Physico-Chemical Behavior of Nanoparticles at CO2-Water-Rock Interfaces

Recently, to help mitigate global climate-change and energy problems, much effort has recently been devoted to developing methods for sequestering anthropogenic CO2 from coal-fired power plants. One of the most promising methods is geological CO2 sequestration (GS). Some prior studies of geological CO2 sequestration have mainly examined the physical processes that occur during the sequestration of CO2. However, most of the relevant studies are based on hydrological transport, using simulation models rather than studying actual interfacial chemical reactions in the ground. The mechanisms, kinetics, and environmental impact of interfacial reactions among CO2-H2O-mineral surfaces at the molecular scale have not been well understood. Changes in the porosity of the mineral phases at the geological formation sites, especially the dissolution of the mineral phase or precipitation of secondary minerals in the pores, will affect the fate and transport of CO2 and the integrity of seals and the matrix within the reservoirs. So far, little is known about the kinetics of the possible geochemical reactions of supercritical CO2 in brine and pre-existing mineral interfaces, or about the ultimate fate and transport of the injected CO2. We investigated the physico-chemical property changes of reference mineral samples (clay minerals) as well as field site samples (sandstone and caprock from the Illinois Basin) by chemical reactions at CO2-H2O-mineral interfaces. We investigated whether reactions between caprock and CO2 can change the integrity of caprock. Our experimental results with caprock samples (CONSOL coal mine sites, West Virginia) indicate that after 14 days in contact with 1 atm CO2 saturated saline water at 80°C, the concentrations of dissolved metals have increased from zero to as high as 47,000 ppm. In our experiments with caprocks and sandstones from GS sites of the Midwest Geological Sequestration Consortium, we found that the most significant extent of dissolution occurs within a day. This result implies that monitoring the earliest stage of reactions of sandstone with CO2 is crucial to understanding the CO2-water-rock interactions in deep saline aquifers. The second experiments investigated the effects of different reaction conditions on the reaction pathways and the extents of reaction. Freshly cleaved surfaces of phlogopite (KMg3(Si3Al)O10(F,OH)2) were prepared as thin layers. Using in situ high pressure (1000-1500 psi) and temperature (55-95oC) small angle x-ray scattering (SAXS) reactor, the real time reactions were monitored. After CO2 was injected into the microreactor, the scattering pattern changed due to surface morphology changes with time. This reaction occurs significantly, even within one hour. After each experiment, each sample was measured by atomic force microscope (AFM). By incorporating aqueous chemistry with in situ SAXS and AFM, we monitored real-time nanoscale reactions resulting from dissolution of pre-existing minerals and precipitation of new mineral phases at CO2-water-rock interfaces. Those morphological changes can be key information to understand the changes in porosity and permeability in GS sites.