Many heavy oil formations in Alberta and Saskatchewan have underlying water sands. Thermal recovery applications under such conditions depend strongly on the extent of bottom water, oil viscosity, vertical permeability, oil saturation, and fluid injection rate.
This paper discusses in-situ combustion, cyclic steam stimulation, and steamflooding under bottom water conditions, from the mechanistic standpoint, with field examples cited to illustrate the expected performance. While the emphasis is on thermal processes, selected non-thermal techniques are also considered in a qualitative sense. It is seen that although bottom water is undesirable in most instances, it may serve the purpose of providing initial injectivity in very viscous oil formations.
The major portion of the paper is devoted to a discussion of field case histories where thermal recovery methods (in-situ combustion, cyclic and continuous steam injection) were applied in bottom water formations. The results are described to derive general conclusions. Firefloods under bottom water conditions seem to have been largely unsuccessful in that air-oil ratios were above the marginal value of 2700 sm3/sm3, although the extent of bottom water was less critical than in steam injection. Cyclic steam stimulation performed poorly in the reported field tests, except in one case where the bottom water was less than one-fifth of the oil zone thickness. Steamfloods under bottom water conditions are few and far between, but in selected instances they performed more favorably because of the careful prior choice of the operating conditions.
Many heavy oil formations in Alberta and Saskatchewan are characterized by a water saturated sand directly below the oil zone and often in communication with it. The water sand may be merely a high water saturation zone, or a transition zone. In the following, "bottom water" will refer to all these situations. The extent of bottom water would vary from reservoir to reservoir. It may be thin sand, a few meters in thickness, or it may be an aquifer.
In any oil recovery process, the presence of bottom water is likely to have a detrimental effect on oil recovery. The injected fluid – air, steam, solvent, etc. – will have a tendency to migrate into the low resistance water sand, resulting in poor displacement and sweep efficiencies. The magnitude of such channeling will depend on a variety of factors, notably oil viscosity, relative water sand thickness, injection rate, oil saturation in the water sand, if any, and vertical permeability. The latter factor is of considerable importance, for even a limited barrier would drastically reduce the channeling tendency. Thus any shale breaks, silt zones, or other heterogeneities at the base of the oil sand would serve to improve the oil recovery performance.
The above simplified picture is complicated by the flow of heat from the water zone into the oil sand, if a hot fluid is injected. If air is the injected fluid, the situation may be even more complex. It is quite likely that in the case of a limited water sand, a thermal recovery process could be designed around the water zone, even utilizing it under certain conditions, as a means of initial injectivity.
