Geochemical Modelling for Carbon Dioxide Removal Applications

Water-rock and aqueous reactions affect CO₂ geological storage in several ways, including through carbonate mineralization, dissolution and reprecipitation, silicate dissolution, and acid-base buffering. However, complex water chemistry compositions and multiple rock mineral types make the quantitative characterization of these reactions difficult. Here, predictive models of geochemical reactions were developed taking place within strong to moderately reactive storage formations where pH-sensitive silicate dissolution and carbonate precipitation dominate. To do this, the TOUGHREACT thermal-hydrological-chemical multiphysics subsurface reactive transport simulator was used to develop well-calibrated models based on field monitoring data.

This study first benchmarked a model against a single-well CO₂ push-pull field test conducted in a pH 11.02, shallow peridotite formation in Oman, described in Matter et al. (2025). The simulation included the 13.7-hour carbonated water injection, the subsequent 45-day shut-in, and then 11.2 days of pumping. Calibration of the porosity, permeability and formation mineral assemblage primarily occurs against recorded ion concentrations during the pumping period. The model suggests calcite precipitation dominated at the margins of the 6.4 m radius mineralization zone, with dolomite at intermediate distances and magnesite in the immediate vicinity of the well. Magnesite precipitation is associated with lower pH conditions near the well where there is sufficient available Mg²⁺ dissolved from the host rock, whereas dolomite and calcite are deposited at higher pH and sufficient available Ca²⁺. During the storage period, our model underpredicts mineralization (52%) compared to that inferred by Matter et al. (88%), likely due to underprediction of dolomite or magnesite. The precipitated carbonates remain stable upon re-equilibration of the groundwater.

The model was then applied to a hypothetical doublet storage operation in a CO₂-rich hydrothermal system at Ohaaki, New Zealand. The goal was to predict CO₂ phase evolution subject to long-term geochemical reactions as well as boiling of the fluid phase. Simulations show that ions primarily controlled by a single mineral (Ca²⁺, Na⁺, K⁺, and Fe²⁺) all reach their peak concentrations within five years, whereas subsequent geochemical evolution is influenced by the dynamic equilibrium of aqueous complexes, such as CaSO₄(aq), NaCl, MgHCO₃⁺, and FeCl⁺. Driven by the injection of aqueous solutions with high carbonic acid concentrations, the mineral volume fraction at the injection well changes at a rate 2–11 times greater than that observed in the rest of the simulation domain. Under high-temperature and low-pressure conditions of the production well, a CO₂ boiling zone forms in the reservoir, with the peak gas saturation of CO₂ exsolved from the liquid phase reaching 7.6 wt% over the simulation period. This research shows that the geochemical reaction simulation holds significant scientific value for CO₂ storage applications in strong to moderately reactive storage formations.