Speaker
Description
We investigate the combined influence of scCO2-brine and mineral interfaces on local pH gradients and carbonate precipitation under diffusive conditions using microfluidic flow cells in a pressure reactor. The controlled studies will yield relationships for reactive transport modeling of scCO2-driven precipitation in vesicular basalts and other reactive media. We hypothesize dissolution and diffusion of CO2 in pore water will generate local pH gradients as a function of pore morphology and water saturation, especially in poorly-connected vesicles where snap-off phenomena trap bubbles and advective transport is minimal. Hence, in these dual-porosity systems there are pore-scale regions at a certain distance from scCO2-brine interfaces and metal ion-sourcing mineral interfaces where pH is ideal for carbonate formation. In those regions, the concentration of dissolved CO2 ions is high enough to form carbonate, but, critically, low enough to not over-acidify the fluid, rendering carbonates soluble. This hypothesis, termed “Goldilocks Zone”, was introduced by Shen et al. (ES&T, 2025) in pore-scale modeling of scCO2 injection in sidewall cores from the Wallula Basalt CO2 Injection Project conducted by PNNL.
To test this hypothesis, we isolate the impacts of scCO2 diffusion and metal ion sourcing on spatial pH and mineralization behavior with diagnostic single-outlet microfluidic devices with embedded MgO crystal inclusions. The devices feature a simple Archimedean spiral channel or isolated reaction chambers bonded to a polished crystal substrate and are filled with buffered “formation fluid” and pressurized to 90 bars in a Parr vessel. The chamber headspace is filled with scCO2, creating a scCO2/brine interface at the channel’s entrance. Across the interface, CO2 dissolves and diffuses down the channel, reacting with MgO and forming magnesium carbonates in hours to days.
We investigate precipitation behavior under different pH regimes by varying the initial buffering capacity of the fluid and determine pH computationally with 1D diffusion-reaction models in PHREEQC using reaction coefficients from literature. Post-reaction, the volume, morphology, and mineralogy of carbonate precipitants is analyzed with µCT and microscopy as well as XRD, SEM, and Raman. Our results show a spatial preference of carbonate growth midway into the channel achieved through local pH-driven precipitation and re-dissolution of Mg-carbonates in different reaction stages, which supports our Goldilocks Zone hypothesis. The findings from this work will enhance the understanding of how flow regimes can be used to optimize precipitation behaviors in reactive reservoirs to enhance in situ mineralization or separations or to maintain accessibility.
| References | Multiphase Flow Regime Controls Carbonate Precipitation Morphologies during CO2 Injection in Subsurface Basalts. Environ. Sci. Technol. 2025, 59 (38), 20345–20356. https://doi.org/10.1021/acs.est.5c03397. |
|---|---|
| Country | United States of America |
| Student Awards | I would like to submit this presentation into both awards |
| Acceptance of the Terms & Conditions | Click here to agree |
