## Compaction-driven fluid flow in viscoelastic rock

### Details

Serval ID

serval:BIB_0412F64F27D9

Type

**Article**: article from journal or magazin.

Collection

Publications

Fund

Title

Compaction-driven fluid flow in viscoelastic rock

Journal

Geodinamica Acta

ISSN-L

0985-3111

Publication state

Published

Issued date

1998

Peer-reviewed

Oui

Volume

11

Pages

55-84

Language

english

Abstract

Compaction driven fluid flow is inherently unstable such that an

obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled

waves of porosity, propagated by dilational deformation due to an

effective pressure gradient within the wave. Viscous porosity waves have

attracted attention as a mechanism for melt transport, but are also a

mechanism for both the transport and trapping of fluids released by

diagenetic and metamorphic reactions. We introduce a mathematical

formulation applicable to compaction driven flow for the entire range of

rheological behaviors realized in the lithosphere. We then examine three

first-order factors that influence the character of fluid flow: (1)

thermally activated creep, (2) dependence of bulk viscosity on porosity,

and (3) fluid flow in the limit of zero initial connected porosity. For

normal geothermal gradients, thermally activated creep stabilizes

horizontal waves, a geometry that was thought to be unstable on the

basis of constant viscosity models. Implications of this stabilization

are that: (1) the vertical length scale for compaction driven flow is

generally constrained by the activation energy for viscous deformation

rather than the viscous compaction length, and (2) lateral fluid flow in

viscous regimes may occur on greater length scales than anticipated from

earlier estimates of compaction length scales. In viscous rock, inverted

geothermal gradients stabilize vertically elongated waves or vertical

channels. Decreasing temperature toward the earth's surface can induce

an abrupt transition from viscous to elastic deformation-propagated

fluid flow. Below the transition, fluid flow is accomplished by short

wavelength, large amplitude waves; above the transition flow is by high

velocity, low amplitude surges. The resulting transient flow patterns

vary strongly in space and time. Solitary porosity waves may nucleate in

viscous, viscoplastic, and viscoelastic rheologies. The amplitude of

these waves is effectively unlimited for physically realistic models

with dependence of bulk viscosity on porosity. In the limit of zero

initial connected porosity, arguably the only model relevant for melt

extraction, travelling waves are only possible in a viscoelastic matrix.

Such waves are truly self-propagating in that the fluid and the wave

phase velocities are identical; thus, if no chemical processes occur

during propagation, the waves have the capacity to transmit geochemical

signatures indefinitely. In addition to solitary waves, we find that

periodic solutions to the compaction equations are common though

previously unrecognized. The transition between the solutions depends on

the pore volume carried by the wave and the Darcyian velocity of the

background fluid flux. Periodic solutions are possible for all

velocities, whereas solitary solutions require large volumes and low

velocities. (C) Elsevier, Paris.

obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled

waves of porosity, propagated by dilational deformation due to an

effective pressure gradient within the wave. Viscous porosity waves have

attracted attention as a mechanism for melt transport, but are also a

mechanism for both the transport and trapping of fluids released by

diagenetic and metamorphic reactions. We introduce a mathematical

formulation applicable to compaction driven flow for the entire range of

rheological behaviors realized in the lithosphere. We then examine three

first-order factors that influence the character of fluid flow: (1)

thermally activated creep, (2) dependence of bulk viscosity on porosity,

and (3) fluid flow in the limit of zero initial connected porosity. For

normal geothermal gradients, thermally activated creep stabilizes

horizontal waves, a geometry that was thought to be unstable on the

basis of constant viscosity models. Implications of this stabilization

are that: (1) the vertical length scale for compaction driven flow is

generally constrained by the activation energy for viscous deformation

rather than the viscous compaction length, and (2) lateral fluid flow in

viscous regimes may occur on greater length scales than anticipated from

earlier estimates of compaction length scales. In viscous rock, inverted

geothermal gradients stabilize vertically elongated waves or vertical

channels. Decreasing temperature toward the earth's surface can induce

an abrupt transition from viscous to elastic deformation-propagated

fluid flow. Below the transition, fluid flow is accomplished by short

wavelength, large amplitude waves; above the transition flow is by high

velocity, low amplitude surges. The resulting transient flow patterns

vary strongly in space and time. Solitary porosity waves may nucleate in

viscous, viscoplastic, and viscoelastic rheologies. The amplitude of

these waves is effectively unlimited for physically realistic models

with dependence of bulk viscosity on porosity. In the limit of zero

initial connected porosity, arguably the only model relevant for melt

extraction, travelling waves are only possible in a viscoelastic matrix.

Such waves are truly self-propagating in that the fluid and the wave

phase velocities are identical; thus, if no chemical processes occur

during propagation, the waves have the capacity to transmit geochemical

signatures indefinitely. In addition to solitary waves, we find that

periodic solutions to the compaction equations are common though

previously unrecognized. The transition between the solutions depends on

the pore volume carried by the wave and the Darcyian velocity of the

background fluid flux. Periodic solutions are possible for all

velocities, whereas solitary solutions require large volumes and low

velocities. (C) Elsevier, Paris.

Create date

09/10/2012 20:50

Last modification date

18/11/2016 12:33