CO2 Enhanced Oil Recovery

Oil Production Video

Oil is extracted in three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. Primary and secondary recovery operations often leave more than two-thirds of the oil in the reservoir. Injecting CO2 into the reservoirs can recover some of that remaining oil. This practice is called CO2 enhanced oil recovery (EOR).

How EOR Works

Carbon dioxide is injected into the oil reservoir. Once there, the CO2 dissolves into the oil upon contact and thins the oil so it can flow more easily. The CO2 also repressurizes the reservoir. As a result, an incremental portion of the oil is freed to move through the minute pathways in the reservoir rock to the surrounding production wells. In many cases, water is injected after the CO2 to help physically push and drag the CO2-infused oil to the production wells. This alternation of water and CO2 injection is repeated until additional oil cannot be produced economically.

EOR Model

Fate of the Injected CO2

When the CO2 is injected, a portion becomes trapped in the pores of the reservoir rock, but another portion returns to the surface dissolved in the crude oil. At the surface, the dissolved CO2 is separated from the oil, and the CO2 is then compressed and reinjected.

Each time CO2 is injected, some of the CO2 remains underground, permanently trapped in the oil-bearing rock formation. This trapped CO2 does not return to the surface in the produced crude oil.

To make up for the CO2 that remains trapped in the oil reservoir, additional CO2 is purchased. The new CO2 with CO2 that has been separated from the crude oil is then injected, and the process is repeated again and again over the decades-long life of the CO2 EOR project. At the end of the project, essentially all of the CO2 that was injected is ultimately trapped or "sequestered" underground in the pores of the reservoir rock. As a result, hundreds of millions of tons of CO2 are permanently stored underground in oil fields in the United States and around the world.

Natural CO2 vs Anthropogenic CO2 for EOR

Historically, most of the CO2 for EOR has come from natural underground accumulations of CO2. The CO2 from these geologic deposits is limited in volume, and economics restrict its use to within a couple of hundred miles of where it occurs. In contrast, anthropogenic (or human made) sources of CO2 are widespread. As more and more oil fields mature, the need for CO2 EOR will increase, and the potential for EOR using anthropogenic CO2 will increase as well.1,2 EOR using anthropogenic CO2 represents a prime opportunity for reducing the CO2 emissions from large-scale sources like power plants, gas- and oil-processing plants, and ethanol facilities.


References
  1. Bradley, T., The CO2 enhanced oil recovery story: Kinder Morgan CO2 Company.
  2. https://www.edf.org/sites/default/files/10254_Bradley.pdf (accessed March 2018).
  3. National Resources Defense Council, 2008, Tapping into stranded domestic oil - enhanced oil recovery with carbon dioxide is a win-win-win. https://www.nrdc.org/sites/default/files/eor.pdf (accessed March 2018).