Tertiary Recovery

What Is Tertiary Recovery?

Tertiary recovery, also known as enhanced oil recovery (EOR), is the third stage used to extract oil from an oil reserve. 

Because it is more costly and expensive than the primary recovery and secondary recovery stages, tertiary recovery is only carried out when the price of oil is sufficiently high to justify the investment.

How Tertiary Recovery Works

The primary recovery stage of extracting oil from a reserve works by exploiting the natural disparity in pressure between the surface of the oil well and its subterranean reserves. Typically, this is done by increasing the pressure inside the oil reserve using injections of steam or natural gas.

Although there are many specific tertiary recovery techniques, all variations of this stage of extraction generally rely on directly influencing the chemical composition of the oil remaining in the reserve. The process of tertiary recovery relies on injections that are designed to decrease the viscosity of the remaining oil, thereby making it easier to extract.

While the primary and secondary recovery phases typically extract between 10% and 40% of the available well, tertiary recovery is used to recover the remaining portion. Because of its increased cost, however, resource-extraction companies may deliberately abandon oil wells without progressing to tertiary recovery if the price of oil is not high enough to justify the expense.

There are three primary methods of tertiary recovery, involving the use of heat, gas, and chemical injections, respectively.

In the thermal recovery method, the reservoir is heated through the injection of water, which quickly converts into steam. The steam then warms the oil, causing it to lose viscosity and therefore flow more easily toward the lower-pressure area of the surface. 

The gas injection method functions by pumping gases, such as carbon dioxide, nitrogen, or natural gas, into the reservoir. These gases then expand, increasing the pressure of the reservoir and therefore pushing the oil toward the surface.

Lastly, chemical injections involve pumping polymers into the reservoir so as to lower the surface tension of the oil. As with the other methods, this approach allows the oil to flow more freely toward the surface. Due to its additional complexity and environmental concerns, however, it is far less common than the thermal or gas injection methods.

Sometimes carbon dioxide is used in tertiary recovery. In the past, the carbon dioxide used for this type of recovery came from naturally-occurring carbon dioxide reserves. However, it is now possible to harvest carbon dioxide from natural gas processors, and from fertilizer and ethanol production plants. Pipelines can then transport carbon dioxide to the injection site, thereby making tertiary recovery more widely accessible and efficient than it was in the past. The use of carbon dioxide in tertiary recovery shows significant potential for increasing the practicality of these recovery methods.