Miscible flooding involves injecting fluids that are miscible with residual oil trapped in the reservoir, allowing the fluids to fully mix at the molecular level. There are two main types of miscible flooding processes: single-contact processes where miscibility occurs immediately upon contact, and multiple-contact (dynamic) processes where miscibility occurs gradually through chemical exchange between phases. Multiple-contact processes include high-pressure vaporizing processes where a vapor phase is injected to vaporize components from the oil, and enriched-gas condensing processes where intermediates are condensed out of the injected gas into the oil.

1. Miscible Flooding
-Microscopic efficiency depends on interfacial tension.
-A miscible process is one in which the interfacial tension between trapped oil
and the displacing fluid is zero; that is, the displacing fluid and the residual oil
mix to form one phase.
-Microscopic displacement efficiency is maximized.
Miscible
Flooding
Single-Contact
Miscible
Processes
Multiple-Contact
Miscible
Processes

2. Ternary Diagrams of HCs
Two phase
region
-Three pseudo-components at the corners of the
triangle. C represents the critical point
-A gas could be mixed with either a liquid or a
vapor in appropriate percentages and yield a
miscible system. However, when liquid is mixed
with a vapor, often the result is a composition in
the two-phase region.
-A mixing process is represented on a ternary
diagram as a straight line.
-Mixture M is the resulting composition of mixing
between any two phases, it lies on the st.line of
their mixing process.
G
L
V or Dry
Gas
-If two phases are formed, their compositions, V1 and L1, would be given by a tie line
extended through the point M to the phase envelope.
-The two phase region area decrease with increasing pressure/decreasing temperature.
-Therefore, miscible mixtures lie outside the two phase region.

3. Miscible Flooding cont’d
Single-Contact Miscible Processes
-Injection fluids as liquefied petroleum gases (LPGs) and alcohols.
-Miscible with residual oil immediately on contact.
-Points O, P, and V representing the oil, LPG, and dry gas, respectively.
-The oil and LPG are miscible in all
proportions.
-At the front side of LPG, there is a mixing
zone (mixture M1) that continues to grow
as the LPG advances through the
reservoir.
-At the rear of the LPG slug, the dry gas
and LPG are miscible and a mixing zone
will also be created at this interface
(mixture M0).
-Dry gas is injected as it is much cheaper
than LPG.

4. -It is important that the amount of LPG that is injected be large enough that the
two mixing zones do not come in contact, because it yields mixtures inside the
two-phase region (Line M1M0) Nevertheless, the volume of LPG must be kept
small enough to avoid large injected-chemical costs.
-Reservoir pressures must be sufficient to achieve effective miscibility by LPG
processes (P>1500 psia).
-Reservoirs with pressures less than this might be amenable to alcohol
flooding C1–C4 range, another first-contact miscible process, since
alcohols tend to be soluble with both oil and water (the drive fluid in this case).
-Alcohols are expensive and become diluted with connate water during a
flooding process.
Miscible Flooding cont’d

5. Multiple-Contact (dynamic) Miscible Processes
-The injected fluids in this case are usually methane, inert fluids, or an enriched
methane gas supplemented with a C2–C6 fraction.
-The injected fluid and oil are usually not miscible on first contact but rely on a
process of chemical exchange between phases to achieve miscibility.
-It includes two processes:
1) High-pressure (lean-gas) vaporizing process.
2) The enriched-gas-condensing process
Miscible Flooding cont’d

6. Miscible Flooding cont’d
High-pressure (lean-gas) vaporizing process
-Vapor V (C1 and some intermediates)
serves as the injection fluid.
-Oil composition is at O.
Steps of miscibility
1) fluid V comes in contact with crude
oil O. They mix, and the resulting overall
composition is M1 with liquid phase
composition L1 and vapor phase
composition V1.
2) The liquid L1 has been created from the
original oil O by vaporization of some of the
light components. The oil was immobile,
L1 was extracted form O, so L1 is still immobile but V1 will move away from the
oil and be displaced downstream (as Krg becomes > 0).
3) V1 will come in contact with fresh crude oil O, and again they mix. The overall
composition yields two phases, V2 and L2. L2 again remains immobile and V2 moves
downstream, where it comes in contact with more fresh crude.

7. Miscible Flooding cont’d
4) The process is repeated with the vapor phase composition V1–V2–V3 moving along
the dew point curve, until the critical point C is reached and miscibility takes place.
-This process requires a crude oil with significant percentages of intermediate
compounds (To be vaporized, added to the injected vapor, and achieve
miscibility with oil).
-This requirement means that the oil composition must lie to the right of a tie
line extended through the critical point on the diagram.
As a rule, P ≥ 3000 psia and oil API ≥ 35 are required for miscibility in the high-
pressure vaporizing process.

8. Miscible Flooding cont’d
-Inert Gas Injection is an example of a High-pressure vaporizing process
-CO2 and N2 are the most common, the ternary diagram is the same except
that either CO2 or N2 becomes a component and C1 is lumped with the
intermediates.
-One-phase region is largest for CO2, with N2 and dry gas having about the
same one-phase size. The larger the one-phase region, the more readily
miscibility will be achieved.
-Pressures in case of CO2 are lower (1200-
1500 psia) than N2 or dry gas P ≥ 3000 psia.
-The capacity of CO2 to vaporize hydrocarbons
is much greater than that of natural gas.

9. Miscible Flooding cont’d
The enriched-gas-condensing process
-In this case, however, the
intermediates are condensed from
the injection fluid to yield a “new” oil
miscible with the old oil.
Steps of miscibility
1) Injection of fluid G rich in
intermediates mixes with oil O.
2) Mixture M1 is formed separates
into L1 and V1.
3) Vapor V1 moves ahead of the
immobile liquid, so that L1
becomes in contact with G and
mixing occurs, giving L2 and V2.
4) The process is repeated until reaching C and the liquid becomes miscible with G.
-In contrast with the high-pressure vaporizing process, it is possible that no residual oil
can be left behind as long as there is a sufficient amount of G injected to supply
the condensing intermediates.

10. Miscible Flooding cont’d
-This process may be applied to reservoirs containing crude oils with only small
quantities of intermediates.
-Reservoir pressures are usually in the range P=2000–3000 psia.
-Intermediates are expensive, so dry gas is injected after a sufficient slug of
enriched gas has been injected.