Ensuring the safe geological storage of CO2
Proper reservoir selection is paramount to ensuring the safe geological storage of CO2. Only reservoirs well suited to storing CO2 for long periods of time should be considered. To determine their suitability, the geological structures at the potential reservoir site must first be accurately and thoroughly characterized. Numerical modelling is then used to simulate long-term CO2 behaviour and assess the site’s safety in terms of storage.
An in-depth understanding of all proposed storage sites will reveal potential CO2 migration paths in advance, thereby allowing the safest site to be identified in terms of gas behaviour. This process also allows us to evaluate and prevent any potential effects on people and ecosystems (CO2GEONET, 2008).
By studying naturally existing gas reservoirs, we already have a good understanding of the conditions under which gases remain stored underground. These gases have been trapped naturally in a stable state for millions of years. Major gas leaks are rare in natural reservoirs and are generally caused by significant seismic or volcanic activity, like earthquakes or volcanic eruptions. This underscores the importance of choosing a storage site within a geologically stable area.
The combination of a number of specific conditions are needed before leakage can occur. Consequently, it is highly unlikely taht a well-chosen and carfully engineered CO2 geological storagesite will leak. (translated from CO2GEONET, 2008).
How will sites be monitored?
A number of monitoring methods can be used before, during and after the injection of CO2 to make sure a site remains safe and to verify the effective storage of CO2 within the reservoir. Monitoring is therefore continuous throughout the storage process.
A site’s initial state must be characterized before injection begins by taking several types of measurements. One important step is to measure the natural concentrations of CO2 in the soil, air, groundwater and surface water. This allows the concentrations in the area during and after the injection to be compared to the initial starting conditions, thus identifying any changes caused by operating the storage site (IPCC, 2005).
Monitoring is at its highest level during and immediately following an injection when the risk is greatest. Several complementary techniques used to monitor the injection site include:
- tracers added to the flow of injected CO2, and monitoring of these tracers within the reservoir and at the surface;
- pressure and temperature measurements in wells;
- a recording network to continuously monitor seismic activity;
- seismic reflection;
- borehole geophysical methods.
Some of the above techniques will operate on a continuous basis, whereas others represent punctual measurements that provide snapshots at specific moments in time. The geophysical methods can help determine, with various degrees of resolution, the path of the CO2 plume through the reservoir. The same techniques are used in the oil and gas industry, so they represent well established methods.
The information obtained during the CO2 injection phase provides a means of evaluating how well the numerical reservoir model predicts real CO2 behaviour. In cases where the predictions are inconsistent with reality, the model must be refined to produce a more accurate estimate of the gas plume’s diffusion within the geological reservoir.
As the Research Chair work progresses, more information will be available about the monitoring aspect.
Do CO2 leaks pose a risk?
The risk of a massive leak is virtually nil because geologically stored CO2 must break through several barriers to reach the atmosphere. Several mechanisms come into play to stop CO2 from breaching the surface:
- stratigraphic and structural barriers: CO2 movement is blocked by impermeable caprocks, for example;
- residual saturation: some of the injected CO2 is immobilized in the water-saturated pores of rocks;
- geochemical barriers: some of the CO2 is dissolved in water and/or transformed into new minerals.
Consult the storage page for more details about CO2 trapping mechanisms.
These mechanisms occur at different time scales (immediately after the injection starts in the case of dissolution, and up to several thousand years in the case of mineralization) and they may compete among themselves. Nevertheless, as time goes on, the CO2 becomes increasingly trapped within the reservoir.
A very small risk of diffuse leaks is always present, but the more time that passes, the lower the risk. A leak is caused by a combination of specific factors, such as the location of a reservoir in a geothermal, volcanic or seismic area, the presence of highly fractured zones, the presence of active fault lines, etc. However, an initial site assessment would identify these types of features, and any such site eliminated from the selection process. For these reasons, it is highly unlikely that leaking would occur if proper site selection and design is followed.
Effects of CO2
Even though the risk of a leak from a geological storage site is negligible, it is nonetheless important to know the potential effects of such a leak. CO2 is naturally present in air but becomes dangerous to humans and other living creatures when concentrations exceed 5%. When the CO2 concentration in ambient air approaches 5%, it causes headaches, dizziness and nausea. Above this threshold, the gas can cause death by asphyxiation when oxygen concentrations in the air fall below 16%. However, if CO2 were to escape into an open or flat area, it would disperse rapidly in the air, even on a windless day (CO2GEONET, 2008).
The environmental impact of a CO2 leak depends on whether the site is onshore or offshore. Only onshore storage sites are currently being studied in Québec. In these cases, the effects of a CO2 leak on vegetation would be focused around the gas leak. CO2 can have a lethal effect on plants when its concentration in air exceeds 30% (CO2GEONET, 2008).
The potential impact of a CO2 leak on drinkable groundwater should not to be overlooked, even if the effects would only be local. CO2 can change the chemical composition of water, making it more acidic.
What happened at Lake Nyos?
On August 21, 1986, Lake Nyos in Cameroon “exploded” in a limnic eruption. The eruption released about 1 cubic kilometre of CO2, or about 1 billion cubic metres, killing 1,800 people and 3,000 animals.
Lake Nyos sits inside a volcanic crater. Carbon dioxide, escaping from underground volcanic chimneys, is continuously dissolves and concentrates at the bottom of the lake. In 1986, an earthquake triggered a landslide, which disrupted the stratified layers of water and “overturned” the lake. The CO2-rich water from the bottom rose to the surface and the gas burst out. Heavier than air, it flowed down the sides of the volcano and into adjacent valleys where it asphyxiated all living beings in its path.
Other lakes of this type exist around the world. In most of these cases, preventative degassing of the deeper waters is now underway.
It is important to understand that this scenario would never happen at a geological storage site where CO2 gas is stored at least 800 metres underground and is therefore isolated from the atmosphere.