Investigations of calcium carbonate mineral precipitation using 3D-printed micromodels

Carbon storage in deep aquifers relies on permanent mineralisation of carbon dioxide as solid carbonate mineral. As injected CO2 leaves the well and travels further into the aquifer it mixes with the reservoir brine and initially becomes a low pH acid. However, as mixing and dilution continues, the pH increases, and the carbonate species become supersaturated and fall out of solution. This precipitation then fills the pore space of the reservoir with permanently stored carbonate mineral. The CO2 injection strategy must be designed so that precipitation is favourable at long time scales far away from the injection well, but not so fast that the major flow pathways through the aquifer are clogged and expensive additional well-drilling is required. The interplay between mixing, pore-space heterogeneity, particle nucleation and transport, and crystal growth processes are poorly understood. Here we use the combination of 3D-printed micromodels, light microscopy, and pore-scale reactive transport models to investigate flow and transport conditions favourable for calcium carbonate precipitation. 3D-printed micromodels have the benefit of being inexpensive to produce with both high design fidelity and flexibility in design. In these experiments we co-inject CaCl2 and NaHCO3 brines into a 3D-printed micromodel and monitor precipitation using a visible light camera. We vary flow rate, ion concentration, and pore-space heterogeneity. We see that the local saturation index and the pore-space heterogeneity play a key role in the location of precipitated particles. Furthermore, we see considerable positive feedback between the focussing of flow pathways and the growth of crystals in the pore throats. These experimental results are then benchmarked with direct numerical simulations. This work has extensive applications for the carbon storage and enhanced oil recovery industries.