HR: 1340h
AN: V43D-1444    [Abstracts]
TI: Permeability and Degassing of Porous Volcanic Rocks Undergoing Rapid Decompression: an Experimental Determination.
AU: Mueller, S
EM: mueller@min.uni-muenchen.de
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Melnik, O
AF: Institute of Mechanics, Moscow State University, 1-112b, Michurinsky prosp., Moscow, 119192 Russian Federation
AU: * Spieler, O
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Scheu, B
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Dingwell, D B
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AB: The gas permeability of volcanic rocks may influence various eruptive processes. The transition from a quiescent degassing dome to rock failure (fragmentation) may, for example, be controlled by the rock's permeability, in as much as it affects the speed by which a gas overpressure in vesicles is reduced in response to decompression. To measure permeability, we use a modified setup of a shock-tube-based fragmentation apparatus. The method is based on an unsteady-state measuring principle. After sudden decompression above the rock cylinder, pressurized gas flows through the sample. The exponentially decaying pressure trend of a defined gas reservoir below the sample delivers the basis for the permeability determination. A transient 1D filtration code was developed to analyse the experimental data. Hereby the flow is assumed to be isothermal due to the high heat capacity of the matrix skeleton, the intense heat transfer between gas and skeleton, and the nearly constant temperature in the gas volume below the sample. A non-linear friction term is taken into account to describe the rapid filtration processes. Additionally, we provide a simplified method to determine the permeability coefficient as during most of the experiment the system behaves quasi-statically. Values of \emph{k} predicted by both methods are similar, with maximum difference of only 0.3 log units. This means that the steady-state approach provides an easy and fast method to determine permeability. Over 100 permeability measurements have been performed on samples covering a wide range of porosity. The results show a general positive relationship between porosity and permeability with a high data scatter. Our preferred interpretation of the results is a combination of two different, but overlapping effects. We propose that at low porosities, gas escape occurs predominantly through microcracks or elongated micropores and therefore could be described by simplified forms of Kozeny-Carman relations and fracture flow models. At higher porosities, the influence of vesicles becomes progressively stronger as they form an increasingly connected network. Therefore, a model based on the percolation theory of fully penetrable spheres is used, as a first approximation, to describe the permeability-porosity trend.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting