Gas injection is the process of injecting natural gas (miscible and immiscible) or nitrogen (immiscible) to the reservoir, to push the oil to a producing well. The purpose of immiscible natural gas (dry gas) and nitrogen injection is to maintain pressure in the reservoir and create a gas cap. Sometimes nitrogen is used to capture oil that was trapped, as well. Miscible natural gas (lean gas, LPG or rich gases), on the other hand, is used due to its ability to mix with oil, also known as miscible with oil.
Why it is required? Oil and water cannot be mixed into a homogeneous liquid, which in turn makes it less effective when water is used to push the oil. However, if the injected gas is miscible with oil, it will become a homogenous mixture, that will expand and allow the forces of injected lean gas to be used effectively, moving oil more easily towards the well. This method needs a high reservoir pressure in order to work effectively, as well as sources of economically cheap gas, normally lean gases (residual gas -methane and ethane).
It is important to make a distinction between gas injection and gas lift, as these are two different processes. In gas injection, as part of EOR, gas is injected through separate injector wells that are usually spread across the field in a predetermined manner. Gas lift, on the other hand, is an artificial lift method, whereby a pressurized gas, produced by a nearby well, is injected continuously or intermittently into the production tubing to lift the fluids. Injected gas travels to the reservoir through a gas lift valve at a certain depth. The underlying principle of the gas lift method is that gas reduces the density of the fluid by aerating it with gas bubbles. This, in turn, reduces bottom-hole pressure, aiding fluids from the reservoir to travel to the wellbore faster and easier.
While onshore logistics makes it easier to conduct a gas injection Enhanced Oil Recovery (EOR) project, in the offshore environment it is a lot more complex, challenging and costly, due to the fact that existing infrastructure was not designed to accommodate it, in terms of weight, space, handling capacity and power. Depending on the size of the project, a specialized platform might be required to conduct offshore gas injection EOR projects. There are a number of constraints that should be considered and planned in advance, such as tanks and storage capacity, manning capacity to accommodate the required crew, pumping and supply capacity. In general, best practices and facilities for onshore gas injection EOR may not apply offshore.
Feeding gas is a huge challenge that may be too costly to overcome, due to high H2S content in the natural gas available in the region.