Visualizing the effect of CO2 on caprock microstructures

© 2020 EPFL

© 2020 EPFL

Eleni Stavropoulou, scientist in EPFL’s Laboratory of Soil Mechanics (LMS) lead by Prof. Laloui, has been awarded a Swiss National Science Foundation grant to study how the presence of carbon dioxide alters the structure of caprocks. Her research dovetails with efforts to reduce greenhouse gas emissions by storing CO2 deep underground. 

In October 2018, the Intergovernmental Panel on Climate Change (IPCC) published an alarming report on the effects of a 1.5°C rise in global temperatures and called for the world to achieve carbon neutrality by 2050. One of the headline measures proposed in the report was to capture and store CO2 emitted by human activities. 

This approach, used in Norway for almost 30 years and adopted more recently by other countries including Canada, involves capturing CO2 near production plants and other places where it’s emitted in dense concentrations, and then piping it directly into underground rock formations. Common locations include depleted rock reservoirs that once contained oil and gas, sub-ocean saltwater aquifers and coal seams – all of which offer long-lasting storage capability.

In the team of Prof. Lyesse Laloui (LMS), Eleni Stavropoulou is studying shales – a material whose distinctive properties make it part-rock, part-soil – to determine whether it makes a good candidate for caprock, a type of rock formation that sits above an underground reservoir and acts as a barrier, stopping CO2 from migrating to the surface. Her research focuses in particular on how argillite behaves mechanically in the presence of CO2. This behavior is key to reliable, long-term storage without the risk of leaks.

© 2020 EPFL
“Shales have low permeability and are naturally self-sealing, so they’re good candidates for geological barriers,” says Eleni Stavropoulou. “But these same properties make studying samples especially difficult, because the rock is highly sensitive to changes in temperature, humidity and other ambient variables.”

Scaling down to micro

Eleni Stavropoulou is turning to less destructive analysis methods as a way to avoid damaging samples in the lab and still produce conclusive findings. She plans to use X-ray tomography (a technique used in CT scanners) and 3D imaging to generate the first-ever in-situ images of changes in the structure of argillite in the presence of supercritical CO2, the fluid state in which the compound exists underground. 

Whereas research of this kind is normally conducted on samples measured in centimeters, Eleni Stavropoulou intends to analyze samples measuring no more than 5 millimeters across, subjecting them to the same thermal, hydrological, chemical and mechanical conditions found in underground reservoirs. “By scaling down to the micro level, we can produce higher-resolution images and observe slow-acting phenomena at work” explains Eleni Stavropoulou. “More important still, using non-destructive methods means we won’t alter samples from their initial state.”

© 2020 EPFL

The project has received a CHF 100,000 grant from the Swiss National Science Foundation’s (SNSF) Spark program, which supports the development of new scientific ideas and methods that “show unconventional thinking and introduce a unique approach”. Eleni Stavropoulou has 12 months to demonstrate the feasibility of her method.

“With any experimental method, there’s always the risk that things won’t go to plan,” she says. “But you don’t know until you try! This is an extremely complex material and, in real-world conditions, the phenomena we’re studying happen over long timeframes. That’s why we need to shrink the measurement scale while being able to generate more detailed images. What we lose in spatial resolution, we’ll gain in time. Ultimately, these experiments are about developing mechanical laws and models to explain how such heterogeneous materials behave. It’s important to observe these phenomena in a localized manner rather than using conventional experimental techniques, which generally produce averaged responses.”

If Eleni Stavropoulou’s experiments go to plan, her findings could support the research that’s being conducted on CO2 storage and rock reservoirs at the Mont-Terri Rock Laboratory, based in the Jura Mountains.

Funding

The aim of SNF's Spark is to fund the rapid testing or development of new scientific approaches, methods, theories, standards, ideas for application, etc. It is designed for projects that show unconventional thinking and introduce a unique approach. The focus is on promising ideas of high originality, with minimal reliance on preliminary data. Taking risks is very welcome, but not a requirement in itself. The focus is on projects or ideas that are unlikely to be funded under other funding schemes.

The proposals are evaluated in a double-blind reviewing process (i.e. the identity of the applicant will not be disclosed to the evaluators). In this way, the SNSF aims to ensure that the evaluation will focus on the project idea.