Gas injection is a secondary recovery method that requires the drilling of an injection well through which gas is pumped into the reservoir with the ultimate aim of improving hydrocarbon recovery. Gas injection is different from gas lift.
Gas lift does not require the drilling of any new well, the gas is simply injected into the annular space between the production tubing and casing of the target well. Gas injection, on the other hand, requires drilling a new well that communicates with the reservoir, through which gas can be pumped to improve hydrocarbon recovery.
The injection well could be drilled into the oil column to sweep oil towards the wellbore or into the gas cap for pressure maintenance and improvement in oil recovery. Apart from improving recovery, gas injection could be carried out for gas storage purposes. In times of low gas sales, it may be necessary to store the gas in underground formations until it can be sold at a later time.
Overall, gas injection could be carried out for pressure maintenance, oil viscosity reduction, stripping of light ends in gas condensate reservoirs or gas storage.
Types of Gas Injection
Gas injection can be classified into two different types depending on the location of the gas injection wells. They include crestal gas injection and pattern gas injection. In crestal gas injection, the gas is injected right into the primary or secondary gas cap of each target producing well.
Obviously, this method will work best if the reservoir already had a gas cap or for reservoirs having good vertical permeability which will ensure good communication between the injected gas and the oil below it. The gas injected into the gas cap helps slow down reservoir pressure decline and helps in oil displacement into the wellbore, ultimately improving oil recovery.
In pattern gas injection, on the other hand, the injection wells are drilled into the oil column and involves spreading the gas injection wells throughout the oil reservoir in a particular pattern (for example 5 spot or 7 spot pattern also common with water injection wells). This method of gas injection entails dispersing the injected gas within the oil.
The dispersed gas will cause the oil column to swell, lower the oil viscosity and make it easier for oil to flow; ultimately improving oil recovery. More injection wells, however, need to be drilled in pattern gas injection to improve areal sweep efficiency . This makes pattern gas injection more expensive than crestal gas injection.
Sources of Injection Gas
Sourcing for the gas that will be used for a gas injection program could present a challenge on its own. Below are some gas sourcing options. The gas can be sourced from gases produced along with the oil from the same reservoir. This local gas can be re-injected to improve oil recovery. New wells could also be drilled into deeper gas reservoirs to source for gas.
Also, some operators may opt to purchasing gas from nearby fields. In some cases, the gas could be sourced from a combination of any or all of these options. Whichever choice is made, the economics of gas injection has to be justified in the sense that the extra oil recovery from a gas injection program should pay the bills for the injection plan with some profit.