How much CO2 can Hellisheiði reservoir, Iceland store? A multiscale characterisation of porosity and permeability in basaltic rocks.

Carbfix is an Icelandic carbon sequestration company contributing towards global CCS efforts; they have been injecting CO₂ and H₂S into porous basalt at Hellisheiði, Iceland since 2012. The compounds are dissolved in H₂O and injected to depths of 500-800 m where temperatures range from 20-50 °C [1] at a 3:1 ratio of CO₂ to H₂S [2]. Once injected, the CO₂ dissociates into bicarbonate and carbonate ions which react with Ca²⁺, Mg²⁺ and Fe²⁺ ions present in the basalt, causing carbonate minerals to precipitate [3]. 95% of CO₂ mineralises within two years [4], and almost all H₂S mineralises to pyrite within four months of injection [1]. These short timescales make this process a valuable asset for rapidly reducing atmospheric CO₂ levels. The carbonate minerals are locked in solid form for geologically significant timescales, eliminating risk of leakage from the reservoir.

This style of reactive CCS can be considered anthropogenic alteration and analogous to low temperature alteration observed in many mineral deposits. As with mineral deposits, the location of mineral precipitation is governed by fluid chemistry, the reactivity of the rock mass, and crucially the porosity and permeability of the rock mass. Specifically in the case of CCS, we seek to understand not only where these minerals will precipitate, but additionally the potential storage capacity of the reservoir and how this changes over time. Callow et al. [5] concluded that Hellisheiði reservoir has a storage capacity of 0.33 Gt based on analysis of a single core sample. The heterogenous nature of Hellisheiði reservoir indicates that one sample is not representative of the whole reservoir.

This project aims to enhance understanding of the preferential fluid pathways and storage capacity at Hellisheiði reservoir using a range of samples and techniques including X-ray CT scanning and laboratory experiments to understand porosity and permeability, and optical microscopy, XRD and analytical SEM to understand mineralogy. This poster presents current analysis and findings from X-ray CT, optical microscopy and laboratory experiments.

References:

[1] Snæbjörnsdóttir S Ó et al. (2017) Int J of GHG Control 58:87-102

[2] Clark D E et al. (2020) Geochimica et Cosmochimica Acta 279:45-66

[3] Matter J M et al. (2011) Energy Procedia 4:5579-5585

[4] Matter J M et al. (2016) Sci 352:1312-1314

[5] Callow B et al. (2018) Int J of GHG Control 70:146-156