The development of geologic carbon storage technology has created a need for strategies to manage potential CO₂ leakages. Targeted mineral precipitation with calcium silicates may be one possible solution. To achieve permeability decrease, CaSiO₃ nanoparticles are delivered to a region in the subsurface and reacted with the CO₂ to produce pore-filling minerals. Our past experimental results have shown that different pH conditions and CaSiO₃ polymorphs such as wollastonite and pseudowollastonite can precipitate a variety of minerals including CaCO₃, SiO_2(am) and layered calcium silicates. Interestingly, layered calcium silicate hydrates produced from pseudowollastonite-CO₂ reactions show resistivity to acid attack in comparison to CaCO₃, making it advantageous for sealing leaks in a CO₂-rich environment.

To examine the effect of pH and the use of different CaSiO₃ polymorphs for sealing, we developed a 1-D reactive transport model examining CaSiO₃-CO₂ reactions in a subsurface system with a formation porosity of 0.3. Four dissolution-precipitation reactions for CaSiO₃, CaCO₃, SiO_2(am), and the calcium silicate hydrate tobermorite were included in the model. The formation of microporosity from tobermorite precipitation was also modeled in an effort to capture the associated volumetric changes observed in previous experimental work. In agreement with those experiments, our model showed that at 150°C with 1.1 MPa CO₂, pseudowollastonite reacting at 0.1 M NaOH produces CaCO₃, SiO_2(am) and tobermorite formation, coinciding with significant drops in sample permeability. The increase in pH from NaOH addition increases CO₂ solubility, increasing the amount of CaSiO₃ dissolution and promoting tobermorite formation. In contrast, under the same conditions, the reaction of wollastonite, with its lower solubility and slower dissolution rate than pseudowollastonite, forms only CaCO₃ and SiO_2(am) has limited impact on overall permeability. The extrapolation of the model to field conditions in the deep subsurface (40°C, 7.5 MPa CO₂) suggests that targeted CaSiO₃ delivery could be used to alter permeability to seal CO₂ leakages by creating a more stable seal that contains not only CaCO₃ and SiO_2(am) but also layered calcium silicate hydrates.

Dan Plattenberger

Appearances