Injection of CO2 is a technology that has been in use since the 1960's, therefore, scientists have decades’ worth of data to guide present day storage projects.
The subsurface naturally stores a myriad of naturally occurring gases, including naturally occurring CO2, so the concept is to use the natural barriers that have held other gases for millions of years. Through decades of injecting and storing other gases in the subsurface, like natural gas, researchers are very confident that CO2 storage is safe.
CO2 can be stored in:
A good storage site is typically more than 800 metres below the surface so that CO2 can be injected and stored as a liquid. CO2 in a liquid state is more predictable and easier to contain in a liquid state and at depths of over 800 metres geological pressures ensure it stays there as a liquid.
Porous rock formations or deep saline formations offer the best storage mediums for CO2. Contrary to most people's perception of storing CO2 in the subsurface, it does not occupy some giant cave underground, but rather the microscopic pores in porous rock. In a porous rock formation, such as sandstone, the chemical interaction between liquid CO2 and the sandstone over a period of a thousand years or more, will see that CO2 transformed to calcium carbonate - a solid. In areas where moisture is present, like in deep saline formations, the CO2 will dissipate into the moisture over long periods of time.
No matter what storage medium is utilized, a good injection site requires one or more cap-rock formations above the injection zone to ensure the CO2 does not migrate back up to the surface, into ground water, or our land. These cap-rock formations typically have kept naturally occurring gases and liquids underground for millions of years.
Whether seismic events like earthquakes are common in your part of the world or seldom thought of occurrences, when it comes to discussing CO2 injection for the purpose of storage, people always ask about earthquakes. The fact is that storage sites are selected because they are least likely to encounter an earthquake to begin with. Project developers pick stable places to store CO2 because safety is a top priority in any modern engineering project. However, if an earthquake did occur it is highly improbable that a leak would occur. Our best teachings on this come from California and Nagaoka, Japan.
California has many gas and oil deposits near seismically active faults and earthquakes, and over decades of study and innumerable earthquakes, none have caused a leak. As one of the more seismically active places on earth, with a sizable endowment of oil and gas, California shows us that earthquakes don’t cause leaks.
Another illustrative example is from Nagaoka, Japan. A CO2 injection site 1,100 metres below the surface was hit by an earthquake measuring 6.8 on the Richter scale; its epicentre was a mere 20 kilometres away from the injection site. The injected CO2 has been monitored by scientists before, during and after the earthquake and no leaks have been detected to date. Some theorize that this is because an earthquake’s energy is directed at the Earth’s surface as a path of least resistance and CO2 storage occurs below those depths.
Primary Characteristics of Geological Media Suitable for CO2 Storage:
If a storage site does not have the ability to contain the CO2 that site is disqualified!
There are many locations worldwide where naturally occuring CO2 is stored. Some examples are:
Natural CO2 fields give confidence that under the right conditions, CO2 can be stored underground for millions of years, i.e. until well after any greenhouse crisis has passed.