Study Questions Lifecycle Emissions Benefits of Using CO2 for Enhanced Oil Recovery as a Method for Carbon Sequestration
|Net life cycle GHG emissions of five CO2-EOR projects used as case studies. Credit: ACS, Jaramillo et al. Click to enlarge.|
Using CO2 injection for enhanced oil recovery (EOR)—an established commercial practice that currently annually consumes some 50 million metric tons of CO2 (the majority from natural accumulations)—has also been identified as a method of sequestering CO2 captured from industrial sources, such as power plants. Of the $21.6 million the US Department of Energy recently awarded to carbon capture and storage research projects, $5.9 million was for EOR projects. (Earlier post.)
However, a new study by researchers at Carnegie Mellon University assessing the overall life cycle emissions associated with CO2-EOR sequestration under a number of different scenarios has concluded that “without displacement of a carbon intensive energy source, CO2-EOR systems will result in net carbon emissions.” Their paper was published online 30 September in the ACS journal Environmental Science & Technology.
While a number of earlier studies have generally concluded that CO2-EOR can store significant amounts of CO2, thereby reducing the greenhouse gas impacts of power generation, those studies for the most part have ignored the fact that oil is produced as a result, and that 93% of the carbon in petroleum is refined into combustible products ultimately emitted into the atmosphere, the researchers said.
|CO2 flooding for oil recovery. Source: Kansas Geological Survey. Click to enlarge.|
CO2-EOR. Production of an oil reservoir can go through three distinct phases: primary, secondary and tertiary (enhanced) recovery. During primary production, natural reservoir pressure or gravity pushes oil into the wellbore. Only about 10% of the original oil in place is typically produced during primary recovery, according to the DOE.
Secondary recovery techniques generally injecting water or gas to displace oil and drive it to a production wellbore, resulting in the recovery of 20 to 40% of the original oil in place. The use of tertiary techniques can offer prospects for ultimately producing 30 to 60%, or more, of the reservoir’s original oil in place. There are three major categories of EOR that have shown commercial viability:
- Thermal recovery (e.g., using steam to reduce viscosity) accounts for more than 50% of US EOR, mainly in California.
- Gas injection (e.g., CO2, methane or nitrogen) uses gases that expand in the reservoir and push additional oil to the wellbore, or other gases that dissolve in the oil to lower viscosity. gas injection accounts for nearly 50% of EOR in the US.
- Chemical injection, involving 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 1% of US EOR production.
A set of 10 basin-oriented assessments released by the DOE in February 2006 estimated that 89 billion barrels of additional oil from currently stranded oil resources in ten US regions could be technically recoverable by applying state-of-the-art CO2-EOR technologies.
Recent work has also determined that under certain geologic and hydrodynamic conditions, an additional residual oil zone (ROZ) exists below the oil-bearing transition zone beneath the traditionally defined base (oil-water contact) of an oil reservoir. This resource could add another 100 billion barrels of oil resource in place in the United States, according to the DOE, and an estimated 20 billion barrels could be recoverable with state-of-the-art CO2-EOR technologies.
DOE also suggests that CO2-EOR technologies will be important in the future development of technically recoverable domestic oil resources that as yet remain undeveloped or are yet to be discovered. Undeveloped domestic oil resources still in the ground (in-place) total and estimated 1,124 billion barrels. Of this large in-place resource, 430 billion barrels is estimated to be technically recoverable.
The CMU Study. In their study, Paulina Jaramillo, W. Michael Griffin and Sean McCoy used the guidelines established by the International Standards Organization in ISO 14040. They included within the boundaries of the analysis the emissions associated with the:
- Life cycle of the electricity generated within the power plant for CO2 capture;
- Transport of the CO2 from the power plant to the field;
- Oil extraction;
- Transport of the crude oil produced in the field;
- Crude oil refining; and
- Combustion of the refined petroleum products.
The boundaries excluded transport of petroleum products from the refinery to the consumer, as the widely different characteristics of the refined products result in large uncertainties associated with calculating the total transport emissions of all the refined products. These are estimated to be a small percentage, however (approximately 1%). The study boundary also excludes any emissions associated with the construction of the physical infrastructure needed for these projects.
For the study, they used five CO2-EOR projects as case studies: Northeast Purdy; SACROC; Ford Geraldine; Joffe Viking; and Weyburn.
The net emissions from the systems are positive meaning that the GHG emissions are larger than the CO2 injected and stored in the reservoir. The SACROC Unit, Kelly Snyder and the Weyburn Unit cases have the largest net emissions...The largest source of CO2 emissions is related to the ultimate combustion of petroleum-derived products and by itself is larger than the emissions offset by CO2 sequestration.
We calculated that between 3.7 and 4.7 metric tons of CO2 are emitted for every metric ton of CO2 injected. The fields currently inject and sequester less than 0.2 metric tons of CO2 per bbl of oil produced. In order to entirely offset system emissions, e.g., making the net CO2 emissions zero, 0.62 metric tons of CO2 would need to be injected and permanently sequestered for every bbl of oil produced. The only way to sequester this amount of CO2 would be to operate a sequestration project concurrently with the CO2-EOR project. For example, instead of recycling produced CO2, as in typical CO2-flood EOR projects, produced CO2 could be reinjected into the water leg of the same formation (as practiced at the In Salah project) or into another nearby appropriate geological formation.—Jaramillo et al.
The key argument for CO2-EOR as a sequestration method is that the electricity and oil produced within the system boundary displaces oil or electricity from other sources, the researchers said.
Without a detailed economic model that captures the complexity of oil use or electricity production and management it is difficult to be certain what sources, if any, will be displaced. A thorough understanding of ultimate displacement is necessary before anyone can suggest that CO2-EOR is a sequestration technique.
Certainly it is intuitive that a bbl produced by the use of anthropogenic CO2 could replace a bbl of oil recovered using natural CO2. The link to other conventional and unconventional crude oil displacements is much more tenuous. Also, any displacement argument must take into account the overall continual increases of demand of energy to make certain that within a relative time frame important to climate change the displaced energy source remains displaced. It is clear, that without displacement of a carbon intensive energy source, CO2-EOR systems will result in net carbon emissions.—Jaramillo et al.
Paulina Jaramillo, W. Michael Griffin and Sean T. McCoy (2009) Life Cycle Inventory of CO2 in an Enhanced Oil Recovery System. Environ. Sci. Technol., Article ASAP doi: 10.1021/es902006h