CO₂ Mineralization in Basalts

  Geological CO₂ storage is one of the feasible technologies to remove anthropogenic CO₂ emissions on a large scale. Most industrial CO₂ storage projects inject CO₂ into sedimentary rock formations, such as deep saline aquifers and depleted oil and gas reservoirs. Injected CO₂ is sequestered by four trapping mechanisms – structural trapping, residual trapping, solubility trapping and mineral trapping. Structural trapping relies on overlying impermeable rock formations, such as shale layers, to contain the buoyant CO₂ plumes. In residual trapping, nonwetting CO₂ bubbles/ganglia are trapped in the micropore spaces of rocks. Furthermore, CO₂ can dissolve into resident brines of the rock formations, which accounts for the solubility trapping. Eventually, CO₂-charged brine reacts with the host rock and produces solid carbonates, which is the safest stage of the four trapping processes. However, mineralization reactivity between CO₂ and these sedimentary rocks is low, such that effective mineral trapping occurs at a time scale of hundreds to thousands of years. Therefore, CO₂ may remain mobile for a long period of time, causing risks of leakage, seismicity, groundwater contamination, and added cost of post-injection monitoring programs.

  In recent years, two pilot projects – the CarbFix project in Iceland and the Wallula project in Washington state, US, demonstrated that injected CO₂ in basalts (one of the mafic/ultramafic igneous rocks) can be mineralized to solid carbonates in 2 to 3 years. Such rapid mineralization of CO₂, to a large extent, eliminates the risks of leakage and others mentioned above, and significantly increases the safety and effectiveness of geological CO₂ storage. However, the physics and chemistry of basalt/water/CO₂ system under high-pressure high temperature (HPHT) geological conditions are far from well-understood.

  The first step of our research is to build an HPHT reactor assembly for basalt/water/CO₂ reactions that simulates subsurface geological conditions. In the reactor vessel, water-saturated basalt samples will be immersed in supercritical CO₂ and heated and pressurized to above 100 degrees Celsius and around 2000 psi. We will examine the before- and after-reaction properties of the basalt samples through advanced techniques such as NMR (Nuclear Magnetic Resonance) for characterizing pore structures, confocal imaging for rock thin slices characterizations, Scanning Electron Microscopy (SEM) for observing precipitate morphologies, SEM energy dispersive X-ray analysis for mineralogy, etc. The central task is to investigate how the complex pore structures/surfaces of basaltic rocks affect and get modified by the reactions in a CO₂/water two-phase reactive transport process. As supercritical CO₂ percolates into the rock sample, it contacts, displaces, and acidifies resident water, which then dissolves silicate minerals, releasing divalent cations, such as Ca²⁺, Mg²⁺, Fe²⁺, to precipitate the carbonate ions in water. The dissolution/precipitation process couples with multiphase fluid transport and surface processes in the micropores, leading to an evolving pore geometry and precipitate distribution/morphology/mineralogy. In addition, due to the complex mineralogy of basalts, the chemical pathways of the silicate dissolutions and carbonate precipitations are diverse, producing numerous non-carbonate secondary minerals. These minerals may compete for reactive surfaces in the pore spaces and lead to a phenomenon known as surface passivation that slows the carbonation rates. Our study may help identify how these small-scale physicochemical interactions and reactive transport map to overall carbonation rates under HPHT conditions, which is critical for successful future projects of CO₂ storage in basalts.