Geochemical Reactivity Modelling During CO2 Induced Convective Brine Mixing

Abstract

Abstract OMV is planning to store CO2 in a depleted gas field located in Eastern Europe. Within the storage site, the rock assemblage contains several minerals which may react with CO2. Although such mineral dissolution could be locally limited, those reactions may be enhanced by the CO2 triggered convective flow patterns acting to provide a continuous unsaturated water supply at the CO2/water interface affecting the integrity of the storage site. Using the field observed mineralogy, batch and reactive transport models were constructed to provide understanding about the characteristics of mineral dissolutions in the storage site. Batch modelling was undertaken to understand the most significant reacting minerals. 1D modelling captured key characteristics of mineral dissolution within the reactive system under forced carbonated water flow. 2D modelling was used to analyse the mineral dissolution profile at and below the CO2/water interface for a range of permeability conditions. Learnings were applied to a full field 3D model, enabling understanding of the significance of convective flow patterns on mineral dissolution at the field scale. Batch modelling results showed only carbonate minerals are susceptible to reaction with carbonated water; other minerals could thus be discarded from further reaction transport modelling. 2D modelling results identified an interesting and previously unreported feature of temporary calcite dissolution/precipitation within the storage site. It was observed that at favourable permeabilities, dissolved calcite mineral within the CO2 plume region will slump along with the carbonated water, and subsequently reprecipitate immediately beneath the CO2/water interface to form a temporary dense calcite region. This denser accumulation of calcite is, nevertheless, dissolved in the long-term by the effect of the convective flow patterns. 2D modelling results also showed that under natural flux of convective formation water triggered by CO2 dissolution, mineral dissolution will be insufficient to affect storage site integrity. Finally, results of 3D modelling showed that within the target storage site, the impact of convective flow patterns is not expected to be significant enough to trigger extensive geochemical reactions and integrity issues. This is due to the relatively low permeability of the storage site, which retards the formation of strong convective flow patterns below the CO2/water interface. Mineral dissolution is locally limited and does not represent a major risk to storage security. This study illustrates the impact of convective flow patterns when evaluating the integrity of CO2 storage prospects. Although such geochemical reactions may be assumed to be locally limited, they could be promoted by the supply of continuous water triggered by convective flow patterns. This feature is important in integrity modelling studies, and the impact of continuous water supply on the extent of long-term geochemical reactions should be correctly taken into account.