During Canada's recent oil shortages the Province of Alberta offered to substantially raise production rates even though these higher rates might detrimentally affect ultimate oil recovery. Recent studies show that within reasonable limits of economic rates reservoirs that are pressure maintained are at worst insensitive to rate. In fact, results for several pools in Alberta generally show an increasing recovery with rate. The qualifying statement to these results is that good operation practices must prevail. In general though it can be concluded that higher producing rates to meet increased market demand will not impair ultimate oil recovery.
As production rates in Alberta have increased to meet high market demand, it has been proposed1 that in many cases increasing production rate can adversely affect ultimate recovery. This rationale is in turn, partly derived from observations that counter-current imbibition and gravity drainage are time-dependent recovery mechanisms and can more effectively contribute to ultimate recovery at lower reservoir withdrawal rates. This paper describes results of a study to determine the effects on oil recovery of production rate. This study was constrained to determination of rate effects solely fro the point of view of reservoir fluid mechanics. Besides reservoir flow mechanics considerations, operational strategies and constraints will affect ultimate recovery. For example, the type of water-flood pattern implemented, injectivity and productivity limitations, the allocation of production among wells and complex economic limit considerations at abandonment will affect ultimate recovery. Such operational considerationsare outside the scope of this study and do not affect the rate sensitivity conclusions as they relate to reservoir fluid mechanics. Ultimate oil recovery should be independent of production rate if the reservoir were produced for infinite time with no economic limit imposed. However, the question that must be answered is whether oil recovery is rate sensitive with the reservoir operated at reasonable economic field rates over a reasonable period of time. The number of reservoir-fluid variables, the complexities of interacting recover mechanisms and the limited time period of this study prevented an exhaustive or statistical investigation. Accordingly a number of subject reservoirs typical of Alberta's fields were selected. The reservoirs or cases studied include:
Swan Hills Beaverhill Lake A Pool – This pool is a fairly thick and homogeneous reservoir under a pattern waterflood.
Rainbow Reef – This pool is a thick carbonate low quality reef with a bottom water drive.
Countess Upper Mannville B Pool – This reservoir is a high quality sand containing medium-heavy oil underlain by water.
Simonette D-3 Pool – This pool is a thick carbonate high quality reef with a bottom water drive.
Pembina Keystone Belly River B Pool – This pool Ls typical of a pattern flood of a thin heterogeneous sand.
Actual reservoir-fluid data from these pools were used in the simulation studies. Since the recovery process is dependent on permeability, heterogeneity, fluid properties, relative permeability and capillary pressure, runs were done changing the basic reservoir-fluid properties to determine any sensitivity to these properties.