Understanding CO2 Storage

Site selection and project development are central to the secure long-term storage of CO2.

Fluids, gases and rock formations make up the earth beneath our feet and CO2 is naturally present in large volumes below the surface. In fact, many different gases and fluids have been safely and naturally stored underground for millions of years.

We more commonly think about CO2 as part of the natural carbon cycle, where CO2 is absorbed by green plants and converted to oxygen through photosynthesis; however, human activity has increased levels of CO2 in our atmosphere, which most everyone agrees is a growing concern and a major contributor to climate change.

Burgeoning economies around the world increase CO2 emissions through industrialized growth, having a plan to deal with CO2 emissions makes sense and CCS has the potential to mitigate those emissions until we find better ways to create and use energy.

Using sites that have naturally secured oil and gas for millions of years is a logical starting point because so much is known about the geological make-up of oil and gas reservoirs through decades of intense study.  

CCS is a process that stores CO2 emissions deep underground, where other gases have been stored for millions of years, but how do we know it will stay underground?


 (The image on the right is drawn to scale (notice how small the large capture facility is on the surface) shows  a depth of injection up to 2 kilometres.  One the right hand side are aquitards, or solid caprocks and impediments to the upward motion of the injected CO2, and on the right are saline formations (sometimes called deep saline aquifers) made up of porous rock that contains high levels of very salty water.  All these layers act as barriers to the vertical movement of injected CO2.)

Site selection and project development are central to making sure CO2 injection is safe and secure. In Canada, the Western Canadian Sedimentary Basin has features that hold promise for the development of CCS technology, because it is large, tectonically stable, and offers many different options for storage in the sub-surface.


Monitoring CO2 is also very important in understanding what happens thousands of metres below the surface at the selected sites and it provides the strongest indications of what that CO2 is doing in the sub-surface. Through pressure tests, 3D seismic arrays, soil and water testing, as well as a host of other monitoring techniques, scientists, researchers and engineers are confident that the geological traps that previously held gases for millions of years are suitable to keep CO2 deep underground into the future.

How long before we start storing CO2?

Injection of CO2 for permanent geological storage is already underway in Canada; storage will occur once injection has stopped. Currently, the IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project, the world's first and largest CO2 monitoring and storage project, has injected over 17 million tonnes into the Weyburn-Midale oilfields in and around Weyburn Saskatchewan. By the end of operation this project is expected to hold roughly 40 million tonnes of CO2 that would have otherwise been vented into the atmosphere. That's over a tonne of CO2 for every Canadian.

Cenovus’s Weyburn field and Apache’s Midale field, located in southeast Saskatchewan, Canada, host this world-leading project studying CO2 geological storage.


At the Weyburn-Midale Storage Project (WMP):

  • 99% of CO2 injected into a geological formation is “very likely” to be retained for over 100 years Formations occur in basins filled with sedimentary rocks that can accumulate up to thousands of meters of sediment.
  • The pore spaces in these sedimentary rocks are filled with salt water or, in some cases, oil and gas. The rocks in sedimentary basins consist of alternating layers of sand, silt, clay, carbonate, and evaporites that were long ago deposited in oceans, deltas, lakes, and rivers.
  • The sand layers provide storage space. The silt, clay, and evaporite layers provide the seals above the storage reservoir that can trap buoyant fluids such as oil, natural gas, and CO2 for millions of years.
  • Because CO2 has a lower density than water, the presence of an overlying, thick, and continuous layer of silt, clay, or evaporiteis the single-most important feature of a geologic formation that is suitable for geological storage of CO2. These fine-textured rocks physically prevent the upward migration of CO2 by a combination of viscous and capillary forces.
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