Page 75 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

But what does it mean to be miscible? Or more accurately, what conditions
make a CO 2 flood considered as miscible? Theoretically, when CO 2 is injected
into an oil reservoir, there is a minimum pressure level—below that value CO 2
and oil are no longer miscible. Increasing the pressure leads to an increase in CO 2
density, which reduces the density difference between crude oil and CO 2 .Asa
result, the IFT between crude oil and CO 2 vanishes, and they will reach mutual
solubility in each other. This minimum pressure is named as minimum miscibility
pressure (MMP) [19 21]. A large amount of research has been implemented to
determine the MMP parameter, and one could find several correlations and experi-
mental methods as well, which are mainly applied for MMP prediction/measure-
ment. The main factors affecting CO 2 oil MMP are reservoir temperature, oil
composition, and purity of injected gas. Generally, low temperature reservoirs con-
taining light crude oils have smaller CO 2 oil MMP.However,the impactsof
impurity are not general and depend on the type of components [22]. Adding H 2 S
results in MMP reduction, while addition of N 2 leads to an increase in CO 2 oil
MMP value [23,24].
Basically, oil recovery is higher when CO 2 and oil are miscible. In other words,
there is a great deal of interest toward reaching miscibility when injecting CO 2 .To
give an explanation, imagine some oil on a surface. Water will get a little of oil off,
but solvent will remove every trace of oil. This is because solvent can get mixed with
the oil, creating a homogenous solution. Here, it can be stated water is immiscible
with oil and solvent is just miscible [25,26].

3.2.1 Miscible Flooding
As discussed before, the pressure at which miscibility occurs is defined as MMP.
Providing this condition during injection process will lead to a miscible CO 2 EOR,
in which the recovery would be as high as 90%, theoretically. Indeed, oil recovery
increases rapidly as the pressure increases and then flattens out when MMP is achieved
[10,15,27,28].
Dealing with CO 2 injection, there are two types of miscible flooding, known as
follows [15]:
1. First-contact miscibility (FCM): In this process CO 2 and crude oil are mixed in all
proportions upon first contact, making a single homogenous solution.
2. Multiple-contact miscibility (MCM): Generally, CO 2 and crude oil are not misci-
ble on the first contact. Indeed, miscibility occurs dynamically upon multiple con-
tacts within the reservoir. This type of miscibility is called MCM. During this
process, the composition of solutions (injection and reservoir fluids) are changed
through a mass transfer between CO 2 and crude oil. This mass transfer phenome-
non drives miscibility in two ways [12]: