Laboratory Experiments to Understand CO2 Reactivity
Complementary techniques offer unique advantages
Laboratory experiments are being conducted at PNNL to discover and demonstrate general truths about subsurface conditions relative to geologic carbon storage. Experimental results are filling gaps in the current knowledge and leading to the development of an associated suite of tools for application in research and the field. Researchers are gaining a better understanding of caprock-greenhouse gas reactions, acquiring much-needed thermodynamics and kinetics data about wet supercritical carbon dioxide (scCO2)-mineral interactions, and leveraging EMSL capabilities to develop one-of-a-kind instrumentation to observe reactions in situ at scCO2 pressures and temperatures to understand reservoir and caprock properties. EMSL, the Environmental Molecular Sciences Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at PNNL.
Novel high-pressure in situ probes developed under PNNL’s Carbon Sequestration Initiative are unmatched internationally. Their development involves an infrared spectroscopic titration system, a hyperbaric atomic force microscope (AFM), a magic angle sample spinning nuclear magnetic resonance (MAS NMR) spectroscopy capability, and a x-ray diffraction (XRD) system. Together, these capabilities form a powerful toolkit for studying wet supercritical carbon dioxide (scCO2)-mineral reactions in situ. PNNL’s strategy is to integrate results from these techniques, while capitalizing on their advantages.
Study of Wet scCO2-Mineral Reactions
Research at PNNL has focused on in situ investigations of the reactions of variably wet-scCO2 with a range of geologically relevant materials, including 1) water partitioning and clay expansion in the scCO2-H2O-montmorillonite system, and 2) mineral carbonation in the scCO2-H2O-forsterite system.
Water Partitioning and Clay Expansion in the ScCO2-H2O-Montmorillonite System [+ expand/ - collapse]
Caprocks of reservoirs targeted for geologic carbon sequestration, such as the Eau Claire Formation, contain high concentrations of clay minerals, including expandable montmorillonites. While clay expansion will increase solid volume and could lead to closure of fractures, clay shrinkage could open fractures and compromise the seal. PNNL researchers report concentrations measured in situ of water partitioned to montmorillonite or scCO2 at 50°C and 90 bar, and these data are correlated with interlayer spacing. Direct molecular-level information from spectroscopic measurements suggests competition for interlayer residency: increasing concentrations of intercalated water lead to decreasing concentrations of intercalated CO2.
Mineral Carbonation in the scCO2-H2O-Forsterite System [+ expand/ - collapse]
Basaltic formations have high reactive potential for mineral trapping because they contain an abundance of divalent-cation containing silicates, such as forsterite. Results from in situ spectroscopic titrations of forsterite with dissolved water in scCO2 at 90 bar and both 35 and 50°C indicate that at low total water concentrations, only highly structured water and bicarbonate are detected at the forsterite surface. Eventually a critical water concentration is reached where the bicarbonate concentration decreases and magnesite precipitates. PNNL researchers speculate that an adsorbed water film, once thick enough, becomes a conduit to transport ions to the nucleation sites, thereby enabling the construction of the magnesite crystals detected in their spectra and observed by ex situ scanning electron microscopy.
The results of these studies provide important insights into clay hydration and metal silicate carbonation mechanisms in low water scCO2environments. They also constrain thermodynamic models and molecular dynamic simulations used to predict volume changes in high clay content caprocks and mineral trapping extents in basaltic host rocks.