Basalt as a CO2 storage reservoir

Harnessing a natural process, mineralization of CO₂ is the pro­cess by which primary and secondary minerals such as Mg, Fe, and Ca pres­ent in basalts interact with CO₂-enriched fluids to con­vert it into solid carbonates.

The main benefit of employing basalts for CO₂ sequestration is the rapid timescales for CO₂ conver­sion into solid form, which is in the order of months to years as opposed to the thousands or millions of years, if at all, anticipated for most traditional sedi­mentary storage systems. By using basalts for CO₂ sequestration, the risks as­sociated with unexpected plume migration and long-term seal integrity can, therefore, be avoided.

The conventional ener­gy sector has rarely target­ed basalts specifically, and our knowledge of these se­quences as reservoirs is still in its infancy. But that may be set to change.

With pilot projects in the USA and Iceland, where CO₂ has successfully been injected into basalts to test the process of minerali­zation, these extrusive rocks have become more impor­tant in the discussion of permanently storing CO₂ in the subsurface, especial­ly in areas where suitable sedimentary storage is not available.

Economically speaking, the best places to store CO₂ are close to high CO₂ emit­ters, which may not always be close to young, reactive volcanic sequences or safe sedimentary storage sys­tems that are appropriate for conventional storage. For this reason, it is critical to understand whether CO₂ storage is feasible for the range of geology available to us. Furthermore, young and relatively fresh basalts are not ubiquitous, and the reservoir properties of older basalts with more complex or protracted burial histo­ries is not well understood. To form a better under­standing of the variability and heterogeneity of basal­tic reservoirs, I am study­ing the reservoir properties of basalts of different ages from the UK and abroad in terms of porosity, perme­ability and mineralogy to investigate their suitability for CO₂ disposal.

It is estimated that the cost of CO₂ storage in basalt is currently between $10- 30/tCO₂. However, a num­ber of variables, including the pH, temperature, and availability of reactive pore surfaces in the system, have a significant impact on the cost and rate of storage in a particular sequence. There­fore, to assess the suitability of a sequence for CO₂ stor­age from both geological and economic perspectives, I am looking to understand the local vertical and later­al variations in mineralogy and permeability distribu­tions and understand their effect on reservoir quality.

Exploring the feasibility of applying basalt storage techniques in other regions of the world also requires an assessment of other selec­tion criteria, such as prox­imity to CO₂ source, and the availability of transport infrastructure and water. For example, the Carbfix project in Iceland currently requires 22 tonnes of wa­ter to dissolve one tonne of CO₂. Therefore, imple­menting similar projects on other regions of the world requires consideration to the availability of water and the potential costs associat­ed with transporting that water to the injection site.