Safely Storing CO2

CO2 Storage is Safe

Injection of CO2 is a technology that has been in use since the 1960's, therefore, scientists have decades’ wShown is a magnification of how the CO2 fills in the pore space.  The rock sample on the right (about 3orth 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.

 

"With proper geologic selection and project management,
99% of injected CO2 will “likely” be retained for 1000 years"
(IPCC Special Report on CCS, 2005)

How can CO2 be Stored?

CO2 can be stored in:

  • deep geological formations
  • unmineable coal seams
  • deep saline aquifers
  • depleted oil and gas wells
  • ocean storage (not currently planned for Canada)

What are the elements of a good storage site?

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.

Trapping the CO2 Underground

There are 4 main trapping mechanisms used to keep the CO2 underground:

  • co2-in-rock-pores_sm.jpgStructural or Cap Rock Trapping - these are also called Volumetric traps and are the most commonly used to date.  The cap rock is a thick dense layer of rock, approximately 170 metres thick. The supercritical (liquid) CO2 is trapped in pore spaces and is prevented from seeping to the surface by physical or hydrodynamic barriers (in oil and gas reservoirs and deep saline aquifers). Volumetric traps also include man-made cavities such as salt caverns and mine shafts.
     
  • Residual traps – in places where CO2 has migrated through a formation, a portion of the gas is retained in the pore space as a result of capillary forces. Thus, a portion of the gas is trapped by forces other than a simple physical cap rock.
  • Solution traps – where CO2 is either in solution in the formation fluids, or forms ionic bonds with the fluids, such as in the water or oil that saturates the pore space within a rock formation. 
  • Adsorption – options where the CO2 bonds with formation rocks that contain organic material, such as coal or shale. Mineral traps – sites where the CO2 precipitates out as a carbonate mineral. Such reactions can occur when CO2-charged formation fluids react with other formation minerals.

CCS and Earthquakes

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.

QUICK FACTS:


Primary Characteristics of Geological Media Suitable for CO2 Storage: 

  • Capacity: to store the intended CO2 volume 
  • Injectivity: to receive the CO2 at the supply rate 
  • Containment: to prevent, avoid or minimize CO2 leakage

If a storage site does not have the ability to contain the CO2 that site is disqualified!


NATURAL STORAGE OF CO2

There are many locations worldwide where naturally occuring CO2 is stored.  Some examples are:

  • McElmo Done, Colorado
  • Pisgah Anticline, Mississippi
    • contains about 200 million tonnes of almost pure CO2
    • estimated to have been generated in Late Cretaceous times, more than 65 million years ago
  • Eger Basin, Czech Republik

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.

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