The pre-injection characterization of the Coda Terminal CO2 storage site, Iceland

The permanence of underground CO₂ storage using the Carbfix technology relies on neutralization of the injected acidic CO₂-charged water through its interaction with the host rock consisting of basalt. The CO₂ solubility and alkalinity trapping followed by carbonate mineral precipitation has been confirmed in various projects that utilized this technology (e.g., Carbfix, Carbfix2, GECO). However, CO₂ storage through carbon mineralization has only insubstantially contributed to the global CCS activities so far. Therefore, it is necessary to develop projects where CO₂ is injected and stored at a megatonne scale. One of these projects is the Coda Terminal located in Straumsvík in the Hafnarfjörður municipality, SW Iceland. Preparations for the project began in 2021 on the frontend engineering design, licensing processes and reservoir characterization. Full-scale injection of 3 MtCO₂ is planned to be reached in 2032. 

The storage reservoir´s bedrock consists of five lithological units: Holocene basaltic lava flows, basaltic lavas, glassy basalts, hyaloclastites and sediments. These layers are similar in terms of their mineralogical and petrological characteristics. Up to date, seven wells have been drilled in the injection area that intersect all but the hyaloclastite layers. The deep wells include the injection well CSI-01 with a vertical depth of 982 m, and two monitoring wells CSM-01 and CSM-02 with depths of 618 and 700 m, respectively. The main minerals identified in the collected drill cuttings are plagioclase, pyroxene, olivine, zeolites (analcime, chabazite, clinoptilolite) and quartz. The average divalent cations oxides content for CaO, MgO, and Fe₂O₃ is 11, 9, and 12 wt%, respectively and it is similar to the composition of the host rock in storage sites currently operated by Carbfix.

Major feed zones in the wells were identified below 300 m depth based on well logging data and step rate injection tests. The water discharged from CSI-01 is saline with a conductivity of about 40,000 μS/cm. In contrast, water from the main feed zones in CSM-01 and CSM-02 is fresh to brackish with a conductivity of 600-1000 μS/cm. Samples collected at various depths in CSM-01 and CSM-02 using a deep sampler show, however, a variability in the chemical composition with depth compared to the composite water pumped from the wells. The dissolved elemental ratios in the water indicate a substantial depletion of B and Na relative to Cl in brackish to saline samples when compared to seawater. In contrast, an enrichment in B and Na relative to Cl is observed in fresh samples. Enrichment in Ca relative to Cl is seen in both fresh and saline samples. The variability in the reservoir water chemical composition with depth and distance from the coast indicates aquifer chemical stratification. This adds to the complexity of the storage reservoir and its surrounding formation. Data obtained from future wells and the pilot injection will verify and broaden our current knowledge about the aquifer including the origin of the groundwater, water-rock interaction, water residence time in the reservoir, and the depth of saline-freshwater interface.