Many mobile heavy oil reservoirs in Saskatchewan and Alberta are unsuitable for the application of thermal recovery methods, such as steam injection, for a number of reasons including formation thicknesses less than 10 m. Oil recovery from such reservoirs can be accomplished by the use of non-thermal methods, among which chemical flooding has considerable importance. This paper discusses recent laboratory results using chemical flooding techniques. At the same time, limitations of such methods, limited field experience in heavy oil formations, and possible improvements are also considered.
Among the chemical flooding methods, alkaline and surfactant flooding techniques are more important, partly because the chemicals involved are less expensive, and also much has been learnt from past experience in laboratory and field. The laboratory studies discussed consisted of surfactant floods and huff n'puff of two Lloydminster heavy oils. The recoveries in the floods were as high as 33%. The other recovery method discussed involved cyclic stimulation using these chemicals. Oil recoveries as high as 12% were achieved. Though low, such an approach can be cost-effective in special circumstances.
Much of the heavy oil in Saskatchewan and Alberta is mobile under reservoir conditions to the extent that primary production and waterflooding is economically feasible, although the recovery factors are low, 5 to 10% in most cases. Furthermore, the formation thickness is small (85% of the oil in Saskatchewan occurs in formations less than 5 m thick), so that larger spacings are needed, which makes the application of thermal methods, notably steamflooding, doubly unattractive. Non-thermal recovery methods, such as chemical recovery processes and immiscible carbon dioxide WAG (Water-Alternating-Gas) process can be economically viable in such reservoirs, even though the recovery factor is low.
This paper discusses primarily the more promising non-thermal chemical flooding methods, selected laboratory and field results, and their limitations. Results of a few experiments involving chemicals with hot water are also added.
The two important concepts involved in oil recovery are Mobility Ratio, M, and the Capillary Number, N,. Mobility ratio, M, is usually defined as the mobility λing (=k/ µ, where k is effective permeability and µ is viscosity) of the displacing fluid divided by the mobility λedof the displaced fluid (assumed to be oil in this discussion). If M>l, the displacing fluid will flow past much of the displaced fluid, displacing it inefficiently. Thus the mobility ratio influences "displacement efficiency", i.e. the (microscopic) efficiency of oil displacement within the pores. For M> > 1, the displacing fluid will channel past oil ganglia. This is often called "viscous fmgering". For maximum displacement efficiency, M should be ≤1, usually denoted as "favourable" mobility ratio. If M>1 (unfavourable), then, in the absence of viscous fingering, it merely means that more fluid will have to be injected to attain a given residual oil saturation in the pores.