The challenge

As part of the pre-injection assessment of a geological storage, it is essential to estimate the mass of CO₂ to be immobilized under different trapping mechanisms, which are:

1. Structural trapping of free-phase CO₂ in geological traps.
2. Capillary trapping of small bubbles of CO₂ gas that are retained in the porous space by capillary forces.
3. Trapping by CO₂ dissolution into the resident brine.

Mineral trapping by precipitation of carbonated minerals into the porous space.

A Multiphysics approach

This is a multiphysics and multiscale problem. For that reason, we opted for Comsol Multiphysics (www.comsol.com) to build up a multiphase transport model that considers miscibility between CO₂ and the native brine. Structural, capillarity and dissolution process are taken into account.

Results

The injected CO₂ gas forms a bubble that moves upwards and accumulates under the anticline caprock. While the CO₂ plume moves, small bubbles of CO₂ gas are retained in the porous space by capillary forces at the tail of the CO₂ plume. At the CO₂ /brine interface, CO₂ slowly dissolves into the resident brine. The CO₂ -enriched brine is denser than the resident water and tends to sink leading to the formation of gravity fingers. The resulting convective flow facilitates a faster dissolution of CO₂ bubble. With the numerical model we can assess the storage capacity of the anticline/syncline system by quantifying the mass of CO₂ captured for the different mechanisms and its temporal evolution.

Just changing the location of the injection well affects the contribution of the different trapping mechanisms. For example, in Figure 3, we see how the free-phase CO₂ diminishes faster for a well located at the syncline (well C). In that case, the longer CO₂ footprint results in a more efficient capillary and dissolution trapping increasing the storage security.

Acknowledgements

This work has been funded by the 7FP EC project CarbFix and has been conducted at Amphos 21 in collaboration with Albert Nardi and Fidel Grandia. Their contributions are gratefully acknowledged.