Geochemical fingerprinting pushing the frontiers in Iceland (Stuart Gilfillan)

Dr Stuart Gilfillan, University of Edinburgh, was awarded UKCCSRC Flexible Funding in 2014 for “Quantifying Residual and Dissolution Trapping in the CO2CRC Otway Injection Site” and in 2018 for “Resolving CO2 trapping and tracking CO2 migration in the Carbon Management Canada Field Research Station, Canada, using inherent tracing tools“.  These projects are now being built on with a new grant from NERC’s Pushing The Frontiers scheme for “Enabling CO2 mineralisation through pore to field-scale tracking of carbonate precipitation: INCLUSION”.

Hi Stuart, what issue is your research trying to address?

My key research motivation is assuring the safety and security of CO2 storage, which has been the fundamental theme running through all the research I’ve done to date. The tools we use are geochemical fingerprinting tools and that’s for two things. First, to check what’s happening with the CO2 when it’s underground in the storage reservoir; and second, if it makes an unplanned migration out of the storage site, can we detect it?

I’m quite careful in the wording, because I don’t deem leakage to be the same thing as unplanned migration out of the storage site.  What we look at is different sensitivities of different tools to detect that, and then fix it before it becomes a leakage to surface or a major issue. The vast majority of the work I’ve done is about storage security in the reservoir itself.

What exactly is geochemical fingerprinting?

CO2 is made up of carbon and oxygen and the work I’ve done is based on what I term inherent fingerprints – fingerprints that are contained in the CO2 itself. CO2 has different fingerprints – in terms of the carbon and the oxygen, it has different isotopes or variations, so the same element but different mass number, and that’s because of a different number of neutrons in the nucleus. You’ve got carbon-12 and carbon-13, and we’ve also worked a little bit on radiocarbon as well, which is the one people have probably heard about as it’s what archaeologists use for carbon dating.

And then we also have stable isotopes of oxygen, so oxygen 16 and oxygen 18.  It’s using those different tools – the carbon-12 and carbon-13 ratio – to differentiate between the different sources of CO2 in the subsurface, those that are anthropogenic (i.e. injected) and those that are natural. One of the things I was involved in, 15 years ago, was an investigation in Canada where there were allegations of leakage from a CCS project and that was based on matching of the carbon isotopes (1).

The other toolbox I use is the noble gases – I’m trained in noble gas geochemistry – and in any injected CO2 or any CO2 in the subsurface, there are trace amounts of noble gases. The noble gases are conservative tracers in the subsurface, and they allow us to fingerprint different sources of CO2.  They also enable us to understand what happens to that CO2 in terms of different things acting on it, e.g. dissolution or mineralisation, and we’ve also worked on the residual trapping of the CO2 as well.  They’re all different ways of understanding what’s happening to the carbon dioxide when it’s injected underground.

Is the carbon dioxide we inject underground different to what’s naturally there?

It depends.  For the most part, the research we’ve done shows that there is a distinct fingerprint for the vast majority of it, and that’s because the combustion process and then the capture process does give it a reasonably unique fingerprint. That is caveated with different things, so different tools can be used for different sources, but some of the work we did, linked with a project that the UKCCSRC funded, was looking at the fingerprints in captured CO2. There were two good papers that came out of that (2 & 3) and we did determine that, for the most part, there are distinct fingerprints in the anthropogenic CO2 compared to the background CO2 underground.

You had two Flexible Funds awards from us – one in 2014 and one in 2018. What did those two awards enable you to do?

The 2014 project funded work in Australia, looking at a couple of things. The first was noble gas tracers in the subsurface, and enabled us to collaborate with one of the world’s best CO2 injection test sites at Otway in Australia. That project was fantastic to work on, great people and a really nice site set-up.  They’ve done some great science there, looking at different aspects of CO2 injection and testing a lot of different tools.

