Page 106 - Primer on Enhanced Oil Recovery

96 Primer on Enhanced Oil Recovery

Figure 10.1 Project numbers of thermal recovery and gas flooding in USA.

then an early breakthrough of the working agent occurs and the sweep efficiency
will be low. Hydrocarbons have been and are used widely for the gas injections too.
However, hydrocarbons are valuable consumer products and there is an increasing
reluctance to inject them back into the reservoir. In recent years, especially in the
United States, the emphasis has shifted to less valuable, non-hydrocarbon gases,
such as CO 2 , nitrogen and flue gases. Although nitrogen and flue gases displace oil
less efficiently than hydrocarbon gases, overall economic outcome can be some-
what more favorable.
A significant influence on the process of oil displacement by gas injection into
the formation is exerted by its miscibility with oil. As a rule, at atmospheric pres-
sure and room temperatures dry natural gas (methane) and oil do not mix. To
ensure the mixing of oil and gas, the existence of a transition zone between them is
necessary. In this case, the hydrocarbon mixture forming the transition zone must
be dissolved in both gas and oil. This is usually achieved by enriching the injected
gas.
The displacement of oil under conditions when complete mixing with the gas
does not occur is called immiscible displacement. The best results in enhanced oil
recovery by the gas flooding are observed when the oil is displaced by gas in a mis-
cible displacement.
The CO 2 use for EOR significantly developed from the first patent for the
method in 1952. Analysis of the EOR gas methods use of in the United States
shows that there are periods during which interest in this method of EOR has
increased significantly (see Figs. 10.1 and 10.2). The first period was during