Typically, a well will produce at its highest production rate at the beginning of the production cycle; and then production will wane. In an effort to increase production from both oil and natural gas wells, secondary production methods are employed. A type of Enhanced Oil Recovery (EOR), secondary production includes water flooding and gas injection.
Secondary Production: Gas Injection
Secondary production methods are employed to increase production by boosting depleted pressure in a formation. As the oil or natural gas in a formation is produced, the hydrocarbons remaining in the reservoir may become trapped because the pressure in the formation has lessened, making production either slow dramatically or stop altogether.
A form of secondary production, gas injection is used on a well to enhance waning pressure within the formation. Systematically spread throughout the field, gas-injection wells are used to inject gas and effectively sweep the formation for remaining petroleum, boosting production.
Somewhat similar to water injection, or water flooding, gas injection is a pressure maintenance program that can be employed on a reservoir at the start of the production process or introduced after production has already started to lessen. Here, gas is injected into the gas cap of the formation, whereas in water injection, the water is injected directly into the production zone.
Cycling in a Natural Gas Reservoir
Sometimes known as cycling, gas injection can entail re-injection of produced natural gas. In this instance, as the pressure drops in a natural gas field, the condensate separates from the dry gas in the reservoir. The condensate liquids block the pores within the reservoir, making extraction practically impossible.
Cycling is used to prevent the condensate from separating from the natural gas in the reservoir. In this process, the natural gas liquids (condensate) are stripped from the gas on the surface after it has been produced from the reservoir, and the dry gas is then re-injected into the reservoir through injection wells. Again, this helps to maintain pressure in the reservoir while also preventing the separation within the hydrocarbon.
Natural Gas Disposal Solution
Additionally, gas injection can serve as an economical way to dispose of uneconomical gas production on an oil reservoir. While in the past, low levels of natural gas that were produced from oil fields were flared or burned off, that practice is discouraged in some countries and against the law in others.
Now, the low levels of natural gas that are produced from prolific oil fields are re-injected into the formation as form of disposal, as well as pressure maintenance. Here, produced wet gas from oil fields are stripped of their natural gas liquids, compressed and pumped into an injection well.
If the oil field is highly saturated, the natural gas is injected in the free gas cap; but if the oil field is under-saturated, the gas is injected directly into the oil reservoir.
Gas Injection, Gas Lift & Gas Miscible Process
Although the terms are sometimes interchanged, gas injection and gas lift are two separate processes that are used to increase production. While gas injection is a secondary production method, gas lift is a type of artificial lift.
Artificial lift is another way to increase production from a well by increasing pressure within the reservoir. The main types of artificial lift include gas lift and pumping systems, such as beam pumps, hydraulic pumps and electric submersible pumps.
While gas injection is achieved by injecting gas through its own injection well, gas lift occurs through the production wells. In gas lift, compressed gas is injected down the casing tubing annulus of a production well, entering the well at numerous entry points called gas-lift valves. As the gas enters the tubing at these different stages, it forms bubbles, lightens the fluids and lowers the pressure, thus increasing the production rate of the well.
Furthermore, a type of EOR employed on a well in the tertiary production process, a gas miscible process can be used to increase production. The difference in this recovery method is that the gases introduced into the reservoir are not naturally occurring. In a gas miscible process, carbon dioxide, nitrogen and LPG are injected into the reservoir.