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| Titel |
The formation of open fractures in brittle rocks and the evolution of permeability with fault slip- results from analogue and numerical models |
| VerfasserIn |
H. W. van Gent, S. Abe, J. L. Urai, M. Holland |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023819
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| Zusammenfassung |
| The formation of open cavities as a result of (normal) faulting of a brittle material under
low effective stress has profound effects on the hydraulic properties of rocks both
near the surface and at depth. It is however often difficult to access the fault zone
directly. Here we present the results from a series of analogue models of normal
faults in brittle rocks. Fine grained, dry Hemihydrate powder (CaSO4 * 1/2 H2O)
was used as the truly cohesive analogue material. An extensive characterization of
material properties, including the porosity dependency of both tensile strength and
cohesion, showed the increase of strength of the powder with burial in the experimental
box.
In side view observations of the analogue models three structural zones were
distinguished; a pure tensile failure zone at the surface and pure shear failure zone near the
bottom of the box. At mid-depths we observed a transitional zone with mixed mode failure
and the formation of fault cavities. These cavities initiate at local dip-changes of the fault and
can collapse with progressive deformation. The transitions between these zones can be
directly related to the increase of material strength due to burial compaction. The
intercalation of relatively softer sand layers and relatively stronger layers of a hemihydrate
and graphite mixture resulted in a marked increase of the complexity of the fault
zone, but the three structural zones remain clearly visible. The sand layers can form
decollement surfaces and "sand-smears". The observed structures compare well with fault
outcrops and fault related cave systems in carbonates, basalts and consolidated
sandstone.
We used Particle Image Velocimetry (PIV) to quantify deformation and strain and
observed plastic deformation prior to brittle failure at increments to small for visual
inspection. However, the forces involved remain largely unknown. Therefore we have used
the Discrete Element Method (DEM) to numerically model the formation of open fractures in
brittle lithologies three dimensions. We used the results of the material characterization to
define the strength of the brittle-elastic “bonds” between the numerical elements, as well as
the repulsive-elastic and frictional “contact”-interactions after these bonds are broken. The
three-dimensional numerical models allowed for detailed studies of fault initiation and the
evolution of the porosity-connectivity. The DEM-models also are a relatively quick way to
test the effects of different fault dips and changes in the material properties. The results from
simulations are in good agreement with the analogue experimental results and field
observations. |
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