Supercritical CO 2 uptake by nonswelling phyllosilicates.

Interactions between supercritical (sc) CO ₂ and minerals are important when CO ₂ is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO ₂ alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO ₂ on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO ₂ ), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO ₂ uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO ₂ partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO ₂ to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO ₂ , can increase CO ₂ storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO ₂ uptake constitutes a previously unrecognized potential trapping mechanism.
Competing Interests: The authors declare no conflict of interest.
(Copyright © 2018 the Author(s). Published by PNAS.)