Method for estimating minimum miscibility enrichment

1. A method for estimating minimum miscibility enrichment (MME) for an injectant used in gas flooding of a reservoir at a given operating pressure comprising:

performing a plurality of slim tube simulations for the reservoir;

determining minimum miscibility pressure (MMP) for each of a plurality of injected gases in the reservoir;

creating a plot having a recovery factor (RF) vs. 1−(MMP−P)/MMP curve for each of the plurality of injected gases, wherein P is the operating pressure of the reservoir having at least one of the plurality of injected gases, wherein 1−(MMP−P)/MMP is a dimensionless pressure, wherein MMP is the minimum miscibility pressure of one of the plurality of injected gases from the determining step, and wherein the plot has a y-intercept and slope;

combining all of the RF vs. 1−(MMP−P)/MMP curves for each of the plurality of injected gases to form one curve;

obtaining a recovery factor equation RF=i+s(1−(MMP−P)/MMP) from the one curve, wherein i is the y-intercept and s is the slope;

determining a value for i;

determining a value for s; and

calculating the recovery factor.

2. The method of claim 1 further comprising:

calculating MMP ne , wherein MMP ne is the minimum miscibility pressure for at least one of the plurality of injected gases when it has not been enriched;

rewriting the recovery factor equation to represent a recovery factor for the injected gas when it has not been enriched (RF ne ,);

setting RF ne ,=RF max wherein RF max is a maximum recovery factor; and

estimating minimum miscibility enrichment (MME).

3. The method of claim 2 wherein i=0.1828−0.42617X C3 , wherein X C3 , is the mole fraction of the injectant.

4. The method of claim 3 wherein s=0.8172+1.5956X C3 +7.1929X C3 2 .

5. The method of claim 4 wherein rewriting the recovery factor equation further comprises:

rewriting the recovery factor equation as

RF ne =0.1828−0.4262 X C3,ne +(0.8172+1.5956 X C3,ne +7.1929 X C3,ne 2 ) P d

wherein

P d =1−( MMP ne −P op )/ MMP ne

wherein P op is the given operating pressure.

6. The method of claim 5 wherein estimating the minimum miscibility enrichment (MME) further comprises:

obtaining the MME equation

7.1929 P d X MME 2 +(1.5956 Pd −0.4262) X MME +(0.1828+0.8172 P d −RF max )=0

wherein X MME is the minimum miscibility enrichment (MME); and solving for X MME .