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CO2 in enhanced oil recovery
Contents:
1. CO2 in enhanced oil recovery
2. CO2 has been used in EOR in the USA for decades
3. Weyburn & Great Plains Synfuels plant
CO2 in enhanced oil recovery
In the oil industry, enhanced oil recovery (EOR) is used as a generic term for methods to increase the recovery of oil from a reservoir. The oil recovery process is usually broken down into three separate stages.
In the primary recovery stage, natural pressure combined with different lifting techniques (such as pumps) forces the oil to the surface. Between 5 and 40 per cent of the total amount of oil in the reservoir is recovered in this way, the typical fraction being about 10 per cent.
In the secondary stage, water or natural gas is injected into the reservoir to recover another 10-20 per cent of the oil.
In the tertiary recovery stage, CO2 is injected to further enhance recovery. The amount of extra oil that can be recovered will vary depending on local conditions, but Freund and Kaarstad (2007) have estimated that another 10 per cent of the total amount of oil in the reservoir may be extracted using CO2 injection. Other methods than CO2 injection are available in the tertiary phase. Injecting thermal (hot) liquids may reduce the viscosity of the remaining oil, making it flow more easily; polymers can be injected to improve water flow and increase the effectiveness of water injection; other gases than CO2, such as nitrogen, may also be used.
CO2 has been used in enhanced oil recovery in the USA for decades
In USA 30 million tons of CO2 are injected into 82 different oil fields every year. 3100 km of pipelines have been built to transport CO2 from production sites to the fields, with a total transport capacity of 50 million tons CO2 per year. Initially, CO2 separated in industrial processes was used, but this has gradually been replaced by cheaper gas from natural CO2 reservoirs in geological formations. Only 10 per cent of the CO2 used in enhanced oil recovery today comes from industrial sources.
In these projects there has been no environmental motivation for injecting CO2 into the oil reservoirs. Therefore little is known about the storage safety of the reservoirs in Northern America, with the exception of the Weyburn project in Canada. In western Canada CO2 is injected in 44 larger and smaller wells (IPCC 2005b).
Weyburn & Great Plains Synfuels Plant
The Weyburn oil field is situated in Canada, right at the US border. In October 2000 EnCana started injection of CO2 into the field to improve oil recovery. The CO2 is transported through a 325 km pipeline from Dakota Gasification Company’s coal gasification plant in North Dakota. 20 million tons of CO2 will be injected, extending the expected life span of the field by 25 years.
The geological conditions around Weyburn are well suited for long term CO2 storage. The Petroleum Technology Research Center (PTRC) has conducted a four year cross-field study of the storage conditions, in cooperation with field operator EnCana. The study was ordered by the research and development program of the International Energy Agency Greenhouse Gas (IEA GHG). Seismic surveys have charted the distribution of CO2 in the geological formations, and a model to calculate storage capacity has been developed. A risk evaluation concluded that most of the injected CO2 will remain in the reservoir for more than 5000 years.
Great Plains Synfuels Plant (GPSP) produces syngas from lignite, which is manufactured into many different products, such as natural gas, methanol and fertilizer. This is a commercial project where the company separates and sells CO2. The CO2 is separated in the industrial process, so no separate capture equipment is needed. The plant has been running since 1984, using 6 million tons of lignite a year. It is the first gasification plant in the world to sell CO2 for enhanced oil recovery.
Emission balance
For the use of CO2 in EOR to be regarded as a global warming mitigation solution, three considerations must be made.
1) How much CO2 is captured and permanently stored, and how much more CO2 will be emitted from the consumption of the additional oil recovered?
Firstly, this will depend on how good the oil recovery responds to CO2 injection. On the Norwegian continental shelf EOR with CO2 injection can recover an additional 3-8 per cent of the total amount of oil in reservoirs.
Secondly, as CO2 injections are used only in the very late stages of oil recovery, the established infrastructure for further CO2 storage may be operative for many decades after the oil production has been shut down. The extra revenue from enhanced oil recovery can finance pipelines, installations and power plants with CO2 capture, which will enable CO2 injections without enhanced oil recovery for a much longer time than the extended production time.
Thirdly, the initial CO2 capture will be accommodated for the optimal injection rate at the beginning of the EOR process, but the demand for CO2 will gradually decrease over time, as CO2 that comes back up with the oil is recycled at the installation and reinjected. The capture will remain the same, however, and the extra CO2 is then injected for permanent storage purposes only.
Consept studies made by the Norwegian research organization SINTEF conclude that more CO2 will be stored than the CO2 emitted from burning the additional oil.
2) Alternative production without CO2 capture
An important reason why CO2 capture hasn’t been used in EOR on a large scale before is that cheaper CO2 from other sources has been used. In many cases it has been more profitable to invest in new projects rather than to engage in costly EOR projects. If the oil industry does not invest in CO2 capture and infrastructure, the money is spent on other oil project without any CO2 capture and storage.
3) Developing capture capacity and infrastructure
Carbon capture and storage is still a young technology, and establishing the necessary capture capacity and storage infrastructure will be very capital-demanding. Using CO2 in EOR contributes to the development of the first capture plants, CO2 pipelines and injection installations. Considering the vast amounts of CO2 that will need to be captured, most CO2 storage will have to be for climate change mitigation purposes only, but in the early stages of CCS development EOR projects may prove invaluable to get the technology up and running on its own.
Also, one shouldn’t forget that oil use can also be clean, for instance when applied in production of hydrogen with CO2 capture. On the long term, oil production and CO2 storage may form a closed circuit, where the energy from the world’s petroleum reserves can be utilized without emissions.
References
IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage
