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| Titel |
A tool for computing time-dependent permeability reduction of fractured volcanic conduit margins. |
| VerfasserIn |
Jamie Farquharson, Fabian Wadsworth, Michael Heap, Patrick Baud |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135513
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| Publikation (Nr.) |
 EGU/EGU2016-16389.pdf |
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| Zusammenfassung |
| Laterally-oriented fractures within volcanic conduit margins are thought to play an important
role in tempering eruption explosivity by allowing magmatic volatiles to outgas. The
permeability of a fractured conduit margin—the equivalent permeability—can be
modelled as the sum of permeability contributions of the edifice host rock and the
fracture(s) within it. We present here a flexible MATLAB® tool which computes the
time-dependent equivalent permeability of a volcanic conduit margin containing ash-filled
fractures. The tool is designed so that the end-user can define a wide range of input
parameters to yield equivalent permeability estimates for their application. The
time-dependence of the equivalent permeability is incorporated by considering
permeability decrease as a function of porosity loss in the ash-filled fractures due to
viscous sintering (after Russell and Quane, 2005), which is in turn dependent on the
depth and temperature of each fracture and the crystal-content of the magma (all
user-defined variables). The initial viscosity of the granular material filling the
fracture is dependent on the water content (Hess and Dingwell, 1996), which is
computed assuming equilibrium depth-dependent water content (Liu et al., 2005).
Crystallinity is subsequently accounted for by employing the particle-suspension
rheological model of Mueller et al. (2010). The user then defines the number of fractures,
their widths, and their depths, and the lengthscale of interest (e.g. the length of the
conduit).
Using these data, the combined influence of transient fractures on the equivalent
permeability of the conduit margin is then calculated by adapting a parallel-plate flow model
(developed by Baud et al., 2012 for porous sandstones), for host rock permeabilities from
10−11 to 10−22 m2. The calculated values of porosity and equivalent permeability
with time for each host rock permeability is then output in text and worksheet file
formats. We introduce two dimensionless scale limits: the first to determine the
applicability of the compaction model over the width of each fracture, and the second to
indicate whether the fracture can outgas (or if pore pressure within the fracture
will increase). The computational tool warns the user when the scale limits are
violated.
We outline applications of our tool: both independently and in concert with larger-scale
models of volcanic outgassing. For example, in testing the tool, results suggest that
fractures in a highly permeable edifice have marginal—if any—influence on the overall
permeability of a volcanic system. On the other hand, in low permeability systems,
even narrow fractures can allow significant outgassing to occur. Similarly, shallow
fractures will serve to increase outgassing capability relative to deep fractures that heal
more rapidly. We highlight the eminent flexibility of our tool, which enables it to
be adapted to a wide range of specific user-defined requirements and scenarios. |
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