Tae Sup Yun

Evolution of Pore Orientation in Granular Media under Biaxial Compression

Pores in granular media tend to align parallel to maximum stress upon direct shear and creep conditions. We herein numerically investigate the evolution of pore orientation under bi-axial compression using 2D particle assemblies in conjunction with image analysis, in particular for the case of particle clumps bounded by flexible wall. The discrete element method produces the dense and loose packings where two particles are clumped to closely simulate the non-spherical particles followed by bi-axial compression. The Delaunay triangulation and polygonization allow constructing unit pore space whose irregular shape is fitted by an ellipse. This helps characterize the orientation of each unit pore and quantifying the pore size distribution at each strain regime. The simulated results are analyzed within the framework of local void ratio and elongation factor. The initial void ratio uniquely determines the development of elongation factor with strain and pores are aligned toward the direction of maximum compression. Its manifestation is pronounced near the regime of shear band attributed to the formation arching for stabilizing the macro- scale particle network.

Electrical Conduction of Granular Media: Experimental and Numerical Studies

This study presents the evolutionary behavior of electrical conduction for granular mixtures dominated by the volumetric fraction of each constituent, relative sizes, and stress condition. The chrome balls and glass beads are mixed with varying volumetric fraction and relative size ratio. The two-electrode method is used to obtain electrical conductance at different loading stages for each mixture. Three relative sizes of glass beads with respect to chrome balls are tested. As the size ratio determines the spatial configuration and corresponding connectivity of conductive granules in mixtures, the electrical conductance shows the unique evolution with varying volumetric fraction. The discrete element method is also implemented to capture the inter-particle connectivity of conductive granules. Experimental results and numerical simulation conclude that not only the fraction of conductive particles but also long range of interconnectivity of particles construct the electrical percolation path that enables carrying the electric current. This study highlights that energy transfer in granular packing can be optimized by controlling suggested dominant factors.

Pore directivity of soils subjected to shearing: Numerical simulation and image processing

ABSTRACT: Soils subjected to shearing experience dilation or contraction depending on their initial porosity, and the relative displacement of individual particles determines a soil’s unique particle-pore microstructure during volume change. It has been suggested that soil microstructure tends to be stabilized as pores are aligned parallel to the loading direction as particles are mobilized. We explore the evolution of internal pore fabric and directivity during direct shear conditions in which a constrained boundary hampers the full mobilization of particles. Two representative volumetric responses for dense and loose granular soils during direct shear are simulated via the discrete element method. The arbitrarily shaped pore structure in 3D space is quantified using best-fitting ellipsoids to evaluate pore characteristics. Changes in pore fabric are analyzed based on local porosity, pore size distribution, and geometrical configurat ion of fitted ellipsoids. Results show that initial porosity determines the characteristic pore evolution during shearing. Numerical results also demonstrate that a pore elongation oriented in the direction of the shear manifests under dense packing, while randomly distributed pore directivity is observed under loose packing.

Liquid CO2 Fracturing: Effect of Fluid Permeation on the Breakdown Pressure and Cracking Behavior

Liquid CO2 fracturing is a promising alternative to hydraulic fracturing since it can circumvent problems stemming from the use of water. One of the most significant differences between liquid CO2 and hydraulic fracturing processes is that liquid CO2 permeates into matrix pores very rapidly due to its low viscosity. Here we study how this rapid permeation of liquid CO2 impacts a range of features during the course of the fracturing process, with a focus on the breakdown pressure and cracking behavior. We first conduct a series of laboratory fracturing experiments that inject liquid CO2, water, and oil into nominally identical mortar specimens with various pressurization rates. We quantitatively measure the volumes of fluids permeated into the specimens and investigate how these permeated volumes are related to breakdown and fracture initiation pressures and pressurization efficiency. The morphology of the fractures generated by different types of fluids is also examined using 3D X-ray computed tomographic imaging. Subsequently, the cracking processes due to injection of liquid CO2 and water are further investigated by numerical simulations employing a phase-field approach to fracture in porous media. Simulation results show that rapid permeation of liquid CO2 gives rise to a substantial pore pressure buildup and distributed microcracks prior to the major fracture propagation stage. The experimental and numerical results commonly indicate that significant fluid permeation during liquid CO2 fracturing is a primary reason for its lower breakdown pressure and more distributed fractures compared with hydraulic fracturing.

Water Permeability Reduction in THF Hydrate-Bearing Sediments

Water permeability in hydrate-bearing sediments is a key parameter in gas production affecting the effective depressurization boundary from a wellbore and contributing heat transport associated with fluid flow. The experimental measurement of water permeability in the presence of hydrates is associated with many difficulties such as dynamic hydrate dissolution and formation during fluid flow and long induction time. In this study, we formed tetrahydrofuran (THF) hydrates in a core-scale chamber to explore water permeability as a function of hydrates. Wave velocities during the permeability measurement were also measured. The results show that water permeability decreases as hydrate saturation increases. Shear and compression wave velocities increase with increasing hydrate saturation, but the velocity decreases a little during the repetitive permeability measurement at a given hydrate saturation.

Gas Bubble Nucleation and Migration in Soils—Pore-Network Model Simulation

Sediment can be de-saturated by introducing gas bubbles, which is found in various applications such as methane gas generation in landfill, microbial-induced gas bubble formation, air sparing method for soil remediation, heavy oil depressurization for carbon recovery, and gas production from hydrate bearing sediment. The gas introduction method (e.g., nucleation and injection) and migration and trapping of gas bubbles affect the hydraulic conductivity, residual gas saturation, and the stability of these gassy sediments. In this study, the pore-network model is used to investigate gas bubble migration in porous media. Gas bubbles are introduced by mimicking either nucleation or injection. Based on the known gas bubble behavior available in the literature, numerical algorithms are developed to simulate the migration and trapping of gas bubbles in pore-network model. The effect of gas bubble size distribution and pore size distribution on residual saturation is investigated. The results show that gas bubble size distribution becomes wider as gas bubbles coalesce to each other during migration. And the residual gas saturation increase with increasing bubble size and permeability reduction becomes apparent as the gas bubble size and the number of generated gas bubble increase.

Thermal and Electrical Properties of Water-repellent Sands

The behavior of water-repellent soils has been less concerned in spite of the importance of engineering significance compared to the other characteristics of soils. In general, the surface property of soils in natural state is recognized as easy to wet and this concept has been applied to unsaturated soil mechanics. However, there are also water-repellent soils (hydrophobic) generated naturally or produced artificially. The thermal and electrical conductivity are typical indices to assess physical behavior of materials associated with surface wettability. This research presents thermal and electrical properties of wettable and water-repellent sands in unsaturated condition. The wettable sands are in its natural state and the water-repellent sands are chemically treated using organic silane. The microscopic observation using the scanning electron microscopic and the atomic force microscopic images confirms the existence of homogeneously grafted organic materials on the silicate mineral surface. Prepared materials are used in the thermal and electrical experimentations with varying degree of saturation. The transient plane source is used for the measurement of the thermal conductivity. The two-electrode system is selected in the electrical conductivity measurement. The surface wettability that affects the spatial distribution of water in pore space causes the noticeable difference of thermal and electrical properties in two materials.