We were able to deploy our tracer tools on residual trapping tests, what’s called a push-pull test.  So, they injected CO2 into the reservoir, then followed that up with CO2 saturated water. The idea was to drive the formation underground into residual saturation and then pull the CO2 back out, in order to compare the fingerprint of what was injected and what came out, then determine how much residual trapping had occurred. Again, there were two papers (4 & 5) we published about using oxygen isotopes as the tool in that. We did see changes in those but, given the nature of the test, it was quite quick and piggybacking off something else. We still learned from it and saw that the oxygen isotopes did change on a short-time scale.

Then we followed that up with a couple of experiments (and another paper that I published last year (6)), where the postdoc on the project looked at how fast the CO2 equilibrates with oxygen in the lab, and how the oxygen isotopes change. Off the back of that, there was also a PhD student looking at Australian CO2 fields and the origin of CO2, because a lot of the work I’ve done in the past has been looking at how nature has stored CO2 in the natural CO2 reservoirs, how long they’ve stored CO2 for, and how that CO2 has been stored over geological timescales.

(L) Sascha Serno and Gareth Johnson checking gas levels at CO2CRC Otway site; (R) Stuart Gilfillan measuring gas samples in the geochemistry lab at the CO2CRC Otway site

Did your 2018 project lead off that one or was it different?

Having spent my early career looking at these natural CO2 reservoirs, and then following that up by looking at these test sites, I guess I became a specialist in working on test sites. It was a good opportunity to travel and I also really enjoyed the site work and doing the hands-on stuff. With the 2018 project in Canada, we got invited out because we’d been involved in a study previously in Canada on leakage allegations.

We went to a test site that, at the time, wasn’t really constructed.  What’s been fundamental in a lot of the work we’ve done, or what’s caused problems in the past, is not having great baselines. You need to know what your geochemical baseline is so you can do a comparison of before and after. Because the Canadian site was still being developed, we were able to collect a number of different samples from different wells which gave us a really good understanding of what the baseline was.

Steph Flude and Stuart Gilfillan at the Field Research Station, Canada

Interestingly, as well, they were injecting captured CO2, commercial CO2, and one of the things we found in that study was enrichment in the krypton and the xenon in the noble gas fingerprint (7).  That could be quite a sensitive tracer for the injected CO2, and the great thing about that is it’s not something that needs to be added, it’s already present.

That’s presumably because captured carbon has been processed to some degree?

It is.  Usually, in terms of noble gas fingerprinting, it’s generally solubility capture tools that are used, like amine solvents. They concentrate up the heavier noble gases, particularly xenon, because it’s actually very soluble. So that also gets stripped out and enriched in the captured CO2.  The other good thing is that there’s very little xenon in the subsurface, so that’s actually something we’ll need to work on and think about a bit more, to fully understand how we can use this to track the injected CO2.

How did those projects lead on to the big one you’ve got now with NERC – Pushing The Frontiers?

As I said, part of the research I’d been doing had been hands-on work on the different CO2 test sites. Carbfix came on the radar – they’ve been doing their injection tests since about 2012, 2013 – and had seen some of the work we’d done in the past, particularly on the natural CO2 reservoirs.  They thought it would be really good to deploy this in Iceland and that led to a PhD student, Chris Holdsworth, being partially funded by Carbfix.

Off the back of that, they learned through his PhD project how to do things and what the setup was. I’d worked a lot on natural gas fields, hydrocarbon fields and CO2 reservoirs, but geothermal fields are a whole other kettle of fish in terms of complexity, different fluids and interesting sampling. We developed, then used, all the different tools – geochemical, fingerprinting – and applied them to what Carbfix were doing, trying to understand if we can track the mineralisation that they expect to be happening underground and find out if we can actually see that in the different fingerprinting tools.

