CO₂ geological storage: Hydro-chemo-mechanical analyses and implications

The injectivity of CO₂ and the integrity of the reservoir-caprock system are affected by CO₂ invasion, water-CO ₂-mineral reactions, and ensuing mineral dissolution and precipitation. We identify different zones around an injection well and investigate the effects of these hydro-chemo-mechanical interactions. Geochemical analyses combine a comprehensive mass balance formulation with chemical calculations using published equations and PHREEQC. This analysis framework is used to assess near-well, pool, and far-field conditions, and to determine species concentrations, pH, changes in brine density, and changes in mineral and fluid volume in the reservoir. Results show that the brine density may increase by as much as 1.2% and can sustain convective flow of CO ₂ dissolved brine; the characteristic time scale for convection can be as short as a few years in some permeable formations currently being considered for storage. The precipitation of secondary minerals near the injection well increases the mineral volume by a maximum of 5%, yet, only a minor decrease in CO₂ permeability is anticipated. Dissolution may result in unsupported caprock (the span should not exceed 20% of the caprock thickness to prevent failure), and may cause compaction-driven shear failure of the reservoir. Finally, the analysis of lateral capillary trapping shows that the CO₂ pool is only a few meters thick in leveled caprock interfaces and in the absence of geometric traps.