Simulation of Small Chemical Slug Behavior in Heterogeneous Reservoirs

Abstract

Clifford, P.J., U.K. Atomic Energy Authority SPE Member Abstract In simulating field performance of polymer and micellar flooding, it is frequently necessary to calculate the behaviour of chemical slugs which are small compared to the well spacing, and not much larger than the normal gridblock dimension. A polymer slug in a heterogeneous reservoir will then be subject to disruption from numerical dispersion, viscous crossflow and viscous fingering. The problem addressed in this paper is whether oil recovery from the slug can be simulated without an impracticable degree of grid refinement. Analytical modelling is used to explain the process of slug disruption by viscous cross flow in a layered reservoir, and to demonstrate the close relationship between crossflow and oil recovery. Some of the concentration fronts will spread even in the absence of dispersion. Numerical simulation is used to examine the sensitivity of slug behaviour to grid refinement, dispersion, slug size and viscosity and viscous fingering. It is shown how a coarsening of the gridblock structure leads to large qualitative changes in slug flow and the movements of polymer within the reservoir. However, it is also demonstrated that the quantity of incremental oil recovery is much less sensitive to numerical dispersion, over a wide range of likely slug properties. This allows a much more adequate calculation of polymer flood performance in very coarse grids than would be expected from the ratio of slug to gridblock size, and is most important in permitting practicable reservoir simulation of this EOR process. Introduction In many polymer or micellar flood enhanced oil recovery applications (EOR), the volume of the injected chemical is often much smaller than the reservoir pore volume. In stratified reservoirs, polymer solutions are injected as small volume slugs, with the aim of improving vertical sweep efficiency. In this case, the slug will be subject to disruption by several mechanisms acting simultaneously:vertical crossflow of polymer between layers;dilution of concentration fronts by fluid cross flow;dispersion of concentration fronts;viscous fingering of chase water into the slug. Simulation of the behaviour in a coarse-gridded reservoir model also introduces substantial numerical dispersion, which often cannot be removed without excessive computational cost. Given these complexities, it is important to examine which aspects of the behaviour can be adequately simulated in a coarse grid. In particular, is it possible to predict the incremental oil recovery with any accuracy in a practicable model? The mechanisms by which polymer recovers oil in layered reservoirs, and the observed effect of grid coarseness, are first discussed. Simulations are then carried out using the SCORPIO chemical flooding simulator (1), and with simplified reservoir models and polymer properties. The theoretical analysis of flows involved in slug disruption which follows, is based in part on work on characteristics by Zapata and Lake (2) investigating two-phase flow in layered systems. The discussion examines the case of small slug injection, which is more complex than continuous polymer injection. Viscous fingering of polymer in layered reservoirs is examined. It is proposed that the effects of instability in a two-dimensional layered reservoir differ from those in linear displacement. P. 873^