Multiple-contact or dynamic miscible processes do not require the oil and displacing fluid to be miscible immediately on contact but rely on chemical exchange between the two phases for miscibility to be achieved. Figure 11.8 illustrates the high-pressure (lean-gas) vaporizing, or the dry gas miscible process.
Figure 11.8 Ternary diagram illustrating the multicontact dry gas miscible process.
The temperature and pressure are constant throughout the diagram at reservoir conditions. A vapor denoted by V in Fig. 11.8, consisting mainly of methane and a small percentage of intermediates, will serve as the injection fluid. The oil composition is given by the point O. The following sequence of steps occurs in the development of miscibility:
1. The injection fluid V comes in contact with crude oil O. They mix, and the resulting overall composition is given by M1. Since M1 is located in the two-phase region, a liquid phase L1 and a vapor phase V1 will form with the compositions given by the intersections of a tie line through M1, with the bubble-point and dew-point curves, respectively.
2. The liquid L1 has been created from the original oil O by the vaporizing of some of the light components. Since the oil O was at its residual saturation and was immobile due to the relative permeability, Kro, being zero, when a portion of the oil is extracted, the volume, and hence the saturation, will decrease and the oil will remain immobile. The vapor phase, since Krg is greater than zero, will move away from the oil and be displaced downstream.
3. The vapor V1 will come in contact with fresh crude oil O, and again the mixing process will occur. The overall composition will yield two phases, V2 and L2. The liquid again remains immobile and the vapor moves downstream, where it comes in contact with more fresh crude.
4. The process is repeated with the vapor-phase composition moving along the dew-point curve, V1–V2–V3, and so on, until the critical point, C, is reached. At this point, the process becomes miscible. In the real case, because of reservoir and fluid property heterogeneities and dispersion, there may be a breaking down and a reestablishment of miscibility.
Behind the miscible front, the vapor-phase composition continually changes along the dew-point curve. This leads to partial condensing of the vapor phase with the resulting condensate being immobile, but the amount of liquid formed will be quite small. The liquid phase, behind the miscible front, continually changes in composition along the bubble point. When all of the extractable components have been removed from the liquid, a small amount of liquid will be left, which will remain immobile. There will be these two quantities of liquid that will remain immobile and not be recovered by the miscible process. In practice, operators have reported that the vapor front travels anywhere from 20 ft to 40 ft from the wellbore before miscibility is achieved.
The high-pressure vaporizing process requires a crude oil with significant percentages of intermediate compounds. It is these intermediates that are vaporized and combined with the injection fluid to form a vapor that will eventually be miscible with the crude oil. This requirement of intermediates means that the oil composition must lie to the right of a tie line extended through the critical point on the binodal curve (see Fig. 11.8). A composition lying to the left, such as denoted by point A, will not contain sufficient intermediates for miscibility to develop. This is due to the fact that the richest vapor in intermediates that can be formed will be on a tie line extended through point A. Clearly, this vapor will not be miscible with crude oil A.
As pressure is reduced, the two-phase region increases. It is desirable, of course, to keep the two-phase region minimal in size. As a rule, pressures of the order of 3000 psia or greater and an oil with an API gravity greater than 35 are required for miscibility in the high-pressure vaporizing process.
The enriched gas-condensing process is a second type of dynamic miscible process (Fig. 11.9). As in the high-pressure vaporizing process, where chemical exchange of intermediates is required for miscibility, miscibility is developed during a process of exchange of intermediates with the injection fluid and the residual oil. In this case, however, the intermediates are condensed from the injection fluid to yield a “new” oil, which becomes miscible with the “old” oil and the injected fluid. The following steps occur in the process (the sequence of steps is similar to those described for the high-pressure vaporizing process but contain some significant differences):
1. An injection fluid G rich in intermediates mixes with residual oil O.
2. The mixture, given by the overall composition M1, separates into a vapor phase, V1, and a liquid phase, L1.
3. The vapor moves ahead of the liquid that remains immobile. The remaining liquid, L1, is then contacted by fresh injection fluid, G. Another equilibrium occurs, and phases having compositions V2 and L2 are formed.
4. The process is repeated until a liquid is formed from one of the equilibration steps that is miscible with G. Miscibility is then said to have been achieved.
Figure 11.9 Ternary diagram illustrating the multicontact enriched gas-condensing miscible process.
Ahead of the miscible front, the oil continually changes in composition along the bubble-point curve. In contrast to the high-pressure vaporizing process, there is the potential for no residual oil to be left behind in the reservoir as long as there is a sufficient amount of G injected to supply the condensing intermediates. The enriched gas process may be applied to reservoirs containing crude oils with only small quantities of intermediates. Reservoir pressures are usually in the range of 2000 psia to 3000 psia.
The intermediates are expensive, and so usually a dry gas is injected after a sufficient volume of enriched gas has been injected.
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