Seabed Aquifer Sequestration
Seabed aquifers are saline aquifers located under the ocean floor and are composed of basalt or sedimentary formations, which are buried under low-permeability sedimentary rocks that serve as cap-rocks. When CO2 is injected at depths greater than 2700 m below the ocean's surface, it is pressurized to the point that it becomes denser than seawater, so it is very unlikely to escape or even reach the level of the cap-rock (Goldberg, 2009). Some CO2 also combines with water to form CO2 hydrate, an ice-like substance which is also denser than seawater, eliminating the danger of escaped CO2 surfacing (Goldberg, 2009).
- CO2 reacts with basalt in situ to form carbonates, thereby permanently sequestering the carbon in mineral form (Goldberg, 2009).
- Seabed aquifers have a greater potential capacity than those on land (Bradshaw et al, 2007).
- Many high-emission power plants are located along a coast. If there are no terrestrial carbon sinks in the area, storing CO2 in seabed aquifers can be more cost effective than building an extensive pipeline infrastructure for transportation to terrestrial aquifers (NYSERDA, n.d.)
- In general, it is more expensive to sequester CO2 under the seabed than into terrestrial saline aquifers (Hassanzadeh et al, 2008).
- The injection of CO2 may lower the pH of seawater in the aquifer and dissolve calcium carbonate, thus releasing toxic metals such as iron and manganese (Folger, 2009).
- CO2 could also lower the water's pH enough to dissolve the cement used to plug the injection wells. However, tests in the Frio Formation along the US Gulf Coast did not detect any leakage six months after injection, suggesting that the plug stayed intact (Folger, 2009).
- More research and site characterization must be done, however, to determine whether all these sites have the necessary porosity and structural integrity to safely store CO2 (Bradshaw et al, 2007; Goldberg, 2009).
Potential Storage Sites:
- Total seismic sites (basalt formed from seafloor spreading): 2,313 - 11,564 Gt CO2
- Total aseismic sites (basalt formed from undersea volcanic activity): 5,925 - 29,627 Gt CO2
- Worldwide capacity: 8,238 - 41,191 Gt CO2(Goldberg, 2009).
For comparison, 28.9 Gt CO2 represents the global total for emissions in 2007 (IEA, 2009).
(in reference to the Sleipner project)
- Total compression and injection of CO2: $80 million (Gale, 2006).
- CO2 treatment module: $523.88 million (IEA Greenhouse Gas R&D Programme, n.d.)
(In reference to the Snøhvit project)
- Development: $5.2 billion (includes the price of ships and transportation)
- CO2 pipeline and injection well: $110 million (IEA Greenhouse Gas R&D Programme, n.d.)
- Land-based facilities: $1.65 billion (IEA Greenhouse Gas R&D Programme, n.d.).
Seabed sequestration is a technology that is available now.
StatoilHydro, a Norweigan energy company, has implemented commercial-scale projects that are already storing CO2 in saline aquifers undersea.
Sleipner was the world's first full-scale carbon capture and sequestration plant. Carbon sequestration began in the Sleipner aquifer in the North Sea in 1996. Since then, 1 million tons of CO2 have been injected each year (Gale, 2006). By monitoring the wells over more than a decade, scientists have been able to get some measure of the effectiveness of the trapping mechanisms present in seabed saline aquifers. So far, no leaks have been observed, and there has been no seismic activity caused by the drilling and injection process (Bellona, 2007).
The Snøhvit petroleum production plant in the Barents Sea began sequestering carbon in April 2008. Its annual injection rate is about 700,000 tonnes per year. As in the Sleipner project, the injection process is being monitored constantly, and no leakages have been detected (Statoil, 2008).