CO2 Capture and Storage
5. How can CO2 be stored underground?
- 5.1 What are the possibilities of geological storage?
- 5.2 How expensive is geological storage?
5.1 What are the possibilities of geological storage?
Geological formations suitable for the storage
are oil and gas reservoirs,
deep saline formations, and
un-minable coal seams. Storage sites must generally be located
at a depth of 800m or deeper, where prevailing pressures keep
CO2 in either a liquid or a
supercritical state. Under
such conditions, CO2 is less dense than water and it
must be trapped from above to prevent it from moving back up to
the surface. It can for instance be physically trapped under a
well-sealed cap rock and in pore spaces within the rock, or
chemically by dissolving in water and reacting with rock
minerals to form carbonate minerals.
can be injected into porous rock formations below the earth’s
surface using many of the same methods already used by the oil
and gas industry: well-drilling technology, injection
technology, computer simulation of storage reservoir dynamics
and monitoring methods.
Industrial scale storage projects are underway in the North
Sea, Canada, Algeria and in Texas, each storing every year more
than a million tonnes of CO2 that would otherwise be
released into the
Potential geological storage sites exist around the globe both
onshore and offshore. Estimates of the
total storage space available vary widely, but
they generally indicate that space exists for tens to hundreds
of years of CO2 emissions at current levels.
Furthermore, a large portion of existing power plants and other
industrial sources lie within 300 km of areas with storage
While the available storage capacity in geological reservoirs
is “likely” to be sufficient for contributing significantly to
emission reductions, the true amount is yet uncertain.
Leakage of CO2 from storage
entails global risks for the climate, but also potential local
risks for humans,
groundwater in the case of
sudden and rapid CO2 releases. These risks are
expected to be quite small: most of the CO2 should
remain underground for centuries, and leakages should be
controlled well before causing local damage. Monitoring of
storage sites may however be required for very long periods and
methods are expected to evolve as technology improves. A legal
framework with a long-term perspective is also needed as storage
times extend over many generations. The general public appears
not to know much about this technological option, and is
“reluctant” to accept it, sometimes because of the perception
that CO2 storage is required because of a failure to
reduce CO2 emissions in other ways.
5.2 How expensive is geological storage?
Storage in geological formations is the cheapest and most
environmentally acceptable storage option for
The cost of storage in
saline formations and
depleted oil and gas fields would typically be between 0.5–8
US$/tCO2 injected, with additional monitoring costs
of 0.1–0.3 US$/tCO2 The lowest storage costs are for
onshore, shallow, high permeability reservoirs, and/or storage
sites where wells and infrastructure from existing oil and gas
fields may be re-used. The geological storage of CO2
could even yield net benefits, for instance in the case of
enhanced oil or gas recovery where CO2 could be
injected underground to displace and recover the fuel.