Characterization of granite matrix porosity and pore-space geometry by in situ and laboratory methods

Schild, M.; Siegesmund, S.; Vollbrecht, A.; Mazurek, Martin (2001). Characterization of granite matrix porosity and pore-space geometry by in situ and laboratory methods. Geophysical journal international, 146(1), pp. 111-125. Blackwell Publishing Ltd 10.1046/j.0956-540X.2001.01427.x

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Most available studies of interconnected matrix porosity of crystalline rocks are based on laboratory investigations; that is, work on samples that have undergone stress relaxation and were affected by drilling and sample preparation. The extrapolation of the results to in situ conditions is therefore associated with considerable uncertainty, and this was the motivation to conduct the ‘in situ Connected Porosity’ experiment at the Grimsel Test Site (Central Swiss Alps). An acrylic resin doped with fluorescent agents was used to impregnate the microporous granitic matrix in situ around an injection borehole, and samples were obtained by overcoring. The 3-D structure of the porespace, represented by microcracks, was studied by U-stage fluorescence microscopy. Petrophysical methods, including the determination of porosity, permeability and P -wave velocity, were also applied. Investigations were conducted both on samples that were impregnated in situ and on non-impregnated samples, so that natural features could be distinguished from artefacts. The investigated deformed granites display complex microcrack populations representing a polyphase deformation at varying conditions. The crack population is dominated by open cleavage cracks in mica and grain boundary cracks. The porosity of non-impregnated samples lies slightly above 1 per cent, which is 2–2.5 times higher than the in situ porosity obtained for impregnated samples. Measurements of seismic velocities (Vp ) on spherical rock samples as a function of confining pressure, spatial direction and water saturation for both non-impregnated and impregnated samples provide further constraints on the distinction between natural and induced crack types. The main conclusions are that (1) an interconnected network of microcracks exists in the whole granitic matrix, irrespective of the distance to ductile and brittle shear zones, and (2) conventional laboratory methods overestimate the matrix porosity. Calculations of contaminant transport through fractured media often rely on matrix diffusion as a retardation mechanism.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Rock-Water Interaction

UniBE Contributor:

Mazurek, Martin

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0956-540X

Publisher:

Blackwell Publishing Ltd

Language:

English

Submitter:

Martin Mazurek

Date Deposited:

18 Sep 2014 16:46

Last Modified:

01 Nov 2019 06:35

Publisher DOI:

10.1046/j.0956-540X.2001.01427.x

BORIS DOI:

10.7892/boris.47490

URI:

https://boris.unibe.ch/id/eprint/47490

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