COMPACTION-DRIVEN FLUID-FLOW IN VISCOELASTIC ROCK

Compaction driven fluid flow is inherently unstable such that an obstr uction to upward fluid flow (i.e. a shock) may induce fluid-filled wav es of porosity, propagated by dilational deformation due to an effecti ve pressure gradient within the wave. Viscous porosity waves have attr acted attention as a mechanism for melt transport, but are also a mech anism for both the transport and trapping of fluids released by diagen etic and metamorphic reactions. We introduce a mathematical formulatio n applicable to compaction driven flow for the entire range of rheolog ical behaviors realized in the lithosphere. We then examine three firs t-order factors that influence the character of fluid flow: (1) therma lly activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For n ormal geothermal gradients, thermally activated creep stabilizes horiz ontal waves, a geometry that was thought to be unstable on the basis o f constant viscosity models. Implications of this stabilization are th at: (1) the vertical length scale for compaction driven flow is genera lly constrained by the activation energy for viscous deformation rathe r than the viscous compaction length, and (2) lateral fluid flow in vi scous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inver ted geothermal gradients stabilize vertically elongated waves or verti cal channels. Decreasing temperature toward the earth's surface can in duce an abrupt transition from viscous to elastic deformation-propagat ed fluid flow. Below the transition, fluid flow is accomplished by sho rt wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patt erns vary strongly in space and time. Solitary porosity waves may nucl eate in viscous, viscoplastic, and viscoelastic rheologies. The amplit ude of these waves is effectively unlimited for physically realistic m odels with dependence of bulk viscosity on porosity. In the limit of z ero initial connected porosity, arguably the only model relevant for m elt extraction, travelling waves are only possible in a viscoelastic m atrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes o ccur during propagation, the waves have the capacity to transmit geoch emical 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 t he background fluid flux. Periodic solutions are possible for all velo cities, whereas solitary solutions require large volumes and low veloc ities. (C) Elsevier, Paris.