(L) Chris Holdsworth at Nesjavellir; (R) Stuart Gilfillan and Chris Holdsworth sampling at Hellisheidi

In conventional CCS, and some of the early work I did showed that, even in the natural CO2 reservoirs that have stored CO2 for millions of years, we don’t actually see a lot of evidence for mineralisation of that CO2. That’s simply because in a sandstone rock or carbonate reservoir, there’s not a lot for the CO2 to react with, like iron, magnesium or calcium. However, in Iceland, in these basaltic rocks where Carbfix are injecting the CO2, a couple of things happen. First of all, the CO2 makes carbonic acid underground, which then dissolves some of the rock and releases calcium, magnesium and iron, particularly. They then combine with the CO2 to mineralise and form new carbonate minerals (8).

What we worked on way back, when I started publishing in the first PhD study, was distinguishing between CO2 dissolution and CO2 mineralisation in the subsurface. That work showed that we weren’t seeing evidence of mineralisation on a big scale in the natural reservoirs, but now, taking the same methods, we can use them to say that we are seeing significant amounts of mineralisation happening in the subsurface in Iceland.  That paper (9) came out earlier this year and shows that we can track the mineralisation that’s happening with Carbfix, and it aligns with previous estimates as well.

So is the focus of this new project to take all the skills and learning, but then use them in a different environment?

Yeah, I guess one of the things that’s always intrigued me as a researcher is doing things that are novel and challenging, so the way I see it, with what I term conventional CCS, is that that’s really getting towards the deployment stage. In terms of the science, it’s not like there isn’t more to do, but I think a lot of the fundamental science has already been done, and we’ve been working on that for quite a while. So, this was looking to see, right, we’ve got these tools, but can we now apply these to a different problem, knowing that they should work?  It’s never quite as straightforward or as easy as you think it will be, and certainly the fieldwork has been quite challenging at times, but fundamentally the science is robust. We’ve tested that in a variety of different places, and we’ve now tested and developed that in Iceland with Carbfix.

We’ve got Chris’s main paper from his PhD now published (9) and a patent application from that, then we’ve just done another one on the geothermal fluid chemistry and understanding a bit more about that. We’re also developing the more detailed time series of the mineralisation and seeing how that’s changing. One of the concerns with CO2 mineralisation is how long can you keep doing it for?

The goal of this project is to show that this is a useable toolkit – these inherent tracers are tools that you can actually use to monitor, report and verify CO2 mineralisation. One of the things that’s key for how Carbfix operate, and particularly their combination with Climeworks, is that they’re getting carbon credits for the CO2 that they’re locking away underground.  What we’re trying to do here is develop and help them to verify that that CO2 is being disposed of and permanently stored underground.

Papers

1. Using noble gas fingerprints at the Kerr Farm to assess CO2 leakage allegations linked to the Weyburn-Midale CO2 monitoring and storage project, International Journal of Greenhouse Gas Control, August 2017

2. Inherent Tracers for Carbon Capture and Storage in Sedimentary Formations: Composition and Applications, Environmental Science and Technology, July 2016

3. The inherent tracer fingerprint of captured CO2, International Journal of Greenhouse Gas Control, October 2017

4. Using oxygen isotopes to quantitatively assess residual CO2 saturation during the CO2CRC Otway Stage 2B Extension residual saturation test, International Journal of Greenhouse Gas Control, September 2016

5. Oxygen isotopes as a tool to quantify reservoir-scale CO2 pore-space saturation, International Journal of Greenhouse Gas Control, August 2017

6. Constraining O isotope equilibrium exchange between CO2 and water during fluid cycling in a closed system, International Journal of Greenhouse Gas Control, February 2025

7. Multi-Isotope Geochemical Baseline Study of the Carbon Management Canada Research Institutes CCS Field Research Station (Alberta, Canada), Prior to CO2 Injection, Earth Science, Systems and Society, January 2023

8. Reconstructing the temperature and origin of CO2 mineralisation in CarbFix calcite using clumped, carbon and oxygen isotopes, Applied Geochemistry, February 2024

9. Monitoring CO2 mineralisation and dissolution at CarbFix2 using inherent isotopes of CO2, H2O and noble gases, International Journal of Greenhouse Gas Control, March 2026