Site selection for geological storage of CO2 in Switzerland

Carbon capture and storage operations (CCS), also known as sequestration, is now a widespread practice and a viable path for reducing atmospheric carbon dioxide (CO₂) concentrations with implication on climate change and global warming. Successful sequestration of CO₂ in deep formations (called sinks) 800 metres or more below ground provides a permanent storage solution for anthropogenic CO₂ emitted from large industrial sources (e.g. cement factories, waste incinerators or coal-fired plants, see figure 1). CO₂ sequestration is vital for ensuring environmental sustainability by cutting down on greenhouse gas emissions. Potential subsurface storage targets for CO₂ sequestration encompass depleted hydrocarbon reservoirs, aquifers (saline formations), un-mineable coal seams, organic-rich shales and basaltic formations.

In Switzerland, despite the promising theoretical storage capacity potential of 2670 million tons of CO₂ for the Swiss Molasse Basin (Chevalier et al., 2010), the possibility for suitable large-scale underground storage for CO₂ still needs to be assessed accurately. A major challenge is an accurate regional screening leading to the identification of candidate sites suitable for pilot-scale or commercial-scale storage of CO₂ in Switzerland. Especially, the scarcity or lack of available or reliable geological data hamper this challenge even more. Another challenging issue is the vertical and lateral subsurface geological complexity and heterogeneity. Despite those challenges, the scientists will carry out a site screening activity under the context of the ELEGANCY project to identify potential areas suitable for underground CO₂ storage in Switzerland.

Figure 1: Schematic diagram for CO₂ storage at a potential site in Switzerland.

The goal of this study is to identify candidate sites in Switzerland that fulfil the necessary conditions for the safe injection and long-term storage security of CO₂ in accordance to stipulated global best practices (e.g. Aarnes et al., 2019; NETL, 2017). This involves quantifying the important properties necessary for CO₂ injection and storage sites. Once the quantifications are done, the scientists will model the CO₂ plume in the targeted storage formation and assess the risk and associated uncertainties. To achieve this, they will examine existing subsurface geological data. More specifically, they will search for subsurface structures with regional 2D-seismic data. Additionally, they will examine a large set of non-geological and cultural data (i.e. proximity to CO₂ point sources such as incinerators and cement factories, available volumes of CO₂, conflicts of subsurface use, etc.).

The scientists proposed a screening workflow considering the subsurface geological framework of Switzerland, its underground usage in terms of existing, on-going and future geo-energy, nuclear waste repository projects, the subsurface manifestation of fluids and the envisage regulatory framework. The workflow they developed in this study (see figure 2) should serve as a guideline and would be ideally applicable for any promising geological CO₂ storage site in Switzerland.

Potential sites currently under investigation in Switzerland include several locations where the underground geology is known thanks to the existence of old deep wells previously drilled for hydrocarbon explorations (see figure 2). Those previous findings of hydrocarbon in the Swiss subsurface might be an interesting indication of suitable subsurface characteristics such as reservoir occurrence and transmissivity. However, further investigations need to be carried out to accurately estimate connectivity, storage capacity and related risks (i.e. induced seismicity) associated with CO₂ injection.

In general, regions with depleted natural gas reservoirs are potential sites and suitable for CCS based on the availability of storage capacity, proven caprock/trap, solid knowledge of the storage reservoir and existing surface infrastructure. In addition, reservoirs at depth below 3000 metres underground are associated with geothermal temperatures above 100° Celsius. This may offer the possibility to generate electricity by using CO₂-Plume-Geothermal (CPG) systems where the CO₂ injected in the reservoir is then employed at the surface to operate the turbine and generate electricity (Randolph and Saar, 2011, see figure 2).

The site screening and selection studies are currently on-going in the context of the ELEGANCY project to identify geological structures capable of trapping CO₂ as a pilot or commercial scale CCS operation. The final workflow will include robust conceptual geological models for the identified sites, which will lead to the building of 3D static geo-cellular models. The scientists will then model the identified aquifer or reservoir intervals with relevant porosity and permeability values. Given the broad range of subsurface uncertainties and reservoir parameter variability, they will perform probabilistic based volumetric assessments of storage capacity. Ultimately, the scientists will use the 3D static models as input for dynamic simulation to understand the fate of the CO₂ plume and as a basis for geo-mechanical modelling to predict induced seismicity associated with injectivity tests.

Dr. Ovie Emmanuel Eruteya

Prof. Dr. Andrea Moscariello