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 to the 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. Three major categories of EOR have been found to be commercially successful to varying degrees:

• Thermal recovery, which involves the introduction of heat such as the injection of steam to lower the viscosity, or thin, the heavy viscous oil, and improve its ability to flow through the reservoir. Thermal techniques account for over 50 percent of U.S. EOR production, primarily in California.
• Gas injection, which uses gases such as natural gas, nitrogen, or carbon dioxide that expand in a reservoir to push additional oil to a production wellbore, or other gases that dissolve in the oil to lower its viscosity and improves its flow rate. Gas injection accounts for nearly 50 percent of EOR production in the United States.
• Chemical injection, which can involve the use of long-chained molecules called polymers to increase the effectiveness of waterfloods, or the use of detergent-like surfactants to help lower the surface tension that often prevents oil droplets from moving through a reservoir. Chemical techniques account for less than one percent of U.S. EOR production.

Each of these techniques has been hampered by its relatively high cost and, in some cases, by the unpredictability of its effectiveness.

CO2 Injection Offers Considerable Potential Benefits

Graphic of CO2 enhanced oil recovery. Courtesy of Occidental Petroleum Corp.

The EOR technique that is attracting the most new market interest is carbon dioxide (CO2)-EOR. First tried in 1972 in Scurry County, Texas, CO2 injection has been used successfully throughout the Permian Basin of West Texas and eastern New Mexico, and is now being pursued to a limited extent in Kansas, Mississippi, Wyoming, Oklahoma, Colorado, Utah, Montana, Alaska, and Pennsylvania.

Until recently, most of the CO2 used for EOR has come from naturally-occurring reservoirs. But new technologies are being developed to produce CO2 from industrial applications such as natural gas processing, fertilizer, ethanol, and hydrogen plants in locations where naturally occurring reservoirs are not available. One demonstration at the Dakota Gasification Company's plant in Beulah, North Dakota is producing CO2 and delivering it by a new 204-mile pipeline to the Weyburn oil field in Saskatchewan, Canada. Encana, the field's operator, is injecting the CO2 to extend the field's productive life, hoping to add another 25 years and as much as 130 million barrels of oil that might otherwise have been abandoned.

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A turning-point in CO2-EOR advances is a project funded by DOE in the Hall-Gurney field in Kansas that seeks to demonstrate this technology's time has come - providing energy, economic and environmental benefits. A companion project underway in the Hall-Gurney field involves testing the feasibility of 4-D high resolution seismic monitoring of CO2 injection in thin, relatively shallow mature carbonate reservoirs.  Incorporating such time-lapsed monitoring data into CO2-EOR programs could dramatically improve the efficiency and economics of using the technology in many Midcontinent fields.

New breakthroughs in CO2-EOR recovery technology could further enhance oil recovery in Texas and other oil producing states. One DOE-industry partnership project is investigating gravity-stable CO2 injection in the Permian Basin in West Texas, where the goal is to increase oil recovery in the Scurry Canyon Reef field.

DOE Basin-Oriented CO2-EOR Assessments

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In February 2006, a series of technical reports released by the Department on Energy (DOE) Office of Fossil Energy highlight the significant potential for state-of-the-art and advanced oil recovery technologies to significantly contribute to the development of the large volume of remaining undeveloped domestic oil resources in the United States.  Ten basin-oriented assessments- four new, three updated and three previously released- estimate that 89 billion barrels of additional oil from currently "stranded" oil resources in ten U.S. regions could be technically recoverable by applying state-of-the-art CO2-EOR technologies.

U.S. Basins/Regions Studied for Future Potential for CO2-EOR.

Additional work has examined potential improvements in CO2-EOR technologies beyond the state-of-the-art that can further increase this potential.  This work evaluating the potential of "game changing" improvements in oil recovery efficiency for CO2-EOR illustrates that the wide-scale implementation of next generation CO2-EOR technology advances have the potential to increase domestic oil
recovery efficiency from about one-third to over 60 percent. 

The presence of an oil bearing transition zone beneath the traditionally defined base (oil-water contact) of an oil reservoir is well established.  What is now clear, and as recently documented in a series of DOE Office of Fossil Energy reports, is that, under certain geologic and hydrodynamic conditions, an additional residual oil zone (ROZ) exists below this transition zone, and this resource could add another 100 billion barrels of oil resource in place in the United States, and an estimated 20 billion barrels could be recoverable with state-of-the-art CO2-EOR technologies.

Large volumes of technically recoverable domestic oil resources remain undeveloped and are yet to be discovered in the United States, and this potential associated with CO2-EOR represents just a portion, albeit large, of this potential. Undeveloped domestic oil resources still in the ground (in-place) total 1,124 billion barrels.  Of this large in-place resource, 430 billon barrels is estimated to be technically recoverable.  This resource includes undiscovered oil, "stranded" light
oil amenable to CO2 enhanced oil recovery (EOR) technologies, unconventional oil (deep heavy oil and tar sands) and new petroleum concepts (residual oil in reservoir transition zones).