Gas injection is 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 (s) at certain depth. The underlying principle of the gas lift method is that gas reduce 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 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 planed 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 huge challenge that may be too costly to overcome, due to high H2S content in the natural gas available in the region.
Supply & Demand Dynamics
Global demand for gas injection EOR services will continue to be second preferred method, after the thermal recovery option. However, with more emphasis on the environment and improved technology, the economics of CO2 EOR will result in higher utilization of this particular method and reduce the demand for gas injection EOR.
In the GCC, this change will be particular evident due to power generation requirements. As a result of growing population in the region and efforts to maintain and increase oil production, it is becoming more challenging to meet the demand for natural gas to cater for power generation, gas re-injection projects and steam generation. Hence, focus on other EOR methods, such as CO2, will be evident in the years to come and greatly affect any appetite for gas injection EOR projects.
Application of gas injection (miscible) projects in the GCC is limited to Harweel field in Oman and to a lesser extent in Abu Al Bukhoosh field in UAE.
Cost & Price Analysis
EOR production is considered as a project in itself and requires a large number of equipment and packages. Studies, pilot projects, engineering design, equipment procurement, feedstock planning, drilling and other materials, play important role in EOR projects. Upfront capital costs in EOR projects are significant.
Availability of gas, its opportunity cost and infrastructure re-development play one of the key roles in cost basis for any gas injection EOR.