Numerical simulation of viscoelastic buckle folds: Implications for stress, fractures, porosity and fluid flow

Over the past several decades, buckle folds have been exclusively studied by numerous methods. However, lots of assumptions and simplifications are made, which may not result in realistic in-situ stress conditions leading to rock failure. This study represents the first numerical simulation of folding under the consideration of gravity and pore pressure to simulate the structural development of buckle folds. The first topic covered in this dissertation is the fracture associated to the single layer fold. It is concluded that burial depth, viscosity, and permeability are critical for the initiation of major fracture sets at the hinge zone with varying degrees. Moreover, this study provides a detail research on the stress and strain distribution in the multilayer folds and it is concluded that the stress/strain state within the folding layer(s) are determined by the buckling process, fold geometry and material parameters. The second topic covered in this dissertation is the numerical simulation of multilayer folds. This study demonstrates that the shapes of the multilayer folds are influenced by the various parameters. In addition, the numerical simulations provide a general understanding of the stress/strain distribution in the multilayer system. The third topic covered in this dissertation is the numerical simulation of parasitic folds. This study demonstrates that the shapes of the parasitic folds depend on the buckling of both the large- and small-scale folds and are influenced by the various parameters. The numerical modeling results show a large variability in porosity changes due to the complex distribution of the volumetric strain. In addition, the numerical simulations provide a general understanding of the influence of the various model parameters on the resulting porosity distribution --Abstract, page iv

Image-Based Pore-Scale Modeling of Inertial Flow in Porous Media and Propped Fractures

Non-Darcy flow is often observed near wellbores and in hydraulic fractures where relatively high velocities occur. Quantifying additional pressure drop caused by non-Darcy flow and fundamentally understanding the pore-scale inertial flow is important to oil and gas production in hydraulic fractures. Image-based pore-scale modeling is a powerful approach to obtain macroscopic transport properties of porous media, which are traditionally obtained from experiments and understand the relationship between fluid dynamics with complex pore geometries. In image-based modeling, flow simulations are conducted based on pore structures of real porous media from X-ray computed tomographic images. Rigorous pore-scale finite element modeling using unstructured mesh is developed and implemented in proppant fractures. The macroscopic parameters permeability and non-Darcy coefficient are obtained from simulations. The inertial effects on microscopic velocity fields are also discussed. The pore-scale network modeling of non-Darcy flow is also developed based on simulation results from rigorous model (FEM). Network modeling is an appealing approach to study porous media. Because of the approximation introduced in both pore structures and fluid dynamics, network modeling requires much smaller computational cost than rigorous model and can increase the computational domain size by orders of magnitude. The network is validated by comparing pore-scale flowrate distribution calculated from network and FEM. Throat flowrates and hydraulic conductance values in pore structures with a range of geometries are compared to assess whether network modeling can capture the shifts in flow pattern due to inertial effects. This provides insights about predicting hydraulic conductance using the tortuosity of flow paths,which is a significant factor for inertial flow as well as other network pore and throat geometric parameters

3D Multi-Scale Behavior of Granular Materials using Experimental and Numerical Techniques

Constitutive modeling of granular material behavior has generally been based on global response of laboratory-size specimens or larger models with little understanding of the fundamental mechanics that drive the global response. Many studies have acknowledged the importance of micro-scale and meso-scale mechanics on the constitutive behavior of granular materials. However, much knowledge is still missing to develop and improve robust micromechanical constitutive models. The research in this dissertation contributes to this knowledge gap for many potential applications using novel experimental techniques to investigate the three-dimensional (3D) behavior of granular materials. Critical micromechanics measurements at multiple scales are investigated by combining 3D synchrotron micro-computed tomography (SMT), 3D image analysis, and finite element analysis (FEA). At the single particle level (micro-scale), particle fracture was examined at strain rates of 0.2 mm/min and 2 m/s using quasi-static unconfined compression, unconfined mini-Kolsky bar, and x-ray imaging techniques. Surface reconstructions of particles were generated and exported to Abaqus FEA software, where quasi-static and higher rate loading curves and crack propagation were simulated with good accuracy. Stress concentrations in oddly shaped particles during FEA simulations resulted in more realistic fracture stresses than theoretical models. A nonlinear multivariable statistical model was developed to predict force required to fracture individual particles with known internal structure and loading geometry. At the meso-scale, 3D SMT imaging during in-situ triaxial testing of granular materials were used to identify particle morphology, contacts, kinematics and interparticle behavior. Micro shear bands (MSB) were exposed during pre-peak stress using a new relative particle displacement concept developed in this dissertation. MSB for spherical particles (glass beads) had larger thickness (3d50 to 5d50) than that of angular sands (such as F35 Ottawa sand, MSB thickness of 1d50 to 3d50). Particle morphology also plays a significant role in the onset and growth of shear bands and global fabric evolution of granular materials. More spherical particles typically exhibit more homogeneous internal anisotropy. Fabric of particles within the shear band (at higher densities and confining pressures) exhibits a peak and decrease into steady-state. Also, experimental fabric produces more accurate strength and deformation predictions in constitutive models that incorporate fabric evolution

Impact of Fracture Aperture Distributions on Effective Permeability of Fractured Reservoirs

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Intermittency and roughening in the failure of brittle heterogeneous materials

Stress enhancement in the vicinity of brittle cracks makes the macro-scale failure properties extremely sensitive to the micro-scale material disorder. Therefore: (i) Fracturing systems often display a jerky dynamics, so-called crackling noise, with seemingly random sudden energy release spanning over a broad range of scales, reminiscent of earthquakes; (ii) Fracture surfaces exhibit roughness at scales much larger than that of material micro-structure. Here, I provide a critical review of experiments and simulations performed in this context, highlighting the existence of universal scaling features, independent of both the material and the loading conditions, reminiscent of critical phenomena. I finally discuss recent stochastic descriptions of crack growth in brittle disordered media that seem to capture qualitatively - and sometimes quantitatively - these scaling features.Comment: 38 pages, invited review for J. Phys. D cluster issue on "Fracture: from the Atomic to the Geophysics Scale

3D FEM model of ground deformation in Deception Island (Antarctica)

Ground deformation has been demonstrated to be one of the most common signals of volcanic unrest. A variety of processes can cause ground deformation in active volcanic areas (e.g. magmatic processes, pore pressure variations in the hydrothermal systems, etc), and being able to recognize and distinguish them is crucial for evaluating the potential occurrence of future eruptions. Ground deformation can be measured using remote sensing or geodetic techniques like GPS or tiltmeters. However, even if geodetic monitoring networks may be capable of recording the ground deformation signal at surface, it is difficult to directly identify where and how are the pressure sources responsible for the observed deformation. Deception Island is the most active volcano in the South Shetland Islands, which last destructive events took place in 1967, 169 and 1970. Since the installation of the monitoring network in the island, it has experienced three uplift/downlift episodes, where ground deformation has been measured with GPS stations. . The objective of this work is to evaluate the location, shape, pressure source responsible for surface ground deformation recorded in Deception Island during the period 1995 - 2000 using Finite Elements (FE) linear elastic models. First, we have considered a 2D model where we have studied the effect of the different parameters in ground deformation. Second, 3D models simulating the real topography of Deception Island have been considered. The results of the 3D models are compared with the GPS data registered in some points of the island to approximate the shape, depth, excess pressure of the reservoir. Results obtained are crucial to understand the current magmatic situation of the island and the potential outcome of a future eruption