Carbon capture and storage is a processes for reducing GHG emissions into the atmosphere by first extracting CO2 from gas streams typically emitted during electricity production, fuel processing and other industrial process. Once captured and compressed, the CO2 is transported by pipeline or tanker to a storage site. This often involves injection into an underground storage site (or geological formation), where it will be safely stored for the long-term.
CO2 is a natural substance in the air that is essential to life. As part of the natural carbon cycle, people and animals inhale oxygen from the air and exhale CO2. Meanwhile, green plants absorb CO2 for photosynthesis and emit oxygen back into the atmosphere. CO2 is also widely used for many industrial purposes such as carbonating drinks and filling fire extinguishers.
As a greenhouse gas, CO2’s presence in the atmosphere traps heat from the sun. Normally, this keeps the climate warm enough for life to continue. However, the burning of fossil fuels is increasing CO2 levels in the atmosphere above naturally-occurring levels, contributing to global climate change.
The careful selection and characterization of storage sites ahead of CO2 injection assures that it will be safely stored. Many of the deep geological locations chosen as potential storage sites have already had liquids (such as oil or saline water) or gases (natural gas and CO2) trapped within them for literally tens of millions of years. These are most often the targeted sites for injection of CO2, ones that have already had a long geological history of trapping.
Safe storage is assured by characterization of the site prior to injection. This often involves seismic imaging, examination of the geological layers of the location, and understanding through computer modeling the movement of fluids or compounds laterally and horizontally in the formation (all of which have kept liquids and gases in place in these formations in the past). Caprocks must be in evidence in the formation to assure that the CO2 does not move vertically but, rather, spreads horizontally.
It is important to note that the CO2 will be injected in a condensed, liquid state in most cases. When it is injected into deep saline rock formations or depleted oil and gas fields, the CO2 will not be entering into a large, underground cave or vacant space. Oil/gas fields and deep saline formations are made up of solid, but porous rock (such as sandstone) that contains microscopic pores where water and oil reside. The CO2 is miscible (it can mix and absorb with) water and oil, and so a portion of the CO2 becomes part of the liquids in the formation and is no longer in a form that can leak as a gas.
Safe storage in the long-term will require on-going measurement and monitoring – easily done through seismic arrays and downhole equipment – to assure the continued safety of the seals in place. Often the most important safety consideration is the wellbores themselves (particularly in oil fields where operators may be dealing with several hundred if not several thousand wells). But the enclosure of these wells is already dictated by rigorous regulations in place governing the oil and gas industry. Studies are underway to assure that sealing compounds in CO2 well closures are adequate to deal with the small corrosive elements of CO2 in water.
A safe storage site is one where the CO2 will be injected into a layer deep underground with porous rocks where CO2 can be stored in the pores. There must be a solid layer (cap rock) on top of the porous layer to prevent the CO2 from moving upwards.
These mechanisms are the same ones that have stored oil and natural gas below the ground for millions of years. The fact that oil and natural gas have been trapped underground for millions of years is a very good indication that injected CO2 will remain safely stored.
There can never be a guarantee that CO2 will not leak, but it is possible to store CO2 at locations where the risk of leakage is very low, and in formations where the CO2 itself converts on a broad scale into a form that is no longer susceptible to gaseous release. In the unlikely case of leakage we will have the mechanisms in place to detect the leak and initiate remediation actions. These same mechanisms are already in use to assure that sour gas, natural gas, and other compounds do not leak from the subsurface.
It is thought that injected CO2, after hundreds or thousands of years, will start to react with other minerals and form limestone (a solid rock), which is the safest form of CO2 storage. This means that as time goes by the CO2 storage will likely become even safer.
If CO2 should leak there are ways to stop the leak. The highest risk for leakage is through cracks in injection wells or, in the case where injection has occurred in a depleted oil field, through enclosed oil wells. If this happens the well must be closed down and sealed with cement, or re-sealed if the wells are already enclosed. Leakage in such situations is unlikely to be at levels that would be toxic. Detection devices would catch such leaks at a very early stage, just as they do with the release of natural gas and other more potentially harmful gases.
Leakages can also be stopped if the injection of CO2 is stopped and the pressure in the underground formation is reduced. Again, remember that CO2 in formations often changes state, or mixes with water, resulting in smaller likelihoods that gases would escape in large quantities.
Site selection of CCS projects is a rigorous process which aims to remove the possibility of large earthquakes impacting storage operations to begin with. Yet even if an earthquake should occur at a CO2 storage site, research projects have shown that the CO2 will not leak. There are many underground geological formations near earthquake zones that have experienced countless, large earthquakes over many thousands of years and yet have not released oil or gas in any capacity. For more information on eathquakes and CCS click here.
The Carbon Capture & Storage Association in London, England also had this earthquake FAQ listed on their website and you can click here to read their response as well.
The U.S. Department of Energy (DOE) has made an assessment of the potential sequestration capacity across the United States and parts of Canada and has determined that there exists sufficient volume to store approximately 600 years of CO2 produced from all U.S. fossil fuel set-point emitters such as power plants and refineries (at current rates).
Renewable energy, energy efficiency, and other tools will all be needed to find solutions for climate change. CCS can play an important role in mitigating CO2 because it is the most immediate solution existing for large point emitters of CO2.
With global emissions estimates ranging between 40 to 60 per cent from these set point sources, CCS represents a practical approach to reducing emissions from fossil sources in the near term.
As our social and economic success is dependent on access to energy, CCS has the potential to mitigate existing CO2 production that may be necessary to maintain our shared quality of life. CCS has vast potential to buy us more time in bringing other energy solutions on line.
CCS has been in use by the oil and gas industries as a means to enhance the production of oil and gas for over 40 years, and more recently has been injected into deep saline formations. Although oil and gas companies are the current drivers behind CCS technology, it is the scientific study of CO2 in the subsurface that is important to researchers. The learnings from these projects are already being transitioned to proposed CO2 storage projects without an Enhanced Oil Recovery (EOR) component.
The Sleipner project in Norway (injecting CO2 into a deep saline formation since 1996), and Weyburn-Midale in Saskatchewan, Canada (injecting CO2 into a depleted oil field since 2000) have set an excellent standard for sequestering CO2 with no incidents.