Kinetic Modelling of CO2–Brine–Rock Reactions and Mineral Trapping in Deep Saline Aquifers

Abstract

This study presents a sequentially coupled modelling framework for analysing Carbon (IV) Oxide (CO2) injection and long-term storage in deep saline aquifers, with emphasis on the role of geochemical kinetics in controlling mineral trapping. Multiphase flow, pressure evolution, and phase saturation were simulated over 200 years using a compositional reservoir model, while geochemical reactions were evaluated using a kinetic framework constrained by reservoir-derived pressure, temperature, and CO2 saturation conditions. Mineral dissolution of calcite, anorthite, and kaolinite was represented using Transition State Theory rate laws with Arrhenius temperature dependence, and aqueous speciation in the CO2–brine system was resolved using temperature-corrected equilibrium constants and the B-dot activity model. The results show that CO2 storage behaviour evolves through a sequence of kinetically controlled processes. Dissolution trapping develops rapidly during and after injection, with dissolved CO2 mole fractions reaching up to 0.036 in high-saturation zones and extending beyond the free-phase plume. In contrast, mineral trapping evolves gradually and remains limited over the 200 years. Calcite dissolves rapidly, reaching complete depletion within approximately 0.6 years, and dominates early acid buffering and calcium release, contributing to cumulative Ca2+ concentrations of up to ~30,000 mmol. Anorthite dissolves more slowly but persistently, with dissolution extending for more than 150 years and controlling long-term geochemical evolution. Kaolinite, despite its significantly higher reactive surface area (~799 m2), remains kinetically limited and contributes minimally to total dissolution. The persistence of acidic conditions, with pH stabilising around 5.2–5.3 in CO2-invaded zones, indicates that silicate dissolution is insufficient to neutralise the system within the simulated timeframe fully. These results demonstrate that mineral trapping is fundamentally rate-limited and that storage performance in saline aquifers is governed by the temporal sequence of mineral-specific kinetic responses rather than equilibrium assumptions. The paper provides a tractable approach for linking reservoir-scale flow dynamics with geochemical kinetics and offers improved insight into the evolution of CO2 trapping mechanisms over long timescales.