Research highlight (IFPEN and GEUS) - Gravitational instabilities

Amongst the CO2 trapping mechanisms, dissolution is a slow process occurring at the pore scale that is difficult to simulate effectively in reservoir scale models.

Once accumulated below the caprock, the CO2 dissolves over time only at the interface between the gas and liquid phases. Initially, part of the brine in contact with the CO2 will quickly become saturated and dissolution will be limited by the slow molecular diffusion of CO2 in the brine. This is known as the purely diffusive phase. 

However, because the CO2-saturated brine is denser than the underlying indigenous brine, the brine-CO2 contact zone gradually becomes unstable. Therefore, after the purely diffusive phase, a complex convection process starts where the CO2-saturated water sinks and the CO2-free water rises. This ‘buoyant convection’ accelerates the global dissolution, i.e. the bulk transfer of CO2 from the free gas to the aqueous phase.  

CO2 molar concentration maps at different times for different permeability maps

 

Once CO2-saturated brine reaches the bottom of the reservoir, CO2 transfer progressively decreases and molecular diffusion again drives the final homogenisation until equilibrium is reached and CO2 transfer stops.

The works performed within UltimateCO2 project is the development of an “up-scaling” method of these gravitational instabilities by representing at a coarse grid scale the phenomena described above, that happens at the refined scale. In a coarse grid, only a purely diffusive regime can be simulated, the second and third phases are not represented due to the large size of the cells. An “up-scaling” pseudo-diffusion coefficient at coarse grid scale is derived from the fine scale diffusion coefficient according to the model discretization and the reservoir heterogeneity.

 

 

Dernière mise à jour le 18.08.2015