Microanalysis of natural and engineered CO2 mineralisation at CarbFix2 site, Iceland

CO2 capture and storage (CCS) is the only industrial scale technology that can directly reduce the CO2 emissions produced by the combustion of fossil fuels or industrial process emissions. Given global reliance on fossil fuels for energy and industrial manufacturing needs, CCS is an essential technology for the global drive to reach net zero CO2 emissions to the atmosphere4. The success of geologic CO2 storage critically depends on the long-term security of the storage site. Buoyant gas phase CO2 stored in sedimentary reservoirs requires long duration monitoring of storage. Alternatively, secure storage can be guaranteed through the permanent conversion of injected CO2 into new carbonate minerals within the storage site.

Several tests of the sequestration that occurs when CO2 is injected into reactive basalts have recently been undertaken, notably the CarbFix1 project, located at the Hellisheidi geothermal field in Iceland[1]. Measurements of dissolved inorganic carbon and tracers injected with the CO2 indicate that mineralisation of >95 % of the injected CO2 (175 tonnes) was achieved within two years1. However, feasibility of the method at an industrial scale is still uncertain, as the key question of how long viable injectivity can be maintained is unknown. To address this question, the CarbFix2 experiment was launched in 2014, and currently 10,000 tonnes of CO2 and 5,000 tonnes of H2S are being injected per year into the geothermal field[2]

Key research questions:

-Identify where natural CO2 mineralisation has occurred in the field prior to injection based on observations made during drilling and tracer tests by Reykjavik Energy.
-Resolve the conditions under which natural CO2 precipitation took place during growth of individual crystals using world-class microbeam analysis and geochemical modelling.
-Establish baseline geochemical fingerprints prior to engineered injection of CO2 and H2S
-Identify isotopic signatures of CarbFix1 carbonates from downhole samples.
-Undertake field visit to surface exposures of natural analogues. Well-studied Jurassic ophiolite complexes in Liguria (Italy) host ophicalcites of carbonate mineralised ocean floor, which provide spatial information and constraints on the preservation of mineralisation pathways and geochemical fingerprints over time.
-Reproduce the mineralization process on downhole cutting samples experimentally and image it with synchrotron-based X-ray microtomograph. From these experiments, dissolution and re-precipitation of minerals will be mapped and quantified.
-4D microtomography experiments, varying temperature and salinity, will assess if CO2 mineralisation can be enhanced through purposeful injection strategies into hotter or colder parts of the reservoir with changed brine salinity.

Methodology and timetable:

This project will utilise the world-class ion microprobe (SIMS) instruments within the School of GeoSciences (Cameca 1270 and new Cameca IMS 7f-geo). You will make micro-analytical traverses across individual crystals from borehole cuttings and from mineralised basalt core samples. The micrometre growth zones within each crystal, like the rings on a tree, record the isotope and trace element signatures of porewater during the crystal growth. The growth duration, temperature, CO2 and water origins of natural carbonates will be reconstructed.

These analyses will be complemented by mineralisation experiments in a novel x-ray transparent reaction cell that allows monitoring fluid-rock interaction with time-resolved synchrotron x-ray microtomography. These will use downhole cuttings samples to directly map and quantify dissolution and re-precipitation of minerals at reservoir conditions[3]. The knowledge generated will be used to ascertain how CO2 mineralisation can be enhanced through varied injection strategies and inform how to accurately incorporate such controls into predictive models of CO2 mineralisation in this reservoir.