Page 50 - Fundamentals of Enhanced Oil and Gas Recovery

38 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

1.15.5 Secondary Recovery
1.15.5.1 Miscible Gas Injection
Gas injection is one of the oldest methods of oil production and is known as an
important method in EOR industry. As a technique that yields the highest rate of
oil recovery, steam injection has also a special place in oil recovery industry, yet it
comes second after the most important technique of oil recovery, i.e., gas injection.
EOR using carbon dioxide not only reduces the production cost of oil and its price
fluctuations, but it is the only technique that has developed steadily during recent
years (13). After steam injection, injecting hydrocarbon has rendered the highest
rate of oil recovery, and since there have been many efforts to reduce gas combus-
tion, gas injection is considered as one of the most important techniques in oil
recovery industry, both now and for the future. After many years of enhanced oil
recovering from different reservoirs, gas recovery techniques are well understood
now, and the recovery parameters are discussed with more certainty (13).
Comprehensive and extensive studies have been conducted on carbon dioxide
(CO 2 ) and nitrogen (N 2 ). But the range of gravity, pressure, and depth differs for
each of these methods. Therefore these techniques are preferable for deep reservoirs,
and the final decision often depends on local circumstances as well as the existence,
amount, price, and availability of the given gas.

1.15.5.2 Hydrocarbon Injection
This technique is the oldest technique in EOR industry. Before the discovery and def-
inition of concepts of “minimum miscibility pressure” and “minimum miscibility per-
centage,” the injecting hydrocarbon technique has been practiced for years. It
occupies a position between nitrogen, which needs a high-level pressure, and carbon
dioxide, which requires an average level of pressure. This is true about methane as
well. Nevertheless, in a reservoir with low depth that requires less pressure, one can
do this process by increasing C 2 C 4 . Various techniques that are used in hydrocarbon
gas injection include:
1. Enriched gas drive
2. Lean gas drive
3. High pressure gas injection
4. LPG injection

1.15.5.3 Nitrogen and Generated Gases
Compressed air, nitrogen, and generated gases are the cheapest gases available, and this
is considered a great advantage. These gases can be injected simultaneously, and
because their minimum miscibility pressure is close to each other, they can be used
alternatively for recovering oil.