ADDING KINETICS TO EQUILIBRIUM-BASED GEOCHEMICAL MODELING OF CO2 SEQUESTRATION

Geochemical modeling is a powerful tool for rapidly assessing the potential of various lithologies for CO₂ sequestration via precipitation in carbonate minerals.  Modeling of perturbed systems such as CO₂ injection schemes must account for the relatively slow kinetics of mineral-CO₂-water interactions at aquifer temperatures and pressures, unlike many natural systems where equilibrium may reasonably be assumed.  We have therefore compiled parameters conforming to a general, four-term (H₂O, and H⁺-, OH^--, and CO₂-catalyzed mechanisms) Arrhenius-type rate equation, for over 70 minerals, including phases from all of the major classes of silicates, most carbonates, and many other non-silicates. The compiled dissolution rate constants, far from equilibrium, at 25 °C, and pH near neutral, range from  –0.21 log moles m^-2 s^-1 for halite, to –17.44 log moles m^-2 s^-1 for kyanite. These data have been added to a computer code that simulates an infinitely well-stirred batch reactor, allowing computation of mass transfer as a function of time.

The final code will be general and applicable to modeling mineral-CO₂-water interactions at temperatures (0-350 °C), pressures (1-1000 bars) and water salinities (1000-230,000 ppm TDS) suitable to investigate CO₂ sequestration in saline aquifers and depleted petroleum reservoirs.  Preliminary model results using different rock types with initial 0.1 mm diameter spherical grains, in 1.0 molal NaCl with excess CO₂, at 100°C and 90 bar, yield times for system equilibration ranging from 6.0 years for serpentinite (above surface sequestration using crushed rock), to 330 years for Ca-bearing arkose.  Equilibration rates in aquifers are expected to be much slower, primarily because CO₂ sequestration in aquifers involves flow through porous media, which allows the development of concentration gradients in the aqueous phase near mineral surfaces, resulting in decreased absolute chemical affinity and slower reaction rates.  Differences in CO₂ sequestration rates in the formations as compared to the models may also occur because of variation in grain size, aquifer inhomogeneity, minerals coatings, preferred fluid flow paths, and precipitation of secondary minerals that may lead to decreased porosity and clogged pore throats.