Containment Impact of Calcite Pathways in the Primary Caprock of CO2 Storage in a Depleted North Sea Gas Field

We present results from forward geochemical modelling, coupled to fluid flow, of CO2 injection in a depleted gas field in the UK sector of the North Sea (a candidate storage site). For the reservoir a long-term porosity decrease after shut-in is predicted. Similarly for the caprock a reduction in porosity is predicted (for the average mineralogical composition), suggesting an increase in containment integrity. We consider the risks posed by local composition variations, in the form of possibly through-going calcite pathways (prone to dissolution). Our analysis shows that 1) the dissolution process is transport limited; 2) in the region above the gas water contact the transport process is diffusive, resulting in negligible calcite dissolution; 3) in the region below the contact it would take more than 200,000 years to dissolve a calcite streak through the primary caprock and the resultant flow path would then facilitate the transfer of brine, not continuous phase CO2. The long timescale emerging from this analysis is not very site specific.