Carbon Capture and Storage (CCS)
Geological storage of carbon dioxide
Carbon dioxide (CO2) can be stored in geological formations including:
- Oil and gas reservoirs
- Unmineable coal seams, and
- Saline formations
Carbon dioxide can also be used in creation of valuable by-products.
Graphic courtesy of CO2CRC
1. Oil and gas reservoirs:
In some cases, production from an oil or natural gas reservoir can be enhanced by pumping CO2 gas into the reservoir, under pressure, to allow oil and gas that would otherwise not be produced to be recovered. This is known as enhanced oil recovery (EOR) or enhanced gas recovery (EGR).
 There are around 70 EOR projects using CO2 currently underway, mainly in North America. Enhanced oil recovery represents an opportunity to store carbon at low net cost, due to the revenues from recovered oil/gas. It is also possible to inject CO2 into depleted oil and gas fields for storage only.
Pictured - at right and above: Elements of the Weyburn CO2 Capture & Monitoring Project installation.
2. Unmineable coal seams:
Certain coal seams contain large amounts of methane adsorbed onto the surface of the coal. The current practice for recovering coal bed methane is to depressurize the bed, usually by pumping water out of the reservoir. An alternative approach is to inject carbon dioxide gas into the bed. CO2 is roughly twice as efficient at adsorbing on coal as methane, giving it the potential to efficiently displace methane and remain stored in the bed.
CO2 displacement of coal bed methane has been demonstrated in limited field tests, but much more work is necessary to understand and optimize the process. The recovered methane could provide a revenue stream to offset the costs of the carbon sequestration process. Coal bed methane can be used in gas turbines to produce electricity.
3. Saline formations:
Storage of CO2 in saline formations does not produce value-added by-products, but it has other advantages.
First, the estimated carbon storage capacity of saline formations is large, making them a viable long-term solution. Second, many existing large CO2 emission point sources in Australia are assessed to be within reasonable access of a potential saline formation injection point.
Understanding the environmental implications and safety of CO2 storage in saline formations is a key component of current R&D effort. The oil industry routinely injects brines that were associated with recovered oil into saline reservoirs, and the US Environmental Protection Agency (EPA) has permitted some projects that inject liquid wastes into deep saline formations.
The Norwegian oil company, Statoil, is injecting approximately one million tonnes per year of recovered CO2 into the Utsira Sand, a saline formation around 800 meters beneath the sea floor associated with the Sleipner West Heimdel gas reservoir. The amount being stored at Sleipner is equivalent to the output of a 150-megawatt coal-fired power plant.
Will injected carbon dioxide stay underground?
Only certain geologic formations are considered suitable for carbon dioxide storage. In general, CO2 can be stored in a layer of permeable rock that has a layer of impermeable rock above it. Permeable rock has pores through which fluids can flow and gather. Fluids cannot pass through impermeable rock, and so the top layer traps CO2 that is injected into the permeable formation. Such sites are the same type of formation that have trapped crude oil and natural gas over millions of years, providing a natural precedent for long-term CO2 storage.
Does storage of carbon dioxide underground risk contaminating drinking water supplies?
Much research is currently focused on understanding and addressing this risk. Sites would be selected where there are no natural connections between shallow drinking water aquifers and much deeper potential carbon dioxide storage formations. If there were naturally occurring conduits between the two layers of rock, then the brine or hydrocarbons in those formations would have already contaminated the water over millions of years.
The possibility that carbon dioxide could escape where other fluids did not is considered to be very low. Further research is being undertaken to develop technologies to monitor the movement of CO2 once it is injected underground to confirm that it is being safely and securely contained.
Selected References:
CO2 Sequestration site - IEA Greehouse Gas R&D Programme
http://www.co2captureandstorage.info/
National Energy Technology Laboratory - US Dept of Energy:
Carbon sequestration
http://www.netl.doe.gov/technologies/carbon_seq/index.html
Fossil Energy - US Dept of Energy: Carbon sequestration
http://www.fossil.energy.gov/programs/sequestration/index.html
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