The formation and evolution of layered structures in porous media

Schoofs, Stan; Trompert, R. A.; Hansen, Ulrich

doi:10.1029/98jb01391

Cited by 12 publications

(11 citation statements)

References 29 publications

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“…Previous studies, in which the formation and evolution of convective layers in porous media were investigated under idealized £uid conditions, have shown that a force balance on a £uid parcel just beneath the interface between the two £uid layers determines the stability of layered system [17,18]. In the context of the ridge-crest system, this balance states that the stability of the brine layer is mainly determined by (1) the brine salinity established by the thermodynamical conditions, and (2) the hydrological properties of the basaltic rocks that host the boundary.…”

Section: Introductionmentioning

confidence: 99%

Depletion of a brine layer at the base of ridge-crest hydrothermal systems

Schoofs

Hansen

2000

Earth and Planetary Science Letters

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The variable salinity of fluid venting from mid-ocean ridges is indicative of mixing between hydrothermal seawater and fluids that have undergone supercritical phase separation. In order to study the stability of a brine-saturated layer that may form in the lowermost part of the hydrothermal system, we have performed numerical simulations of a system that has returned into the subcritical regime. For typical geological parameters, it is shown that the interface between the brine layer and the overlying fluids is not very stable, but vanishes by one of two dynamical mechanisms: convective breakdown or vertical migration. This contradicts the conventional picture of a steady, layered convective system in which the brine is depleted only by dispersion and diffusion across the interface. The depletion mechanism depends on the fluid-dynamical stability of the brine layer. Convection within the brine layer results either in the convective breakdown (for low excess salinity of the brine, as compared to seawater) or the upward migration of the interface (for higher excess salinities). Consequently, the depletion times are much shorter than for models with pure dispersion/ diffusion across the interface. If the brine layer is static, high-chlorinity liquid is entrained slowly by the convecting overlying fluids, leading to downward migration of the interface. This gradual depletion of the brine layer results in almost constant vent salinities, in agreement with measured salinities of chronic high-chlorinity vents. ß

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Section: Introductionmentioning

confidence: 99%

Depletion of a brine layer at the base of ridge-crest hydrothermal systems

Schoofs

Hansen

2000

Earth and Planetary Science Letters

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“…Ž . Schoofs et al, 1998 . First, transport of heat solute across the interface changes from convective entrain-Ž .…”

Section: Self-limitation Of the Conõectiõe Layersmentioning

confidence: 99%

“…to generate layered structures Schoofs et al, 1998 . In this so-called Hele-Shaw approximation, however, heat and solute advect at the same speed, which is fundamentally different from flow through a geological medium having a low porosity.…”

Section: Introductionmentioning

confidence: 99%

The formation and evolution of layered structures in porous media: effects of porosity and mechanical dispersion

Schoofs

Trompert

Hansen

2000

Physics of the Earth and Planetary Interiors

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Horizontally layered structures can develop in porous or partially molten environments, such as hydrothermal systems, magmatic intrusions and the early Earth's mantle. The porosity f of these natural environments is typically small. Since dissolved chemical elements unlike heat cannot diffuse through the solid rocks, heat and solute influence the interstitial fluid density in a different manner: heat advects slower than solute through the liquid by the factor f, while diffusion of heat through the bulk porous medium is larger by the factor f y1 times the ratio between the thermal and chemical diffusivities. By performing numerical experiments in which a rigid low-porosity medium is heated from below, we have studied the formation and evolution of layers in an initially stably stratified liquid. Growth of a convective layer through convective entrainment, the formation of a stable density interface on top of the layer and destabilization of the next layer are intimately Ž . Ž . linked. By monitoring the heat solute fluxes, it is observed that the transport of heat solute across the interface changes Ž . from convective entrainment towards a regime in which transfer is purely diffusive dispersive . Because this transition occurs before the stage at which the lower layer arrives at the thermal equilibrium, we conclude that the layer growth stops when the density interface on top has grown sufficiently strong to keep the ascending plumes in the lower layer from convectively entraining more fluid from above. A simple balance between the most important forces, exerted on a fluid parcel in the lower layer, is proposed to determine this transition. This force balance also indicates whether a density interface keeps intact, migrates upwards or breaks down during the further evolution of the layered sequence. Finally, mechanical dispersion tends to increase transport of chemically dissolved elements across the density interface. Since this reduces the density difference between the two adjacent layers, the thickness of the lower layer increases. q

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“…At the temperatures and pressures within the marine sediments, the gas (usually Methane) is in solution. Systematic experimental observations of flow and density profile of methane gas are severely limited due to uncontrollable parameters such as changes in pressure and temperature gradients and morphological variations of the host matrix, i.e., the evolution of porous ground [5][6][7]. One of the major problems in modeling such a complex system is to incorporate all the details on a large scale [11].…”

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confidence: 99%

“…The response of the flow rate density to the temperature is nonlinear. Understanding the flow of methane gas below the ocean floor is of considerable interest particularly in context to geochemical and geophysical characteristics of methane hydrate in marine sediments [1][2][3][4][5][6][7][8][9][10]. At the temperatures and pressures within the marine sediments, the gas (usually Methane) is in solution.…”

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