Nano-scale structure of Geofluids in Porous Silica and Montmorillonite Clay

Earth's crust, composed of different rocks with varying degrees of nm- micron scale porosity, is source and reservoir of geofluids, and target for geologic carbon storage (GCS). The specific fluid-rock interactions control formation of fluid deposits, subsurface fluid mobility and mixing, and enhanced recovery processes. Rock pore characterization includes surface chemical identity, pore size distribution, ratio of connected to unconnected porosity and surface roughness. The properties of fluids confined in these pores are altered from bulk due to surface-fluid interactions and confined geometry effects. Changes in density, freezing temperature, and diffusion properties of pore fluids have been observed. Using a combination of neutron scattering and excess sorption measurements the physical properties of pore fluids can be quantified. We study both model systems with well-defined pore morphologies and natural rocks with fractal pore characteristics. Synthetic Porous silica glasses possessing tunable pore sizes of 8 - 50 nm serve as proxies for quartz-rich rocks, including sandstones. Natural rocks studied are sandstone, limestone, and shale. Excess sorption isotherms to silica aerogel and mesoporous CPG10 were measured using a high-pressure sorption balance and a vibrating tube densimeter. Strong adsorption of CO2 to the silica surfaces was found at low fluid pressure, followed by formation of a maximum in the excess sorption isotherm. The excess sorption took small and finally even negative values at high pressure. An inverse temperature dependence of the sorption strength was found in the adsorption region at low and intermediate pressure, while the excess sorption showed little temperature dependence at high pressure. A shift of the excess sorption maximum to higher fluid density was observed with increasing pore width. From small-angle neutron scattering data the density and volume of the sorption phase of pore CO2 was calculated using the Adsorbed Phase Model. The sorption behaviour was modelled using Grand Canonical Monte Carlo simulation, which exactly reproduced the excess sorption isotherm data under the assumption of a weakly attractive solid-fluid interaction potential. Caprocks overlying the porous reservoir rock serve to retain buoyant plumes of CO2. Caprocks can be comprised of thick layers of clay or mudstones, thought to be impenetrable to CO2. To quantify the interactions of caprock with CO2, we measured the excess sorption of supercritical CO2 at Na-montmorillonite clay, a proxy for cap rock materials. Very limited amounts of CO2 adsorbed to this clay mineral at low fluid densities. Using neutron diffraction, the change of the clay interlayer spacing was measured as a function of the CO2 density. A jump-like increase of the interlayer spacing upon CO2 addition was found at low pressures, and remained constant with further additions of CO2. These results indicate suitability of montmorillonite clay for carbon storage caprock applications.