Page 119 - Primer on Enhanced Oil Recovery

Gas flooding 109

when just carbonated water is injected. At the same time, too high volumes of CO 2
can lead to an early breakthrough of carbon dioxide into producing wells. In a het-
erogeneous reservoir, the volumes of the slugs should be lower than in a homoge-
neous reservoir. In the case of complete miscibility of carbon dioxide and oil, the
volumes of the slugs can be quite large and reach 20% of the pore volume. At low
miscibility, the volumes of the slugs should be rather small with the frequent
alternation.
Other method modifications are developed in order to alleviate demand for
high volumes of CO 2 . This development stimulated search for the further improve-
ment of the injection technique. To reduce the consumption of carbon dioxide, it is
possible to use alternating injection of carbon dioxide, water and other more readily
available gases. Also, it is possible to add various surfactants to water, which will
ensure the formation of foam in the reservoir. It is also possible to use polymer
additives to the injected water.
The field development process for the application of carbon dioxide injection
practically does not differ from the system of development in the case of ordinary
water flooding. Well placement is not significantly different from the conventional
waterflooding. However, in a presence of high vertical permeability and the high grav-
ity segregation of carbon dioxide and water, the well network should be densified.
Choosing the most economical method for the field development should include
clear goal of the development and the planning ought to take into the account vari-
ous factors of technical and capital investment. Significant volume of information
needs to be gathered, including well documented history of the field. Planning for
the flexibility in all aspects of injection and production is highly desirable. Eventual
handling of big volumes of water and gas in the production wells needs to be pre-
pared for. Re-injection needs to be undertaken as environmental concerns become
more noticeable and CO 2 recycling is economically efficient.
It also needs to be taken into the account that the positive outcomes of CO 2
injection while certain would need time to develop.

10.2 Hydrocarbon injection

It was mentioned before that with the injection of hydrocarbon gases into the reser-
voir, oil can be extracted without mixing or with mixing. Dry natural gas at normal
pressures and temperatures, even in deep oil strata, does not mix with oil easily. To
ensure mixing, a transition zone is needed in which both oil and gas intermix.
However, during the injection of dry hydrocarbon gases, the transition zone is not
easily formed, and the oil is extracted without mixing. To ensure more efficient
mixing, three different methods are used: high pressure gas injection; enriched gas
injection and liquefied gas injection.
The mechanism of mixing displacement of oil by hydrocarbon gas:
Creating miscibility when pumping high-pressure gas and enriched gas (this ensures zero
interfacial tension between the oil and the working agent);