The impact of numerical discretisation on the correct simulation of CO₂ convective flow patterns

The convective flow patterns induced in CO2 storage processes plays an important role in safe and permanent CO2 storage. However, their development depends on the continuous contact between brine and CO2 ensuring constant CO2 dissolution in brine and removal from contact. Numerical gridding of such problems, however, implies that the solution is no longer continuous; rather, it is discretised in the spatial domain. Exceptionally, this discretisation may cause the buoyant CO2 and brine to reside in two vertically neighbouring grid blocks without contact. Although this might be physically correct, since this phenomenon disconnects the contact between the two phases, the CO2 dissolution may stop, as supercritical CO2 and brine are no longer in contact. This artefact is important in situations where the direction of CO2 mass transfer caused by different mechanisms are opposite relative to each other. While CO2 migrates upward, the CO2 saturated brine will migrate downward. We show in this study that, because of the discretisation nature, the simulator may not able to capture the full physics of post-injection CO2 dissolution. A fine grid cross-sectional model was constructed. CO2 was injected followed by long shut-in period allowing CO2 dissolution. A plume was developed after CO2 injection termination. Due to CO2 dissolution the plume-water contact moves gradually upward. However, as soon as it reaches the boundary between two vertical adjacent layers, the dissolution stops, although undissolved CO2 is still present and water underneath is not fully saturated with CO2. The issue was also investigated under different alternative injection configurations, though the same problem was confirmed. The artefact can be overcome by the inclusion of mechanisms allowing CO2 to travel across disconnected neighbouring cells. This could be either explicit inclusion of physical diffusion or capillary pressure that smears the front. Interestingly, the problem is exceptional in that grid refinement does not help since it does not create the contact between the two phases residing in vertically neighbouring cells. Whereas much study has been made of convective mixing effects during CO2 injection, this grid discretisation phenomenon has never previously been identified and reported. The artefact is important in processes where the direction of mass transfer is different due to phase density differences and dissolution effects, such as during CO2 dissolution in water.This problem affects the calculation of long-term CO2 storage, and the fate of CO2 - does it remain in a supercritical phase or is it dissolved into surrounding aquifer water.