Page 99 - Primer on Enhanced Oil Recovery

90 Primer on Enhanced Oil Recovery

Generally, the higher is the pressure the smaller is two phase (or multiphase)
region. This is demonstrated at the Fig. 9.4. It is also possible to say, but this is a
less accurate statement compared to the pressure effect, that the temperature rise
has an opposite effect for phase behavior at higher temperature there is higher
probability of multiphase region existence.
Pseudoternary hydrocarbon diagrams are important for analysis of many pro-
cesses in oil reservoir behavior. The diagrams are used for oil, water, gas and sur-
factants behavior. The diagrams are either drawn on the results of laboratory
investigations or by various modeling software.
Injected into reservoir fluids and gases somehow will interact with the crude.
The exact interaction will define, at the end, the efficiency, in extraction terms, of
the injection. It is possible to divide all interactions into two groups immiscible
and miscible. We will not talk about the emulsification at this point as the process
needs different description model.
At the immiscible situation, two liquids/gases will not form a single phase.
Some insignificant number of molecules will cross the border between two sub-
stances, nevertheless. “Insignificant” means that the number of those molecules
would not be big enough to alter the phase behavior.
Miscible situation is more complicated. Pressure plays very important role. It is
possible that the injected liquid (gas) and oil (reservoir hydrocarbons) will mix
immediately on the first contact. This is so named First Contact Miscibility (FCM).
It is close to impossible to achieve and high(er) pressure helps in this respect. LPG
by many is regarded as First Contact Miscible liquid with a reservoir oil.
Most readily (for a broader spectra of substances) the situation is realised when
the injected gas (liquid) mixes with the crude gradually through the process of
exchanging components. This is so named Multiple Contact Miscibility (MCM).
Some components from the injected gas (liquid) will dissolve in crude first, this
will modify the oil and allow other components to dissolve. Some components of
oil (light hydrocarbons) will dissolve in the injected gas (liquid). The gas (liquid)
will be enriched with the oil light components. The process will continue until the
single phase will be created.
Oil extraction is a dynamic situation. We need to make the reservoir liquids (we
prefer oil) in the reservoir to move towards production wells so we would get the
liquids to the surface. As liquids and gases in reservoir move the properties of sub-
stances at each physical point change dynamically.
Physical and chemical processes in many cases depend on concentrations and
defined by the dominant process. It can be that oil light components predominantly
evaporate into injected gas. In this case the injected gas in enriched with the oil
light fractions. As the injected gas propagates through the reservoir, we can say that
the mixing develops on the front zone and this drives virgin oil displacement. This
process dominates oil displacement in the case of displacement by lean gas (meth-
ane and ethane) and it is named a vaporizing drive (see Fig. 9.5A).
If enriched gas injected to drive the displacement, then heavier hydrocarbons
from the gas dissolve in oil and the mixing develops at the back of propagating gas
slug. This process is named a condensing drive.