Page 128 - Primer on Enhanced Oil Recovery

118 Primer on Enhanced Oil Recovery

instance, it is possible to use nitrogen and flue gas. Flue gas, as is known, consists
of almost 90% of nitrogen and about 10% of carbonic acid. On the negative side,
the carbonic acid will dissolve in connate water and the displacement front will be
only then provided by nitrogen. However, if the injection pressure is high enough,
and the oil contains a sufficient amount of intermediate hydrocarbons, then the
nature of the displacing agent is not so important.
In cases, when it is possible to inject flue gases after hydrocarbon gases, the vol-
ume of the hydrocarbon slug can be as low as 5% of the pore volume. An important
advantage of flue gases is a large compressibility factor, which significantly reduces
the financial cost of compressing gas. The disadvantage of flue gases is that they
must be supplied to the field in a compressed form.
When implementing the injection of enriched hydrocarbon gas, it should be
borne in mind that it is rarely when a field produces a sufficiently fat gas and it is
necessary to obtain enriched gas by mixing dry natural gas with liquefied gas. In
order to reduce the consumption of such a valuable commodity it is necessary to
find the permissible dilution of liquefied gas with a dry hydrocarbon gas at the dis-
charge pressure and reservoir temperature before starting the process.
The system of field development in the injection of hydrocarbon gases is practi-
cally the same as the system of development in the case of the conventional water-
flooding. The main requirement for the system development is that the injection
should be produced in an areal configuration. Well placement is also not signifi-
cantly different from waterflooding. However, with high vertical permeability and
the presence of high gravity segregation of hydrocarbon gas and water, the well net-
work should have higher density.

10.3 Nitrogen and flue gases injection

To replace expensive hydrocarbon gases, relatively inexpensive nitrogen and flue
gases can be used as an oil displacer. Depending on the injection pressure and com-
position of the oil, the displacement process can be miscible and immiscible. Due
to their low cost, large volumes of these gases can be introduced into the formation.
Nitrogen or flue gases can also be used to move the hydrocarbon gases or carbon
dioxide slug. All this significantly reduces the cost of the project.
The mechanism of oil displacement with nitrogen and flue gases:
at a sufficiently high injection pressure, evaporation of light fractions of oil occurs, which
provides a miscible displacement;
ensuring gas-pressure regime, in which a significant part of the reservoir volume is filled
with relatively inexpensive non-hydrocarbon gas.
The advantages of nitrogen and flue gases include low cost and availability, low
compressibility (3 times less compared to carbon dioxide and 1.5 times less com-
pared to methane), which provides significantly lower costs for compression (2 3
times less). Nitrogen has further advantage as it does not corrode, as opposed to
flue gases, the metalworks.