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during later stages when a considerable quantity of gas has already
evolved from the oil.
Gas injection wells should be located near the crest of the
reservoir. Natural oil gas is the best working agent, but if not
available in necessary quantities, air can also be injected provided
there is no gas cap. The injection of air into a gas cap is
undesirable as it will considerably impair the quantity of the gas.
In designing the compressor stations it must be borne in
mind that injection pressure is usually 15-20% higher than the
formation pressure.
The quantity of gas to be injected into a well can be
established experimentally by determining the injectivity index or
it may be calculated approximately. In field practice, depending on
3
the local conditions, 10000 to 25000 m /day of gas is injected into
wells at pressures from 5 to 9 MPa.
To prevent reservoir pressure decline, the quantity of gas
injected should be not less than the total volume of fluid
withdrawals.
In the majority cases, however, only the gas that is
produced is recycled, and even not all of that because some are
consumed to cover the field needs. If 70-80% of the gas produced
is recycled, this is generally regarded as good pressure
maintenance practice. Of course, this does not make up completely
for the expended formational energy but nevertheless, the process
of pressure decline is much retarded.
Considering the high working pressure required for a
gas-injection project, in order to save power the gas is supplied to
the compressor from high-pressure gas-gathering lines and so-
called booster compressors are used.
Gas injection is less efficient economically than water
injection because it is necessary to compress the gas to a pressure
greater than the reservoir pressure. Not much of the power used in
compressing the gas is compensated by its lower hydraulic
resistance, as compared with water.

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