Steam Drive Definition and Enhancement
Authors: Doscher Todd M. , Omoregie Osazuwa S. , Ghassemi Farhad 
Published: December 2007

The classic view of the steam drive is of a steam zone developing across the entire cross section of the reservoir-the oil saturation being reduced to some low level by frontal displacement. Such a view is incorrect for viscous crudes because a bank of viscous oil cannot be displaced at a significant rate under the pressure gradients employed.Results of both field operations and scaled physical model studies demonstrate that the steam channels through the reservoir, usually overlying the oil column, and sweeps or drags the underlying oil to the producer. Our own observations and those of others suggest that emulsification of the oil by the condensing high-velocity steam is one factor contributing to mobilization of the heated oil.Because steam flowing through the reservoir across the top of the oil column will not interact with the base rock for some time, there is no reason to believe that thickness, above some minimum value, will affect the results of a steam drive. Physical model studies and field results indicate a general insensitivity to thickness (Fig. 1).Since it is the dynamics at the steam/oil interface that control mobilization of the oil, it is the viscosity of the oil at steam temperature that affects the production rate of a steam drive. However, as the oil viscosity at reservoir temperature decreases to a sufficiently low level, the steam drive changes its character and frontal displacement occurs. This is apparent from the temperature distribution in the scaled model studies. The displacement of the crude is now controlled by the rate of injection of steam and exhibits little dependence on the oil viscosity. Only in laboratory experiments can a sufficient range of viscosities in otherwise identical systems be investigated to develop proof for these hypotheses (Fig. 2). proof for these hypotheses (Fig. 2). The conceptualization of the steam drive that follows from the foregoing observations leads to the conclusion that a sufficiently low viscosity oil should be as effectively displaced by a high velocity gas drive as viscous oils are displaced by steam. (4 MMscf/D gas would be equivalent to 500 bbl of 200-psi steam, before any condensation. The gas velocity into a 50% depleted, 5-acre pattern of a 50-ft-thick sand would average much more pattern of a 50-ft-thick sand would average much more than 100 ft/D.)To verify this, two suites of experiments were run. In one series, the steam drive was interrupted after steam breakthrough had been achieved and nitrogen was (1) alternated with steam injection and (2) injected simultaneously with a proportionally reduced rate of steam injection. The results (Fig. 3) show that the substitution of nitrogen for some of the steam resulted in an improved oil/steam ratio for the same volume of steam injected. In other words, the nitrogen, as a partial substitute for the steam, was effective in continuing the displacement of the already heated oil at the vapor/liquid interface.Finally, a series of runs was performed in scaled models in which low-viscosity oils were displaced by nitrogen alone. The results (Fig. 4) show the marked effect of viscosity on the displacement of the crude, much as is observed in the steam drive. With the lowest viscosity fluid, although the override is still present, so much of the fluid is displaced in so short a time that the displacement is virtually frontal.Note that the kro/krg ratio that is derived from such experiments (Fig. 5) is dependent on velocity, as would be expected at high velocities where stratified flow occurs. These results can be correlated by including higher order terms in the Darcy equation, or by using an exponential viscosity term to absorb the higher order terms (Fig. 6).These continuing studies on the manner in which steam drive displaces crude oil have indicated (1) the potential for extending steam drive to relatively thin potential for extending steam drive to relatively thin reservoirs and reservoirs containing high-gravity crudes and (2) the possibility of realizing greater profits by substituting some inert gas for steam in the mature life of the operation.
p. 1543