity. Each algorithm uses certain input information derived from X-ray tomographic data of filter samples.
Algorithm 1 creates a binary media structure consisting of the true (i.e. tomographically determined) void distribution plus regions of uniform packing density. The average packing density of the media is maintained constant. Algorithm 2 creates a model fibrous structure of straight fibres of equal diameter and random length positioned randomly in space, while maintaining the true (i.e. tomographically determined) fibre orientation distribution. The number and length of the fibres on average adds up to the packing density of the filter. The model fibrous structure is recreated by a stationary Poisson process of convex bodies. Algorithm 3 distributes pores of random size and location within a homogeneous matrix, such that the average packing density again coincides with the true (i.e. tomographically determined) packing density. This algorithm is also based on a stationary Poisson process of convex bodies.
The capability of each algorithm to recreate the essential structural features of the media was tested against 'reality' by computing the respective overall pressure drop of the filter as well as the velocity distribution in the filter interior, and comparing with the results obtained for the 'true' packing density distribution of a sample measured by tomography. Compared to the assumption of a completely homogenous filter (which gives roughly 2 times the actual Δp), all algorithms are closer to reality. The binary algorithm deviates in Δp by a factor of 1.8; algorithm 3 comes within a factor of 1.6 of the true Δp. The best approximation is by algorithm 2 which narrows the difference in Δp to a factor of 1.4.