This work investigates the effects that Water Alternate Gas (WAG) injection and trapping of oil by water have on the behavior of tertiary miscible displacements. Insights are offered into some of the trade-offs associated with the competition between improved sweep efficiency and reduced displacement efficiency associated with WAG injection. One dimensional calculations show an insensitivity of displacement efficiency to water injection over a limited range of low WAG ratios. When operating in such a manner, changing the amount of injected water has no effect on the basic solvent-oil displacement process. It only modifies what happens behind the displacement front. Mobility considerations indicate that the solvent-oil displacement is unstable. If enough water is injected with the solvent, a low mobility solvent-water bank is created which moves with the same velocity as the back of the oil bank. The WAG ratio at which these equal bank velocities occur is critical, because injection below this WAG ratio is predicted to give good displacement efficiency and poor sweep, while above this ratio mobility control and increased sweep efficiency will result at the expense of reduced displacement efficiency. In theory, injection at this "equal velocity" WAG ratio should give the benefits of both, good sweep efficiency without a reduction in displacement efficiency due to trapping. This optimum WAG ratio is shown to be relatively insensitive to the assumed level of trapping. Detailed two-dimensional simulations confirm the unstable solvent-oil displacement below the optimum WAG ratio. Mobility control and stabilization of fingers at higher WAG ratios are also confirmed. However, if trapping is important, a corresponding decrease in displacement efficiency occurs· These two-dimensional simulations also provide insight about more correct ways to model tertiary miscible displacements with simpler models, such as a mixing parameter model.

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