8. What is CO2-EOR?

EOR stands for "enhanced oil recovery." When an oilfield has begun to slow its oil production (this can happen for many reasons such as a drop in pressure that stops the oil from traveling to the production well, or because the oil that remains in the reservoir is very thick and unable to flow,) different methods can be used to "enhance" the production and increase the amount of oil being produced.

One well-known EOR method is called "waterflooding," whereby water is injected to increase the reservoir's pressure and help to push oil to the production well. Other methods include "steam injection" and "solvent injection" whereby heat and/or solvents are injected to help thin heavier kinds of oil so that they flow more freely.

CO2-EOR involves the injection of compressed carbon dioxide into an oil reservoir. CO2 acts like a solvent, and causes the oil to expand and flow more easily to production wells. In the case of the Weyburn oilfield, CO2 is injected at several injection wells in the field (up to 70 or more at a time). After several days of injecting CO2, the wells are then switched to water injection. This alternating water-and-gas injection (or WAG, as it is called) allows the oil to first expand and become better able to flow because of the CO2, and then the water increases the pressure in the reservoir to flush this newly freed oil to production wells.

The Weyburn oilfield was producing only 8,000 barrels of oil per day by 1990. CO2 began to be injected in 2000, and within 5 years oil production had grown to nearly 30,000 barrels per day [see Figure 7]. Because the CO2 is miscible with the oil (miscible means it blends with the oil and becomes part of the oil mixture as it comes to the surface) a certain amount of CO2 returns to the surface during oil production. The oil company at the Weyburn field separates this CO2 from the oil mixture at the surface, and compresses and re-injects it along with the new CO2 arriving from the United States.

Figure 7. Production graph from the Weyburn oilfield, showing a peak production during CO2 injection of just over 30,000 barrels of oil per day (BOPD). "Infills" mean additional wells drilled into the field, first vertical and then, when the technology was developed in the 1990s, horizontal wells. Both of these measures temporarily increased oil production. The injection of CO2, starting in January 2000, has led to a significant jump in production. (Image courtesy of Cenovus Energy)