When the liquid residing in a horizontal bed of porous material is displaced by another liquid of different density, the resulting hydrodynamic dispersion is modified by the formation of a tongue of denser liquid undershooting the less dense liquid, a phenomenon known as gravity segregation. An earlier account of gravity segregation (Rose and Passioura 1971a) contained a substantial error (that of an incorrect frame of reference for flow) and several printing mistakes (the results of a 7-week postal strike in the UK).
Gravity segregation has been extensively studied by petroleum engineers (e.g.
We start by recalling the theory of gravity segregation of immiscible fluids and then compare this theory with the outcome of published experiments on miscible liquids.
We estimated the values of the gravity segregation number, [beta] (Eqn 8), in the following way.
This observation appears counterintuitive: one might expect that mixing about the displacing front would lead to an earlier start and a later end to breakthrough in miscible than in immiscible displacements, However, Kumar and Kimbler (1970) concluded, from 5 displacements in a 3-dimensional radial model constructed from an artificial consolidated sandstone, that mixing has a retarding effect on gravity segregation in miscible liquids.
Figure 4 shows stages in the development of miscible displacement accompanied by gravity segregation in photographs of 0.08M potassium permanganate solution displacing distilled water from a horizontal column of ballotini.
At the lowest speed during horizontal flow, gravity segregation dominates and results in a dispersed BTC because of greater mixing due to the shallow gradient of the tilted interface between the 2 liquids; during vertical displacement, the BTC is steep because of little hydrodynamic dispersion about the stable interface between the liquids.
11b) flow of [Br.sup.-] and [K.sup.+] through sepiolite, was also due to gravity segregation. This causes large changes in apparent dispersion coefficient in response to only small changes in concentration difference during horizontal displacement (Rose and Passioura 1971a) by markedly changing the value of [beta] because of the concomitant change in density difference, [DELTA][rho].
We have explored in detail some consequences of gravity segregation during miscible displacement, a phenomenon that occurs when saline groundwater intrudes into coastal aquifers, an increasingly common problem worldwide.
The results in this paper are offered to stimulate fresh experimental and theoretical studies of gravity segregation during miscible displacement and to focus future modelling efforts of this important phenomenon.
Craig FF, Sanderlin JL, Moore DW (1957) A laboratory study of gravity segregation in frontal drives.