Abstract
Geo-fluids consisting of C-O-H volatiles are the main mode of transport of mass and energy throughout the lithosphere and are commonly found confined in pores, grain boundaries and fractures. The confinement of these fluids by porous media at the length scales of a few nanometers gives rise to numerous physical and chemical properties that deviate from the bulk behavior. Studying the structural and dynamical properties of these confined fluids at the length and time scales of nanometers and picoseconds respectively forms an important component of understanding their behavior. To study confined fluids, non-destructive penetrative probes are needed. Nuclear magnetic resonance (NMR) by virtue of its ability to monitor longitudinal and transverse magnetization relaxations of spins, and chemical shifts brought about by the chemical environment of a nucleus, and measuring diffusion coefficient provides a good opportunity to study dynamics and chemical structure at the molecular length and time scales. Another technique that gives insights into the dynamics and structure at these length and time scales is neutron scattering (NS). This is because the wavelength and energies of cold and thermal neutrons used in scattering experiments are in the same range as the spatial features and energies involved in the dynamical processes occurring at the molecular level. Molecular Dynamics (MD) simulations on the other hand help with the interpretation of the NMR and NS data. Simulations can also supplement the experiments by calculating quantities not easily accessible to experiments. Thus using NMR, NS and MD simulations in conjunction, a complete description of the molecular structure and dynamics of confined geo-fluids can be obtained. In the current review, our aim is to show how a synergistic use of these three techniques has helped shed light on the complex behavior of water, CO2, and low molecular weight hydrocarbons. After summarizing the theoretical backgrounds of the techniques, we will discuss some recent examples of the use of NMR, NS, and MD simulations to the study of confined fluids.
Highlights
All over Earth’s crust, fluids have a dominating role in transporting Earth’s energy and mineral reserves (Liebscher and Heinrich, 2007)
Exploring the behavior of C-O-H fluids confined in microand mesoporous systems provides a foundation for quantifying geologically relevant systems where mass and energy transport occur
Confinement of these fluids in porous systems gives rise to rich behavior that results from an interplay of geometrical restriction imposed by different pore sizes, the interaction between the porous material and the fluid, and the nature of the confined fluid itself
Summary
All over Earth’s crust, fluids have a dominating role in transporting Earth’s energy and mineral reserves (Liebscher and Heinrich, 2007). The effects of coupled reactive-transport processes seen in geological systems vary by the properties and reactivity of these crustal fluids over wide ranges of different parameters including temperature, pressure and fluid composition. A fundamental goal in geochemistry is to form a comprehensive and fundamental understanding of the thermophysical properties, structures, dynamics, and reactivity of complex geologic fluids and molecules (water and other CH-O-N-S fluids, electrolytes, and organic-biological molecules) at different length scales. Hydrocarbons, such as methane and ethane, CO2, and aqueous solutions can fill the pores or fractures of numerous types of complex heterogeneous Earth materials present in the systems outlined above. There is a common consideration that the combined structure and behaviors of bulk fluids are varied by solid substrates, confinement between two mineral surfaces, or in narrow pores due to the interplay of the length scales of the fluid and the confinement length scale (Gelb et al, 1999)
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