A Study of Gravity Counterflow Segregation

Abstract

It has been customary, in predicting saturation changes, to use the Leverett "fractional flow formula", obtained by eliminating the unknown pressure gradient from the generalized Darcy equations for the separate phases. The formula presents difficulties in the case of counterflow, since the "fractional" flow may be negative, greater than unity, or, in the case of a closed system, infinite. Recently, it has been shown by several authors that the corresponding equations (with capillary pressure and gravity terms) for actual flow of the phase may be used just as well. These equations are in agreement with Pirson's statement that, if the two mobilities differ considerably from each other in a closed system, the flow is largely governed by the lower value. The present study was undertaken because of an apparent lack of experimental data on gravity counterflow with which to test the theory. A 4-ft sandpacked tube in a vertical position was employed. Electrodes for determining saturations by resistivity were spaced along the tube, one phase being always an aqueous salt solution. Air, heptane, naphtha, or Bradford crude oil was used for the other phase. A reasonably uniform initial saturation was set up by pumping the phases through the system, after which the tube was shut in and saturation profiles obtained at definite intervals. Cumulative flows over certain horizontal levels were obtained by integration of the distributions; hence, differentiation of the cumulative flows with respect to time gave instantaneous flow rates. To compare experimental and theoretical flow values, capillary pressures were assumed given by the final saturation-distribution curve. The upper part corresponds to the "drainage" region and the lower part to the "imbibition" region, where trapping of the nonwetting phase occurred. While calculations indicated that the capillary pressure saturation function and, probably, the relative permeability saturation functions changed during the segregation, the relation of the measured rates to saturation distributions are in general accord with the frontal-advance equation. It appears that the Darcy equations, as modified for the separate phases, are generally valid for counterflow due to density differences. The usual method of predicting saturation changes, which involves a continuity equation and the elimination of the unknown pressure gradient from the flow equations, should therefore be applicable. However, the need for advance knowledge of drainage and imbibition "capillary pressures" and relative permeabilities during various stages presents difficulties. Introduction The present study was undertaken because of a seeming lack of experimental data relating to vertical counterflow of fluids of different densities in porous media. In particular, it was desired to determine whether data obtained from these laboratory tests were in accordance with certain mathematical treatments of counterflow which have been proposed. The gravity "correction" has been incorporated into the flow equations (and, hence, into displacement theory) nearly as long as both have been used. Field and laboratory data have generally borne out the validity of the theory as applied, for instance, to downward displacement by gas, with all fluids moving downward. However, the modifications for counterflow have only recently been pointed out. It has been customary to use fractional flow rates instead of actual flow rates in displacement calculations. In the case of counterflow, this results in negative values, values greater than unity and, when rates are equal and opposite, in infinite values. As pointed out by Sheldon, et al, and by Fayers and Sheldon, actual flow rates may be used just as well. The fact that these may be of opposite signs for the two fluids does not present any difficulty. SPEJ P. 185^

Numerical Simulation of a CT-Scanned Counter-Current Flow Experiment

“…To explain this, a concept -total effective mobility needs to be introduced. In a closed system, the counter-current flow velocities of the heavy wetting phase and the light non-wetting phase are equal and given by the following equation (Templeton et al, 1962):…”

“…Total effective mobility represents the ability of fluids to flow through the system. This concept is valid for both co-current flow and countercurrent flow from its derivation (Templeton et al, 1962). From its definition, the total effective mobility is mainly controlled by the less mobile phase at any saturation, which corresponds to the small values of relative permeabilities below cross points in a closed system.…”

“…Early experiments in gravity-driven counter-current flow in a closed system were done by Templeton et al (1962), and by Briggs and Katz (1966).…”

Permeability Hysteresis in Gravity Counterflow Segregation

“…To validate the proposed model we compare the numerical gas-water saturation profile with saturation profiles found in [7] and [20]. Following [7], we consider a sand pack with a permeability of 11.84 Darcys and porosity of 0.389.…”

Steady-state countercurrent flow in one dimension

“…There has been very little work reported in the literature which addresses the countercurrent flow problem. The simplest case of countercurrent flow (steady-state flow in one dimension), was initially addressed by Lelievre (1966) and a few authors have performed investigations regarding unsteady-state countercurrent flow (Lefebvre du Prey, 1978;Bourbiaux and Kalaydjian, 1988;Templeton and Nielsen, 1962), but a thorough theoretical and experimental investigation has not been reported.…”