Multiphase Flow Regime Controls Carbonate Precipitation Morphologies during CO₂ Injection in Subsurface Basalts

Pore-scale CO₂-H₂O multiphase flow simulations were performed on domains extracted from first-of-their-kind postinjection sidewall cores retrieved from Pacific Northwest National Lab's Wallula Basalt Pilot Demonstration (WBPD), the world's first supercritical CO₂ injection test in a basalt reservoir. We integrate Lattice Boltzmann Method computational fluid dynamics (CFD) simulations , pore-scale imaging (micro-CT, optical microscopy, SEM), and transition state theory to investigate basalt multiphase flow dynamics and the influence of multiphase flow parameters (capillary number, Ca#, and water saturation) on the occurrence of small (<35 μm) and large (0.2-2 mm) postinjection carbonate nodules within, respectively, WBPD Zone 1 (connected vesicles) and Zone 2 (semi-isolated vesicles) end-members. Image analysis is augmented by pore size distribution measurements (N₂ adsorption, low-field NMR) to establish the dual-porosity nature of the zones and subvoxel resolution fluid flow pathways to vesicles. In Zone 1, drainage simulations demonstrate how the distribution of scCO₂-water interfaces vary under different capillary and viscous flow regimes and may limit the size of precipitants. In Zone 2, 3D morphological analysis of simulated capillary-driven scCO₂-water distributions and microCT-informed carbonate growth within vesicles reveals that the location and growth extent of large carbonate nodules are likely controlled by the distribution of scCO₂-water interfaces (acidity source) in relation to altered vesicle surfaces (alkalinity source). Toward augmenting geochemical/reactive transport models in basalt lithologies and beyond, we propose stepwise mechanisms describing how morphology-determined local water saturation and pH gradients lead to supersaturated "Goldilocks region" conditions favorable for carbonate growth. This work directly links a posteriori CFD models to postinjection crystallization behaviors in a subsurface testbed.