A Reactive Transport Model of CaSiO3 Reactions for Targeted Mineral Precipitation in Porous Media

The ability to selectively promote precipitation in subsurface environments can enable control of permeability thereby sealing leakages in geologic CO₂ reservoirs, compressed air storage sites, deep waste disposal aquifers, and radioactive waste repositories. Targeted mineral precipitation may be achieved through delivery of reactive nanoparticles to porous or fractured media. CaSiO₃ minerals, including wollastonite and pseudowollastonite, are two potential minerals that can react in a CO₂-rich environment to alter permeability. Wollastonite dissolution is known to lead to CaCO₃ and SiO_2(am) precipitation, and in our previous work we showed that pseudowollastonite can lead to precipitation of the same minerals along with calcium silicate hydrate phases. In the present work, we explore the effectiveness of two CaSiO₃ polymorphs in decreasing permeability in a diffusive sand column.

A 1D reactive transport model was developed to understand wollastonite's and pseudowollastonite's abilities to produce different precipitation products, their effects on permeability change, and to enable interpretation of experimental findings under the same conditions. The dissolution and precipitation of CaSiO₃, SiO_2(am), calcite, and tobermorite (a common calcium silicate hydrate) were modeled with aqueous CO₂ (150°C, 160 psi) diffusing through an inert porous medium (porosity = 0.3) for 48 hrs. Simulation results showed that calcite and SiO_2(am) formed in both systems containing wollastonite and pseudowollastonite. It also indicated that the higher solubility of pseudowollastonite allowed for increased Ca²⁺_(aq) and SiO_2(aq) release into solution, which coincided with a pH increase. These changes allowed for tobermorite precipitation to occur with pseudowollastonite dissolution. No tobermorite precipitation occurred in the simulation with wollastonite. The formation of tobermorite in the pseudowollastonite simulation yielded higher decreases in permeability than the wollastonite simulation. These results emphasize the importance of the initial CaSiO₃ in controlling precipitation and the promising effectiveness of incorporating calcium silicate hydrate precipitation into engineering targeted mineral precipitation.