Primary Oil Recovery – Secondary Oil Recovery – Tertiary Oil Recovery
Crude oil development and production in U.S. oil reservoirs can include up to three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. During primary recovery, the natural pressure of the reservoir or gravity drive oil into the wellbore, combined with artificial lift techniques (such as pumps) which bring the oil to the surface. But only about 10 percent of a reservoir’s original oil in place is typically produced during primary recovery. Secondary recovery techniques extend a field’s productive life generally by injecting water or gas to displace oil and drive it to a production wellbore, resulting in the recovery of 20 to 40 percent of the original oil in place.
However, with much of the easy-to-produce oil already recovered from U.S. oil fields, producers have attempted several tertiary, or enhanced oil recovery (EOR), techniques that offer prospects for ultimately producing 30 to 60 percent, or more, of the reservoir’s original oil in place.
Department of Energy – Enhanced Oil Recovery
Other Carbon Dioxide-Enhanced Oil Recovery links:
National Energy Technology Laboratory – NETL
United States Geological Society – USGS
Carbon Dioxide-Enhanced Oil Recovery – (CO2-EOR)
Once secondary recovery techniques such as waterflooding no longer produce oil economically, tertiary recovery techniques, also known as enhanced oil recovery, may be utilized to produce additional oil. While the primary and secondary oil recovery phases depend upon pressure, tertiary recovery methods work by actually altering the original properties of the oil. One of the most common tertiary recovery methods is Carbon Dioxide enhanced oil recovery, or CO2 EOR. Beyond primary and secondary recovery phases, an additional 5 to 20% of the oil in the reservoir can be recovered by CO2 EOR. The CO2 is commonly sourced from large underground deposits and can also be captured from sources such as electric power plant emissions.
Once captured, it is generally brought to the oilfield by pipeline and injected into the reservoir or held in storage tanks. The CO2 is compressed to a high pressure and injected into the oil reservoir to begin the EOR process. Produced water, or oilfield brine, is injected alternately with the compressed gas in what is known as the “water alternating gas”, or WAG, method. Once the pressurized CO2 and water are injected into the reservoir, the oil reacts very differently with the CO2 than it does with the water. Water injection drives oil out of the formation through pressure by direct force.
CO2 injection, on the other hand, is miscible, or able to mix, with the oil. Miscibility means that CO2 can dissolve into and swell the residual oil within a miscible zone. Once the CO2 has been absorbed, inside the miscible zone, the oil’s viscosity is reduced and it is now able to flow more easily through the formation. Because of its lower viscosity, CO2 may begin to form channels through the oil. This channeling effect allows large amounts of CO2 to force through to the production well, bypassing much of the oil. The WAG method uses the pressure of the water injection to reduce channeling. This creates a stable force that drives oil to the production well. Once the oil, CO2, and water mixture reaches the surface through the production well, the CO2 and water are separated from the oil. The oil is sent to storage tanks where it is stored or transported by pipeline to a refinery.
This flow diagram demonstrates how the CO2 flood cycle works. The reservoir, surface facilities and wells make up a closed loop system where virtually all the CO2 is retained in the project. After the CO2 EOR process is complete effectively all of the injected CO2 remains underground in the reservoir. But because CO2 is a valuable commodity, there is interest in recapturing as much of the CO2 as possible. Therefore, at completion, recycled water is sometimes injected into the reservoir to produce any recoverable CO2. This CO2 is then transported to another location sometimes within the same field for reuse. Current CO2 EOR technology makes an estimated 35-50 billion barrels of oil economically recoverable in the United States. The recovery potential for CO2 EOR is more than double the country’s proved oil reserves.
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