Jiamin Wan led a team in the study for NCGC.

New research from the Energy Geosciences Division at Berkeley Lab shows that carbon dioxide can penetrate the inner layers of some non-swelling clay minerals which make up the dominant clays in the Earth’s deep subsurface. Results of the work performed at the Center for Nanoscale Controls on Geologic CO2 (NCGC) and the national lab’s Molecular Foundry could help inform practices intended to help limit carbon dioxide emissions, such as carbon capture and storage (CCS) and enhanced oil recovery (EOR).

The study led by EESA staff scientist Jiamin Wan represents ongoing efforts by the NCGC to understand how CO2 behaves one kilometer and farther below the Earth’s surface. A collaboration of seven partner institutions led by Berkeley Lab under the direction of Don DePaolo, NCGC is one of the country’s 32 Energy Frontier Research Centers (EFRCs) funded by the DOE’s Basic Energy Sciences (BES) program.

Previous studies have shown that CO2 can alter typical swelling (or expanding) phyllosilicate minerals such as smectite under the high pressures and temperatures of the deep subsurface. Less is known about the effects of CO2 on non-swelling phyllosilicates illite and muscovite, despite them being the dominant clay minerals in deep subsurface shales and mudstones.

Wan believes there is the assumption that CO2 cannot penetrate layers of minerals that do not expand. “Describing a clay mineral as ‘non-swelling’ means that it does not expand,” says Wan. “Because of this, people don’t imagine that CO2 can get into the mineral’s interlayers. Instead, they imagine CO2 uptake by only the outer surface of the minerals.”

This assumption may lead scientists to underestimate the amount of carbon storage capacity available within the deep subsurface. Wan and her team chose to conduct their experiments on the two similar clay minerals muscovite and illite using muscovite, because of the ability to extract it in large, smooth sheets.

Wan believes there is the assumption that CO2 cannot penetrate layers of minerals that do not expand. “Describing a clay mineral as ‘non-swelling’ means that it does not expand,” says Wan. “Because of this, people don’t imagine that CO2 can get into the mineral’s interlayers. Instead, they imagine CO2 uptake by only the outer surface of the minerals.”

In their study, the researchers subjected single muscovite crystals to incubation with supercritical CO2 (scCO2); then characterized the reacted samples using combined atomic force microscopy (AFM),  X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements. The first sign that CO2 had penetrated the muscovite sample came when after depressurization the team observed blisters on the muscovite surface (Fig. 1), indicating gas entering the interlayers.

Blistering on the muscovite surface after exposure.

The scientists then confirmed the presence of CO2 using XPS technology, and later quantified the amount of CO2 present by comparing muscovite samples exposed to scCO2 with unexposed control samples to measure the amount of off-gassing of CO2 from the muscovite samples. The exposed samples yielded approximately seven times more CO2 than control samples.