Using nanofluidics and microscopy to study unconventional pore-scale transport phenomena
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Shales are unconventional geologic media primarily composed of nanopores. Once considered impermeable by conventional reservoir descriptions, these media have received attention in recent years due to their vast energy and sequestration potential. Actuating and quantifying fluid flow through shale matrix remains a formidable challenge. Nanofluidics (nanoscale lab-on-a-chip devices) are a promising approach to studying fluid transport anomalies in tight porous media, including shale, because they allow visualization of fluid phenomena and control of synthetic nanoscale geometry. Readily fabricated nanoscale "reservoir-on-a-chip" devices enable testing of geometry- and nanoconfinement-related hypotheses alongside core data. This dissertation discusses nanofluidic studies in different-sized nanochannels and nano-networks and the fabrication of these devices, including first of their kind "shale-on-a-chip" nanomodels. Most experiments documented herein were performed within two-dimensional (2D) silica nanochannels as small as 30 nm x 60 nm in cross-section; foundational results for other geometries are presented as well. Anomalous fluid transport trends were revealed through nanoscale imbibition experiments. Liquid imbibition was captured with fluorescent microscopy and reflected differential interference contrast microscopy; dynamic flow data are rare in geometries that are nanoscale in two dimensions due to the limited resolution of optical microscopy. Imbibition of various wetting liquids in the arrays of horizontal, 2D silica nanochannels consistently demonstrated substantial divergence from the imbibition speeds predicted by the continuum Washburn equation for capillary flow as a function of hydraulic diameter and liquid type. Non-Washburn or non-diffusive front length-versus-time dynamics were also observed. These findings and other atypical imbibition data presented herein are explained by the enhanced influence the following phenomena at the nanoscale: surface forces at fluid-solid boundaries, the presence of quasi-crystalline thin films or boundary regions, and potential solid surface or boundary layer deformation due to meniscus-induced negative pressures (suction). This dissertation presents an experimental method and corresponding image and data analysis scheme that enable identification of the origin of imbibition irregularities in terms of transport variables: independent effective values of nanoscale capillary pressure, liquid viscosity, diffusivity, and interfacial gas partitioning coefficients were determined from imbibition within the tested nanochannels. The method can also be used in nano-networks and nanoporous materials. Phenomenological models were derived to match the nanofluidics data and include descriptions of effective diameter, effective capillary pressure, and effective liquid viscosity. The scalable implications of these findings and models for tight rocks and nanoporous materials are discussed in the context of fluid transport in shale. A complementary study was conducted into the utility of digital rock physics in three dimensional models of nanoscale resolution rendered from focused ion beam scanning electron microscopy (FIB-SEM) images. Results indicate that FIB-SEM images below ~5000 µm³ volume (the largest volume analyzed) are not a suitable volume for extracting representative shale pore-scale networks, permeability, and other fluid transport properties. These findings strengthen the usefulness of nanofluidics in the study of unconventional rocks: nanofluidics fills the gap in quantifying pore-scale transport mechanisms where digital rock physics and indirect core analysis methods have limited scope and/or resolution.