Principles of Heavy Oil Recovery

Abstract

AbstractRising exploration costs have prompted greater interest in the large knowndeposits of heavy oil on our continent. Because of high oil viscosities in suchreservoirs, recoveries are poor, fluid drives are inefficient and productionrates are uneconomical. Viscosity reduction can best be accomplished by heatingthe reservoir. The basic aspects of reservoir heating are reviewed and thoseprocesses which are of practical importance in heavy oil reservoirs arediscussed.Wellbore heating frequently can be applied to heavy oil reservoirs toincrease production rates. It will only be effective, however, where somereservoir driving energy exists and sanding problems are not severe. Forwardand reverse combustion usually require so much air in heavy oil reservoirs thatthese processes are uneconomical. In hot water flooding, the water requirementsare much higher than in an ordinary waterflood. Steam floods are moreattractive, but operating costs are generally high. Fresh water condensate fromthe steam may also cause severe reservoir damage.Conduction heating processes appear most promising. Among these we includethe cyclic steam-soak process. A simple method is presented for estimating theperformance from the first cycle of steam injection into the formation,assuming gravity as the only driving energy.An example calculation for a typical heavy oil reservoir is given. Theresults yield conservative estimates. The combustion-conduction process isattractive because of the characteristic low air-oil ratios. The productionperformance of the heated reservoir is not readily predicted, however. Fractureheating processes hold great promise, provided that we can develop means forcreating fractures with controlled orientation and direction.IntroductionThe cost of finding new oil reserves has been rising continuously over thepast two decades. The oil industry has therefore put increasing emphasis onimproving recovery from known reservoirs. Recently, attention has been directedto the large deposits of heavy oil on our continent. The U.S. Bureau of Mineshas estimated (1) that in the United States there are more than 150 billionbarrels of oil with a gravity of less than 25°API in known reservoirs. Most ofthem contain more than 1000 bbl/a.ft.; about half of them are at depths of lessthan 3000 ft. Oil recovery from these reservoirs, however, is estimated at 11per cent, compared to an estimated ultimate recovery of 30 per cent fromconventional reservoirs. This makes heavy oil reservoirs especially attractivedespite their low oil mobilities.