Once CO2 is captured and transported, it is injected into a geological reservoir for long-term storage The geological reservoir may be a deep saline aquifer, a depleted oil or gas field (or one that is undergoing depletion), or even unmineable coal seams.
During injection, CO2 is delivered to the geological reservoir at depths of 800 to 5,000 metres where the pressure is greater than 74 bars (about 74 times atmospheric pressure). The CO2 is thus in a “supercritical” state; that is, it is as dense as a liquid although it continues to behave as a gas. Starting at 800 metres, one tonne of CO2 occupies a volume of 1 to 2 cubic metres, which means that enormous quantities of CO2 can be stored in a relatively restricted space. For more information on CO2, click here.
It is important to realize that stable natural geological reservoirs of CO2 have existed for millions of years. In matters of geological CO2 storage, it is important to determine whether reservoirs devoid of CO2 display the same characteristics as CO2-containing natural reservoirs, and if they are suitable for carbon dioxide injection.
CO2 geological storage represents a new method for reducing greenhouse gas (GHG) emissions. But injecting CO2 into oil or gas fields is not in itself a new technique. In fact, the enhanced oil recovery (EOR) process uses CO2 injections to increase the yield from hydrocarbon fields and has been widely practiced since the 1970s.
Geological reservoirs consist of porous rocks (sandstone and some carbonate rocks) containing pores that can be filled with CO2. A thick and impermeable caprock (fine-grained or rock salt) located above the reservoir stops any CO2 from escaping to the surface. This caprock is essential for maintaining a sealed reservoir and for keeping the injected CO2 at a suitable depth.
|Geological storage of CO2 in depleted oil and natural gas fields or those undergoing depletion|
|Geological storage in oil and gas fields is based on the principle of enhanced oil recovery. Injecting CO2 into an oil field that is being depleted will increase the pressure in the reservoir and facilitate oil or gas extraction. The CO2 then fills the empty pores that once contained hydrocarbons. Although this technique is no longer considered innovative, using it as an environmental measure is a relatively new practice (OECD/IEA, 2008).|
|Geological storage of CO2 in deep saline aquifers|
|Deep saline aquifers are rock formations that are porous, permeable and saturated with salt water (much saltier than seawater) that is not fit for consumption. The CO2 injected into these formations gradually migrates laterally and vertically, but movement of the CO2 plume is restricted by impermeable layers surrounding the aquifer.|
|Storage in unmineable coal seams|
|Geologically storing CO2 in coal seams that are too deep or not economically mineable is based on the recovery of methane from coal seams (ECBM: Enhanced Coal Bed Methane). Injected CO2 displaces methane gas present in the pores of the coal seam, allowing the methane to be recovered and leaving the CO2 trapped in the coal’s pores.|
The table below provides a summary of the different types of potential reservoirs for geologically storing CO2.
What happens to the CO2 after it is injected?
Once injected, CO2 fills the pores of the reservoir rock. Over time, different mechanisms come into play to trap the CO2 over the long term.
Because CO2 is less dense than saltwater, it tends to accumulate above the water layer, under the caprocks overlying the reservoir. About 10-20% of the total volume of CO2 becomes trapped in rock pores that are too small to allow the gas to circulate. This portion of the gas is considered to be immobilized in the reservoir rock.
Over time, some of the CO2 dissolves in the salty water that was originally present in the reservoir. This water, with its dissolved CO2, is heavier and sinks toward the bottom of the reservoir where chemical reactions with the rock gradually binds CO2 in the form of minerals. As time passes, the amount of supercritical CO2 in the reservoir drops, effectively increasing the proportion of dissolved CO2 and mineralized CO2.
How can we make sure the CO2 is staying in the reservoir? Check out the page on safety and monitoring.
CO2 storage in Québec
In Québec, the geological storage of CO2 can only occur in two types of geological reservoirs:
- depleted oil and gas fields (or those being depleted via enhanced recovery);
- deep saline aquifers.
In Québec, these types of potential geological reservoirs are found in the St. Lawrence Lowlands as well as in the Anticosti and Magdalen basins. The St. Lawrence Lowlands has good potential for geologically storing CO2. It contains reservoir rocks (saline aquifers and depleted gas fields) at depths of more than 1,000 metres, overlain by extensive caprocks. This impermeable cover layer, which generally consists of Utica Shale or rocks of the Lorraine Formation, is present in a semi-continuous manner throughout the St. Lawrence Platform.
To play a video about CO2 geological storage, please visit the European ZEP project website.