The flow in regions of oil and gas reservoirs that are dominated by gravity segregation can occur through two different mechanisms. First, all reservoir fluid phases, possibly oil, gas, and water, might move through the same pores. One phase might move upward while the others move countercurrently downward, or vice versa, in the same pores. If such a mechanism were valid, Darcy's Law could be applied over large areas of the reservoir to provide average phase velocities, just as if the flows were the result of pressure induced convection. However, a second mechanism is also possible. The phases might be segregated such that some pores carry the upward flowing phases, while other pores carry downward flows. When this mechanism is valid, Darcy's Law applies only locally to region of upward or downward flow.
The two mechanisms result in substantially different phase velocities. Simulators, including those available commercially, generally assume the first mechanism, i.e. uniform, countercurrent flow throughout each simulation cell. This paper describes an experimental study in which two-phase flow velocities were measured as the phases moved through a bed of small, uniform glass beads, as the result of gravitational forces only. The resulting velocities were more consistent with those predicted for segregated flows of mechanism two, than to the those of the commonly assumed homogeneous flows of mechanism one. These results suggest that greater accuracy in reservoir simulation can be achieved by including segregated flow in areas of the reservoir where gravity segregation predominates.
The fact that the interface between phases becomes unstable when high mobility fluids displace fluids of lesser mobility has long been recognized. In 1969, Varnon and Greenkorn [1] described viscous fingering in four different flow regimes. Several authors have used reservoir simulators to model the fingering phenomena[2,3,4]. Some point out that the size of the fingers simulated depends on the size of the simulation grid[5,6]. For that reason, Rubin et al.[7] used pseudo-relative permeabilities to account for fingering. Zolotukhin and Frick[8] resorted to stochastic simulations in which fingers were represented in by a method similar to self-similar fractals.
When gravity forces are the predominant cause of the fluid movements, one phase moves upward while others move downward, or vice versa. It becomes difficult to distinguish which is the displacing fluid, and hence to know if fingering is likely. Furthermore there may be a segregation of the flowing phases because the dense phase is unable to enter pores where the lighter phase is flowing upward, and the light phase is unable to flow counter to the downward flowing heavy phase in other pores. In some processes such as steam assisted gravity drainage (SAG), completely segregated flow of the phases has been assumed, with good results[9]. However, in cases where convection predominates in most areas of the reservoir, the possible effect of segregated flow in local areas where gravity drainage dominates, is not usually considered. The purpose of this paper is to consider the importance of such effects.
