Browsing by Subject "FIB-SEM"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Pore-scale numerical modeling of petrophysical properties with applications to hydrocarbon-bearing organic shale(2013-12) Shabro, Vahid; Torres-Verdín, Carlos; Sepehrnoori, Kamy, 1951-The main objective of this dissertation is to quantify petrophysical properties of conventional and unconventional reservoirs using a mechanistic approach. Unconventional transport mechanisms are described from the pore to the reservoir scale to examine their effects on macroscopic petrophysical properties in hydrocarbon-bearing organic shale. Petrophysical properties at the pore level are quantified with a new finite-difference method. A geometrical approximation is invoked to describe the interstitial space of grid-based images of porous media. Subsequently, a generalized Laplace equation is derived and solved numerically to calculate fluid pressure and velocity distributions in the interstitial space. The resulting macroscopic permeability values are within 6% of results obtained with the Lattice-Boltzmann method after performing grid refinements. The finite-difference method is on average six times faster than the Lattice-Boltzmann method. In the next step, slip flow and Knudsen diffusion are added to the pore-scale method to take into account unconventional flow mechanisms in hydrocarbon-bearing shale. The effect of these mechanisms is appraised with a pore-scale image of Eagle Ford shale as well as with several grain packs. It is shown that neglecting slip flow in samples with pore-throat sizes in the nanometer range could result in errors as high as 2000% when estimating permeability in unconventional reservoirs. A new fluid percolation model is proposed for hydrocarbon-bearing shale. Electrical conductivity is quantified in the presence of kerogen, clay, hydrocarbon, water, and the Stern-diffuse layer in grain packs as well as in the Eagle Ford shale pore-scale image. The pore-scale model enables a critical study of the [delta]LogR evaluation method commonly used with gas-bearing shale to assess kerogen concentration. A parallel conductor model is introduced based on Archie's equation for water conductivity in pores and a parallel conductive path for the Stern-diffuse layer. Additionally, a non-destructive core analysis method is proposed for estimating input parameters of the parallel conductor model in shale formations. A modified reservoir model of single-phase, compressible fluid is also developed to take into account the following unconventional transport mechanisms: (a) slip flow and Knudsen diffusion enhancement in apparent permeability, (b) Langmuir desorption as a source of gas generation at kerogen surfaces, and (c) the diffusion mechanism in kerogen as a gas supply to adsorbed layers. The model includes an iterative verification method of surface mass balance to ensure real-time desorption-adsorption equilibrium with gas production. Gas desorption from kerogen surfaces and gas diffusion in kerogen are the main mechanisms responsible for higher-than-expected production velocities commonly observed in shale-gas reservoirs. Slip flow and Knudsen diffusion marginally enhance production rates by increasing permeability during production.Item Using nanofluidics and microscopy to study unconventional pore-scale transport phenomena(2015-12) Kelly, Shaina Alysa; Balhoff, Matthew T.; Torres-Verdín, Carlos; Daigle, Hugh C; DiCarlo, David; Truskett, Thomas MShales 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.