Verifying in-situ CO₂ mineralisation using inherent isotope tracers

Abstract

Mineralisation of CO₂ in the subsurface is being deployed at scale as a means of durable carbon storage and climate change mitigation. Current monitoring and verification of CO₂ mineralisation uses indirect and added geochemical tracers. In this thesis, I test the capability of inherent CO₂, H₂O and noble gas isotope ratios to monitor and verify in-situ CO₂ mineralisation. These tracers are components of the injectate chemistry, meaning they directly track CO₂ fate without the additional costs of added tracers. Samples from three in-situ CO₂ mineralisation projects in Iceland are analysed. Each project is at a different operational stage (pre-injection/injecting/postinjection) to assess how inherent tracers can be used across a project lifespan.

First, I use clumped isotope analysis of calcite (CaCO₃) precipitated on a downhole pump during the CarbFix project to reconstruct the temperature and origin of CO₂ mineralisation. Clumped isotope values (Δ₄₇) correspond to mineralisation temperatures of 45-51°C. These are 10-16°C warmer than pre-injection water temperatures measured at the depth of calcite growth. I attribute the warmer temperatures to continuous water pumping from the monitoring well, drawing up waters from deeper in the storage reservoir to the pump depth. Calculated parent fluid oxygen isotope values (δ¹⁸O) match local well and meteoric water records, and calcite carbon isotope values (δ¹³C) conform to a CarbFix injection source. These results validate previous CarbFix findings that subsurface fluid migration and CO₂ mineralisation occurred within porous media of the CarbFix storage reservoir.

Next, I use shifts in inherent noble gas, CO₂ and H₂O isotope ratios to quantify CO₂ dissolution and mineralisation at the CarbFix2 project. Reductions in CO₂/³He in the CarbFix2 scrubbing tower are indicative of 50% (± 4%) CO₂ dissolution, similar to previously published capture rates. From this, I calculate the isotopic fingerprint of the injected dissolved CO₂ and compare these values to measured monitoring well data. Lower CO₂/³He and higher δ¹³CCO₂ values are recorded relative to expected values if no mineralisation of CO₂ was occurring in the subsurface. Monitoring well δ¹⁸OH₂O data confirm the presence of CarbFix2 injection fluids. δ¹³C꜀ₒ₂ data intersect the modelled evolution of isotope values resulting from CO₂ mineralisation at reservoir temperatures, indicating similar extents of mineralisation as previously published. Further work is needed to quantify uncertainties regarding background fluids and temporal variation of isotope values. Despite this, these results are promising for the wider application of inherent tracers to in-situ CO₂ mineralisation because future projects will have greater distinction between injected and background CO₂.

Finally, I establish a pre-injection baseline dataset of inherent noble gas, CO₂ and H₂O isotope ratios for the CO₂ injection planned at Nesjavellir geothermal field. From these data I predict the isotopic fingerprint of water-dissolved CO₂ and model how this will evolve once injected. Dissolved CO₂ is expected to be distinct from baseline isotope values measured in production wells connected to injection well NJ-18 (NJ-25 and NJ- 31). However, the 70% mineralisation expected at Nesjavellir reduces this distinction, resulting in similar isotope values to those measured in production wells. NJ-31 may record increased δ¹³C꜀ₒ₂ values by up to 0.8‰ but shifts in NJ-25 are unlikely to be distinguishable from baseline values. Post-injection sampling would test the predictions of this work and evaluate the variability of isotope values.

In summary, the work contained in my thesis contains new evidence about the conditions and efficiency of in-situ CO₂ mineralisation. Inherent isotope tracers should be applied to future mineralisation projects as monitoring and verification tools.