Technology & Cost

How feasible is the storage of CO₂ in oceanic crust given the distance from land?

In competition with alternative technologies the costs associated with CO₂ storage in the ocean crust is of central importance for the feasibility of this approach. Besides the regular economic costs like investments, costs associated with eventual environmental risks and social justice need to be taken into account as well to gain a holistic picture. These costs need to be weighed against the gains from storing respective emissions.

The Carbfix project on Iceland is a benchmark with relatively low costs because of the coastal and shallow marine sites. AIMS³ will do the CCS experiment on Reykjanes Ridge south of Iceland in appx. 1800 m water depth, which will be more expensive than Carbfix but potentially safer given that will be present in its liquid state. At the end of the. Project, we will try a cost estimate of distant MOR vs. Iceland vs. standard CCS in the North Sea, as planned within GEOSTOR.

How long does mineralization take?

Chemical reactions between the basaltic host rock and CO₂ loaded injection water have been shown to be rapid, resulting in over 95% permanent mineral CO₂ sequestration in under two years (Matter et al., 2016) as long as the CO2 concentration in the persolllated waters is low.

Mineralization is so quick because dissolution of CO₂ prior to or during injection ensures that chemical reactions between host rock and injected fluid begin to take place immediately after injection. The high reactivity and chemical composition of the basaltic host rock (up to 25% by weight of calcium, magnesium and iron that can combine with the injected CO₂ to form stable carbonate minerals) play an even larger role in the efficiency of permanent mineral storage in basalts.

AIMS³ will run a long-term CCS experiment where the key variables will be changed in pre-programmed steps. A seafloor lander will inject liquid CO₂and seawater at different mixing ratios as well as different pump rates. We plan to have one injection holes but at least two monitoring holes downstream (i.e. direction perpendicular away from ridge axis) where we measure and sample fluids that will be isotopically spiked.

It is known that in sedimentary basins such as these, seawater circulates at a steady pace and over distances of up to 50 kilometres through the upper basalt crust of the ridge flank. As long as the carbon dioxide is liquid and thus heavier than the seawater, such subsurface circulation would facilitate the storage and mineralisation of carbon dioxide – after all, it would help to distribute the injected carbon dioxide over a wide area in the basaltic rock.

AIMS³ CCS experiment – how does it work?

The CCS experiment at the Reykjanes Ridge is designed in at least three phases.

In phase 1 (summer 2022), preliminary geophysical exploration with bathymetric surveys, sub-bottom profiling and in situ heat flux measurements will take place. Background processes will be investigated with lander deployments and gravity sounding (baseline study). The data will be used to determine suitable drilling locations.

In phase 2 (summer 2023), a second expedition will take place to drill at least three holes with the MARUM MeBo seafloor drilling rig (see figure) through the deck sediments up to several tens of metres into the altered basalts. The boreholes will be cased in the upper part to have long-term access to the basalts. One borehole will be used later for CO₂ injection, the others for monitoring the processes in the crust.

In phase 3 (probably from summer 2025), a seafloor lander with tanks for liquid CO₂ and measuring sensors will be set down next to the injection well, and a submersible robot (ROV) will connect the system to the specially developed wellhead through which the injection will take place. The CCS experiment then begins along a pre-programmed schedule, according to which the CO₂ content and pumping rate are varied in different time steps. The other wells have measurement systems installed (Kopf et al., 2015).

Towards the end of the AIMS³ project, another expedition will remove the devices and replace them with new ones if necessary.