Trapped in Minerals Underground: The Future for CO2?

Scientists have determined that atmospheric CO₂ levels are at an all-time high and are continuing to increase at an unprecedented rate. The consequences are rising global temperatures and sea levels and the subsequent impacts on social, ecological, and climate systems (Ghommem et al., 2012; van den Bergh and Botzen, 2014; Anderson et al., 2016). So how can we minimize the increasing concentration of CO₂ in the atmosphere? One promising approach is to permanently store CO₂ underground in minerals―a strategy that the newly funded EFRC, Geo-processes in Mineral Carbon Storage (GMCS), is currently investigating.

The idea of storing carbon underground is not novel; in fact, the injection of CO₂ into sedimentary basins has been studied at length. In this approach, CO₂ is physically stored, primarily in its gaseous form, in a reservoir under an impermeable cap rock. This mechanism of “structural trapping”, however, has several key challenges: (1) sedimentary rocks are limited in their divalent metals (e.g. Ca²⁺, Mg²⁺) needed to form carbonate minerals, meaning that CO₂ storage through mineral trapping, the most secure storage mechanism for CO₂, could take thousands of years (Metz et al., 2005); and (2) because most of the carbon will remain in its gaseous form for some time, there is a risk that CO₂ could leak to the surface if the integrity of the caprock is not ensured (Snæbjörnsdóttir et al., 2020). Consequently, the development of CO₂ sequestration operations in sedimentary rocks has remained at a standstill.

Mafic and ultramafic rocks, which are comprised of silicate minerals containing high concentrations of the divalent cations Ca²⁺, Mg²⁺, and Fe²⁺, have recently emerged as more suitable rock types for carbon mineralization. These rock types, including the well-known basalt, and lesser-known peridotite (a rock mainly composed of the mineral olivine), are highly reactive because their cations can rapidly react with dissolved carbon in solution to form carbonate minerals, permanently storing CO₂ (Fig.1). Moreover, their widespread distribution and abundance in the Earth’s crust, composing approximately 10% of the continents and a majority of the seafloor (Matter et al., 2016), makes them an ideal reservoir for carbon storage. In fact, this process has already been successfully tested at the pilot project scale. In two years, the Carbfix site in Iceland has mineralized >95% of injected CO₂ (Pogge von Strandmann et al., 2019), while ~60% is estimated to have mineralized at the Wallula Basalt Project near Wallula, Washington (White et al., 2020). But what happens when we scale these projects to store CO₂ at the levels required to minimize the effects of current and future emissions (at the gigaton-per-year levels)?

Carbon mineralization takes place at the fluid–rock interface, and thus the extent and longevity of CO₂ storage hinges on the available reactive rock surface and the ability for the CO₂ charge to flow through the rock mass, i.e., through the reservoir’s rock fracture network and into the pore spaces. An important question that remains in the expansion of current pilot projects is whether long-term carbon mineralization in mafic and ultramafic rocks will “clog” these reservoirs, retarding the flow and reducing the reactive surface area, or whether mineralization will be enhanced by processes such as reaction-driven cracking. In this case, the newly formed minerals would induce enough stress on the surrounding rock to produce fractures and increase the reactive surface area. Thus, to delineate these complex processes to enable accurate estimation of the rate of mineralization and amount of carbon that can be stored in full-scale carbonation operations, it is vital to elucidate the coupled thermal, hydrologic, mechanical, and chemical (THMC) processes that occur at all scales during the injection of CO₂ into mafic and ultramafic reservoirs.

The GMCS EFRC will tackle this complex, multiscale research question by studying the reaction, flow, and fracture processes that occur during mineral carbonation in mafic and ultramafic rocks over three distinct scales. GMCS has identified three themes that guide its investigation (Fig. 2): (1) the porous medium scale, where mineralization reactions take place; (2) the fracture-porous medium scale, where the CO₂ is delivered to the surrounding rock; and (3) the fracture network scale, within which the CO₂ charge is distributed to the fracture network system. Through the development and application of analytical and numerical modeling, laboratory experiments, and sensing techniques at these scales, the center will evaluate the capacity and longevity of CO₂ sequestration in mafic and ultramafic rocks. The research produced by this multidisciplinary center will provide a path forward toward the development of full-scale carbon sequestration operations.