2025 PESA WEBINAR SERIES: Tracking the Fate of CO2 from Switzerland to Iceland with Geophysical Methods

In-situ CO2 mineral storage is an effective way of reducing greenhouse gases and fighting global climate change.This technology is particularly interesting for countries that cannot rely on large CO2 storage capacities. Geophysical techniques can help characterise and monitor the storage reservoir. Seismic time lapse, electrical resistivity tomography (ERT) and gravity measurements are often used to track the CO2 plume. In the few existing in-situ mineral storage sites, monitoring relies almost only on geochemical methods.Geophysical methods are less established in this context. Seismic time lapse measurements are thought to be less efficient because the CO2 is dissolved in water before it is pumped in, so the local water is replaced by the CO2-enriched water, which makes the seismic velocity changes too small to detect.Carbonate precipitation happens slowly and makes very small velocity changes, mainly because the porosity decreases. ERT involves replacing the existing groundwater with CO2-charged water. This should lead to a decrease in resistivity. However, resistivity is only expected to increase when carbonates precipitate. Here, we present the results of testing geophysical methods to measure and check the CO2 storage site in Helguvik, Iceland, operated by CARBFIX. This study is part of the Swiss DemoUpCARMA project (https://demoupcarma.ethz.ch/) and uses a variety of scientific methods that have already been successfully tested in other CO2 storage experiments (e.g. Zappone et al., 2021). The Helgivik site is a pilot were CARBFIX is testing the efficiency to use sea water for dissolving CO2, in preparation for an upscaling of the site, the Coda Terminal that will receive and store 3 million tons of CO2 per year.

The pilot storage site lies on the Reykjanes Peninsula in Southwest Iceland, at about 15 km distance from the seismically and volcanically active rift zone. The CO2 is captured in Switzerland are shipped in isotainers (https://demoupcarma.ethz.ch/) to Helguvik . Here, after mixing with saline water, it is injected into the reservoir through a vertical borehole (CBI-01; Fig. 1) with an open section between 250 m and 420 m depth. Two additional vertical wells, CBM-03 and CBM-01, with depths of approximately 400 metres, have been drilled at distances of 30 metres and 100 metres along a northwest-southeast alignment, respectively, for the purpose of monitoring reservoir processes. Rock samples obtained from drill cuttings have been utilised in the characterisation of the stratigraphic sequence of the reservoir, in conjunction with borehole logging data. Furthermore, drill cores extracted from proximate boreholes have undergone laboratory analysis with respect to porosity network and flow properties, both prior to and following il laboratory exposure to CO2-rich saline water. Prior to and during the injection operations, cross-hole seismic measurements were conducted utilising a P-wave borehole sparker source and hydrophone chains. Simultaneously, single-hole electrical resistivity measurements were performed in all the wells. The background seismicity and the seismicity potentially induced by the injection operations were monitored via a backbone seismic network installed around the injection site, and by a seismic array of 3D nodal geophones. The data were streamed in real time to ETH Zurich and shared with all the project partners.

The multi-disciplinary approach reveals variability of the porosity in the basaltic layers, already evidenced by laboratory measurements (Stavropoulou et al, 2024), depicting a stratified velocity distribution with depth Single-hole electrical resistivity data corroborate the layering and is in agreement with the mineralogical data from cuttings. The remarkable consistency between ERT and crosshole seismic observations (Junker at al., 2025 ) highlights the efficiency of the methods to resolve thin layering structures and giving additional constraints to the borehole logging observations for permeable layers. Characterization with geophysical tools allows the continuity of the underground structures to be observed, in contrast to point borehole measurements. A permeability model, based on the crosshole seismic data is found to be in agreement with borehole spinner tests. Modeling on velocity anomalies due to precipitation of Carbonates reveal that that crosshole seismic can detect the precipitation of c.a. 17 kg/m3, corresponding to c.a. 100 t of injected CO2 at the scale of the pilot project.

Acknowledgements: This study is part of the DemoUpStorage project that was funded by the Swiss Federal Office of Energy (SFOE, project number SI/502429).