Chemical Methods for Heavy Oil Recovery

Abstract Many mobile heavy oil reservoirs in Saskatchewan and Alberta are unsuitable forthe application of thermal recovery methods, such as steam injection, for anumber of reasons including formation thicknesses of less than 10 m. Oilrecovery from such reservoirs can be accomplished by the use of nonthermalmethods, among which chemical flooding has considerable importance. This paperdiscusses recent laboratory results using chemical flooding techniques. At thesame time, limitations of such methods, limited field experience in heavy oilformations, and possible improvements are also considered. Among the chemicalflooding methods, alkaline and surfactant flooding techniques are moreimportant, partly because the chemicals involved are less expensive, and alsomuch has been learned from past experience in laboratory and field. Thelaboratory studies discussed consisted of surfactant floods and huff n'puff oftwo Lloydminster heavy oils. The recoveries in the floods were as high as 33﹪.The other recovery method discussed involved cyclic stimulation using twosurfactants. Oil recoveries as high as 12﹪ were achieved. Though recovery wasow, such an approach can be cost-effective in special circumstances. Introduction Much of the heavy oil in Saskatchewan and Alberta is mobile under reservoirconditions to the extent that primary production and waterflooding iseconomically feasible, although the recovery factors are low, 5 to 10﹪ in mostcases. Furthermore, the formation thickness is small (85﹪ of the oil in Saskatchewan occurs in formations less than 5 m thick), so that larger spacingsare needed, which makes the application of thermal methods, notablysteamflooding, doubly unattractive. Non-thermal recovery methods, such aschemical 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 morepromising non-thermal chemical flooding methods, selected laboratory and fieldresults, and their limitations. Results of a few experiments involvingchemicals with hot water are also added. Principles of Oil Recovery The two important concepts involved in oil recovery are Mobility Ratio, M, andthe Capillary Number, Nc. Mobility ratio, M, is usually defined as the mobility?ing (= k/ µ, where k is effective permeability and µ is viscosity)of the displacing fluid divided by the mobility ?ed of the displacedfluid (assumed to be oil in this discussion). If M >1, the displacing fluidwill flow past much of the displaced fluid, displacing it inefficiently. Thusthe 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 oftencalled "viscous fingering" For maximum displacement efficiency, M should be=?1, usually denoted as a "favourable"mobility ratio. If M >1(unfavourable), then, in the absence of viscous fingering, it merely means thatmore fluid will have to be injected to attain a given residual oil saturationin the pores.

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