Enhanced Oil Recovery
EOR for depleted oil and gas fields is attractive for a number of reasons. Oil and gas formations held gases and liquids for millions of years before they were removed for use signifying a viable capacity to store similar substances. At the depths and temperatures of these fields, supercritical CO2 acts like a fluid. This means that where some recoverable oil or gas resource remains in the reservoir, CO2 may be useful for filling the reservoirs for enhanced recovery of these resources. The geology of these formations is known as they have been mapped and studied through previous oil extraction endeavors. Scientists have a solid understanding of the available storage capacity of these fields.
In the case of an oil field, actual CO2 storage is accomplished in two parts. First, some of the injected CO2 is stored in the immobile oil remaining in the reservoir. The rest of the CO2 is collected from the production well and re-used for EOR. Where resources remain in economically depleted oil reservoirs, using CO2 to engage in enhanced recovery of these resources creates a revenue stream that can improve the economics of undertaking CO2 storage. According to biennial EOR reports compiled by the Oil & Gas Journal; domestic EOR production (in US) equaled 649,000 barrels per day in 2006. The latest tabulation of CO2-EOR activity in the U.S. shows approximately 237,000 barrels per day of incremental domestic oil is being produced by 86 CO2-EOR projects, distributed broadly across the U.S. This amounts to about 5% of total domestic oil production. The Weyburn field in Canada is an example of CO2 injection for EOR. In this project, 1.8 million tons of CO2 per year is being captured from a coal gasification plant and injected into an oil field.
The CO2 approach to EOR is likely to increase in response to increasing oil prices and the emergence of carbon constraints. However, implementation of CO2 capture and storage at the scale of current U.S. emissions from power plants will outstrip EOR opportunities eventually, and may exceed the volume of known petroleum reservoirs in the U.S. in the next 50 to 100 years. In a depleted gas field the injected CO2 would fill the space previously occupied by natural gas. Research is underway to see if CO2 can be used for enhanced gas production – a process mimicking EOR. Because of the economic yields, depleted fields and EOR are likely the most economically attractive options for storage in the short-term, but cannot be the only storage option due to geographic distribution and the ratio of emissions to storage space.