Petroleum engineering: Enhanced oil recovery

Image source

We refer to enhanced oil recovery when we have to resort to a mechanism to extract oil when other mechanisms such as primary and secondary recovery techniques have been exhausted, this is one of the reasons why enhanced oil recovery (EOR) is also known as tertiary recovery, since prior to this, primary and secondary recovery techniques should have been applied.

It is important to highlight that when we talk about enhanced oil recovery (EOR) we are improving the production of the reservoir and not of the well, since the production of each well is going to be individual, the improvement of oil production for an oil reservoir comes from the idea of being able to modify the physical and/or chemical properties of the oil and thus to be able to facilitate its extraction.

When the improvement in oil production is seen individually for each well, the chemical or physical composition of the oil is not altered, simply a production method is applied for that well, which can range from installing a pumping mechanism to apply a method of oil production through an artificial gas lift, also when it is decided to install a stilt pump or also called rocker is applying a production method for a particular well.

As time goes by, neither with the production methods of each oil well will it be possible to continue extracting oil, so it is necessary to resort to a mechanism of enhanced oil recovery (EOR) where the mobility of the oil that is still in the rock of the oil reservoir will be improved.

These are mechanisms that are viable depending on the price of oil, since it has its particular costs, however being attractive oil prices, an analysis can be made to optimize oil production at the reservoir level, i.e. if with primary and secondary recovery mechanisms can be obtained from 20% to 40%, then you can choose to apply enhanced oil recovery (EOR) because you can get an additional production that can reach up to a maximum of 60%.

Image source

What is the objective of enhanced oil recovery (EOR)?

It is necessary to analyze the production behavior of the reservoir prior to enhanced oil recovery, in order to know the reasons why we end up applying enhanced oil recovery.

The primary recovery is the reservoir's own energy, here man's engineering does not intervene, since the energy of the reservoir is such that oil can be produced, the reservoir can provide energy by the pressure generated by an aquifer, or even energy that can provide a layer of gas, in some cases the energy of the reservoir is exhausted, however the oil is still mobilized from the rock to the well, it is only necessary to apply a method of artificial lift, this is still contemplated within primary oil recovery.

In secondary recovery, what is sought is to increase the production percentage, since with primary recovery only 10% of the oil stored in the storage rock can be extracted. The aim is not only to increase the percentage, but also the useful life of the reservoir contribution, for which water or gas is injected to improve the mobility of the oil so that it can be produced by the set of producing wells of such producing reservoir.

However, when we look for enhanced oil recovery or also called tertiary recovery, it is because we want to increase the percentage of oil recovery obtained in the secondary recovery, which is 20% to 40%, and that can be up to 60% of the oil originally found in the storage rock.

To differentiate from a secondary recovery mechanism to a tertiary one, we must know which fluid we are going to inject into the reservoir that will make the oil in the reservoir rock improve its mobility (permeability) even more and can be easily moved from the reservoir to the well.

The fluids injected in tertiary recovery are carbon dioxide (CO2), sometimes pure, and sometimes enhanced with the injection of natural gas and/or nitrogen.

Image source

There are multiple techniques for enhanced oil recovery, however I would like to explain briefly one that can be easily applied in my country Venezuela in the Orinoco Oil Belt, which is thermal injection.

The oil found in the Orinoco Oil Belt is an extra heavy oil, which has a high viscosity, which translates into a low value of API degrees, this can be improved by lowering its viscosity, lowering its viscosity improves its permeability and can be easily extracted.

The first thing to take into account is that some wells can be drilled to work as steam injector wells to heat the oil formation and lower the viscosity of the oil.

The drilling of the injector wells must be horizontal, with the intention of heating the formation and the oil can fall by gravity and be collected by other producing wells parallel to the injectors that will collect the oil that will be heated and fall by gravity, to be collected to the surface.

Image source

References

• Walsh, Mark; Larry W. Lake (2003). A generalized approach to primary hydrocarbon recovery. Elsevier.

• Summary of Carbon Dioxide Enhanced Oil Recovery (CO2EOR) Injection Well Technology (Report). American Petroleum Institute. 2007.