When injected in a reservoir, the CO2 fills the rock’s pore spaces, which in most cases are already filled with brine i.e. salty water. As the CO2 is injected, the following mechanisms begin to come into play. The first is considered the most important and prevents the CO2 from rising to the surface. The other three tend to increase the efficiency and security of storage with time.
1. Accumulation below the cap rock (Structural trapping)
As dense CO2 is ‘lighter’ than water, it begins to rise upwards. This movement is stopped when the CO2 encounters a rock layer that is impermeable, the so-called ‘cap rock’. Commonly composed of clay or salt, this cap rock acts as a trap, preventing the CO2 from rising any farther, and leading to its accumulation directly beneath. Fig.10 illustrates the upward movement of the CO2 through the pore spaces of the rock (in blue) until it reaches the cap rock.
2. Immobilization in small pores (Residual trapping)
Residual immobilization occurs when the pore spaces in the reservoir rock are so narrow that the CO2 can no longer move upwards, despite the difference in density with the surrounding water. This process occurs mainly during the migration of CO2 and can typically immobilize a few percent of the injected CO2, depending on the properties of the reservoir rock.
3. Dissolution (Dissolution trapping)
A small proportion of the injected CO2 is dissolved, or brought into solution, by the brine already present in the reservoir pore spaces. A consequence of dissolution is that the water with dissolved CO2 is heavier than the water without, and it tends to move downwards to the bottom of the reservoir. The dissolution rate depends on the contact between the CO2 and the brine. The amount of CO2 that can dissolve is limited by a maximum concentration. However, due to the movement of injected CO2 upwards and the water with dissolved CO2 downwards, there is a continuous renewal of the contact between brine and CO2, thus increasing the quantity that can be dissolved. These processes are relatively slow because they take place within narrow pore spaces. Rough estimates at the Sleipner project indicate that about 15% of the injected CO2 is dissolved after 10 years of injection. After 20 years monitoring Sleipner, within measurement uncertainty this gives a dissolution rate between 0 % and 2.7 % per year. Flow simulations of the plume development at the Sleipner project suggest that dissolution values up to around 10% are quite likely .
4. Mineralization (Mineral trapping)
The CO2, especially in combination with the brine in the reservoir, can react with the minerals that form the rock. Certain minerals can dissolve, whereas others can precipitate, depending on the pH and the minerals constituting the reservoir rock (Fig. 11). Estimations at Sleipner indicate that only a relatively small fraction of the CO2 will be immobilized through mineralization after a very long period of time. After 10,000 years, only 5% of the injected CO2 should be mineralized while 95% would be dissolved, with no CO2 remaining as a separate dense phase.
The relative importance of these trapping mechanisms is site specific, i.e. it depends on the characteristics of each individual storage location. For instance, in dome-shaped reservoirs, CO2 should remain mostly in a dense phase even over very long timescales, while in flat reservoirs such as Sleipner, most of the injected CO2 will be dissolved or mineralized. The evolution of the proportion of CO2 in the different trapping mechanisms for the Sleipner case is illustrated in Figure 12.
The knowledge of these processes comes from four main sources of information:
Only by constantly cross-referencing and cross checking these four sources of information is it possible to acquire reliable knowledge on all the processes occurring some 1000 m below our feet.
We know that the safety of a CO2 storage site tends to increase with time. The most critical point is to find a reservoir with a suitable cap rock above it that can withhold the CO2 (structural trapping). The processes related to dissolution, mineralization and residual trapping all work in favour of preventing CO2 from migrating to the surface.