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 caprock (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 "caprock". Commonly composed of clay or salt, this caprock 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 caprock.

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:

• Laboratory measurements: small-scale experiments for mineralization, flow and dissolution are conducted on rock samples, giving insight into short-term and small-scale processes.

• Numerical simulations: computing codes have been developed to predict CO₂ behaviour over much longer timescales (Fig. 13). Laboratory experiments are used to calibrate numerical simulations.

• The study of natural CO₂ reservoirs, where the CO₂ (generally of volcanic origin) has been trapped underground for long periods of time, often millions of years. Such a setting is referred to as a "natural analogue". These sites provide us with information on gas behaviour and the very long-term consequences of the presence of CO₂ in the underground.

• Monitoring of existing CO₂ geological storage demonstration projects, such as Sleipner (offshore Norway), Weyburn (Canada), or Snøhvit (Barent Sea, Norway). The results of the simulations in the short term can be compared with real field data and help refine the models.

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 1,000 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 caprock 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.