Pore-scale supercritical CO₂ dissolution and mass transfer under drainage conditions

Journal Article · Wed Feb 01 04:00:00 EST 2017 · Advances in Water Resources

DOI:https://doi.org/10.1016/j.advwatres.2016.12.003· OSTI ID:1344032

Chang, Chun; Zhou, Quanlin; Oostrom, Mart; Kneafsey, Timothy J.; Mehta, Hardeep

Abstract: Recently, both core- and pore-scale imbibition experiments have shown non-equilibrium dissolution of supercritical CO₂ (scCO₂) and a prolonged depletion of residual scCO₂. In this study, pore-scale scCO₂ dissolution and mass transfer under drainage conditions were investigated using a two-dimensional heterogeneous micromodel and a novel fluorescent water dye with a sensitive pH range between 3.7 and 6.5. Drainage experiments were conducted at 9 MPa and 40 °C by injecting scCO₂ into the sandstone-analogue pore network initially saturated by water without dissolved CO₂ (dsCO₂). During the experiments, time-lapse images of dye intensity, reflecting water pH, were obtained. These images show non-uniform pH in individual pores and pore clusters, with average pH levels gradually decreasing with time. Further analysis on selected pores and pore clusters shows that (1) rate-limited mass transfer prevails with slowly decreasing pH over time when the scCO₂-water interface area is low with respect to the volume of water-filled pores and pore clusters, (2) fast scCO₂ dissolution and phase equilibrium occurs when scCO₂ bubbles invade into water-filled pores, significantly enhancing the area-to-volume ratio, and (3) a transition from rate-limited to diffusion-limited mass transfer occurs in a single pore when a medium area-to-volume ratio is prevalent. The analysis also shows that two fundamental processes – scCO₂ dissolution at phase interfaces and diffusion of dsCO₂ at the pore scale (10-100 µm) observed after scCO₂ bubble invasion into water-filled pores without pore throat constraints – are relatively fast. The overall slow dissolution of scCO₂ in the millimeter-scale micromodel can be attributed to the small area-to-volume ratios that represent pore-throat configurations and characteristics of phase interfaces. This finding is applicable for the behavior of dissolution at pore, core, and field scales when water-filled pores and pore clusters of varying size are surrounded by scCO₂ at narrow pore throats.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1344032
• Report Number(s): PNNL-SA--122892; 49072; 48255; KP1704020
• Journal Information: Advances in Water Resources, Journal Name: Advances in Water Resources Journal Issue: C Vol. 100; ISSN 0309-1708
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Environmental Molecular Sciences Laboratory