How Brine Composition Impacts CO2 Storage in Depleted Carbonate Reservoirs

This paper uses reactive transport modelling to analyse the impact of brine composition from a geochemical perspective on CO2 storage in depleted carbonate reservoirs. Two reservoir sector models with similar mineralogies and different brine compositions are used to simulate CO2 storage in a depleted reservoir that produced light oil under seawater-alternating-gas injection for 20 years.

This work indicates the previously unreported finding that a reservoir brine tends to store more CO2 in solution when the calcium concentration is lower, as it dissolves more calcite than a brine with a higher calcium concentration due to the common ion effect. In brines less saturated in carbonate ions, the formation of bicarbonate is facilitated, and the dissolution of calcite and dolomite effectively improves CO2 solubility trapping, thereby enhancing the stability and security of CO2 storage in carbonate reservoirs.

This study also demonstrates that CO2 storage in oil reservoirs may offer the advantage of higher CO2 dissolution trapping capacity when compared to aquifers due to the greater solubility of CO2 in oil, and this capacity can be enhanced in light oil reservoirs with originally low CO2 fraction in the oil phase.