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

Area of Science:

• Geochemistry and Subsurface Science
• Computational Fluid Dynamics (CFD)
• Petroleum Engineering and Carbon Sequestration

Background:

• The Wallula Basalt Pilot Demonstration (WBPD) is the first supercritical CO2 injection test in a basalt reservoir.
• Understanding multiphase flow dynamics in basalt is crucial for predicting CO2 storage security and mineral precipitation.
• Previous studies lacked detailed pore-scale insights into CO2-water interactions and subsequent carbonate formation in basalt.

Purpose of the Study:

• To investigate basalt multiphase flow dynamics using pore-scale simulations and experimental data.
• To determine the influence of flow parameters (capillary number, water saturation) on carbonate nodule formation.
• To link computational fluid dynamics (CFD) models with observed postinjection crystallization behaviors in a basalt testbed.

Main Methods:

• Pore-scale Lattice Boltzmann Method (LBM) CFD simulations on core samples from the WBPD.
• Integration of micro-CT, optical microscopy, SEM, N2 adsorption, and low-field NMR for pore structure and fluid analysis.
• Application of transition state theory to model carbonate precipitation mechanisms.

Main Results:

• Simulations revealed distinct CO2-water interface distributions in connected (Zone 1) versus semi-isolated (Zone 2) vesicles.
• Flow regimes in Zone 1 limit the size of small carbonate precipitates (<35 μm).
• In Zone 2, large carbonate nodules (0.2-2 mm) formation is controlled by the interplay of CO2-water interfaces and vesicle surface alterations, creating favorable conditions for growth.

Conclusions:

• Morphology-determined local water saturation and pH gradients drive supersaturation, favoring carbonate growth in basalt vesicles.
• The study proposes stepwise mechanisms for carbonate precipitation, enhancing geochemical and reactive transport models for basaltic lithologies.
• This work establishes a direct link between a posteriori CFD modeling and observed postinjection crystallization in a real-world subsurface environment.