266008 Aqueous Carbon Dioxide Environments Under Silica Nano Confinement
The CO2 geological capture and sequestration have been considered potential approaches toward the mitigation of the greenhouse effects of CO2 release into the atmosphere, a process that relies on the low hydraulic permeability of caprocks resulting from fluid-substrate interfacial and confinement behavior. Fluids at interfaces and under confinement exhibit microstructural, dynamical, and thermophysical behavior quite different from their bulk counterparts, a feature that also highlights the inherent inability of current modeling approaches to describe properly the fluid-caprock interfacial mechanisms underlying the geological CO2 sequestration.
Experimental and modeling studies most often focus on the interactions of either water-rich CO2 or pure CO2 environments with representative caprock substrates, and the ensuing modeling is based on that assumption, i.e., the impact of the CO2-contaminants are rarely addressed, despite the fact that CO2 always carries small quantities of 'contaminants' including SO2, H2S, NOx whose impact in terms of rock interactions cannot be ignored.
The overarching goal of this investigation is to address key fundamental issues regarding the behavior at aqueous-CO2 solutions at the fluid-silica interface, including (a) how the degree of surface polarity and/or surface mismatch affect/s the interfacial structure, (b) how the overlapping of interfacial structures affects the confined fluid composition, and (c) how contaminants might affect the preferential adsorption and composition of the interfacial layers. For that purpose we carry out a molecular-based study of the microstructural and dynamical behavior of CO2-aqueous solutions at silica surfaces under extreme confinement at conditions relevant to geologic capture and sequestration of carbon dioxide, involving either water-rich CO2 or CO2-rich phases. This effort comprises (globally) isobaric-isothermal and (locally) grand-canonical molecular dynamics simulations of aqueous-CO2 systems whose unlike-pair interactions are described by a recently optimized force-field parameterization that provides an accurate and simultaneous representation of the water-rich CO2 and CO2-rich phases at LLE conditions . Moreover, we employ three types of slit-pore configurations to represent two extreme cases of surface polarity, and a mismatched pair of plates, to interrogate the fluid behavior at and confined between heterogeneous surfaces. Based on this study we illustrate how the interplay between these types of fluid-surface interactions and extreme fluid confinement, i.e., strong overlapping of interfacial structures, can induce a drying out of the pore environment whose immediate consequence is a significant enhancement of the pore CO2 concentration relative to that of the corresponding bulk environment . This behavior highlights some often overlooked implications including (a) the pore fluid environments cannot be represented by that of a bulk counterpart at the prevailing pore state conditions when either interpreting or modeling the process at a macroscopic level, (b) the chemical processes occurring in nano-pore aqueous environments strongly depend on the nature of the confining mineral surfaces, (c) the fluid environment in contact with the mineral surface behaves considerably different from either that a few molecular diameters away from the mineral surface or that representing the corresponding bulk, and (d) the competitive co-adsorption of CO2 contaminants at the mineral surfaces can radically modify the interfacial equilibrium and dynamical behavior, and consequently, the mineral surface chemical stability.