Point sources in cylindrically laminated rods

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1998.Includes bibliographical references (p. 193-194).by Joonsang Park.M.S

Wave motion in finite and infinite media using the TLM

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2002.Includes bibliographical references (p. 449-454).The Thin-Layer Method (TLM) is a semi-analytical technique that is efficient for wave propagation problems involving partially heterogeneous media. While the method has been used widely for horizontally stratified media, e.g. dynamic response of foundations over layered soils, there remain many unexplored aspects. These include a thorough evaluation of the TLM's accuracy and range of applicability as well as its extension to semi-infinite and infinite media. Based on these considerations, we have three main goals to pursue in this study. The first one is to explore and improve the accuracy and convergence of the TLM associated with finite media. The second one is to extend the applicability of the TLM to model and analyze semi-infinite and infinite multilayered media. The third one is to develop two novel TLM's that are useful in analyzing wave motions in cylindrically or spherically laminated solids and shells. We proceed our study by separating it into the following three parts. In the first part, we begin by characterizing numerical dispersion phenomena in the TLM by means of general solutions and frequency spectra for discrete homogeneous full-spaces, which are obtained in closed-form with the aid of a finite difference scheme. Then, we determine the optimal combination of the consistent and lumped mass matrices by introducing tuning factors into the discrete system of equations. As a result, we improve the accuracy of not only the eigenvalues associated with free-vibration problems, but also the modal responses to external dynamic loads.(cont.) To assess the accuracy and convergence of the modal solutions, we compute both the displacements and internal stresses for some canonical examples and then compare with the associated exact analytical solutions. From this exploration, we discover various aspects of the TLM modal solutions in connection with the spatial-temporal characteristics of sources and receivers. We consider both the linear and quadratic expansion TLM' s. Finally, we determine the reasonable numbers of thin-layers per wavelength needed to calculate accurate responses with the TLM. In addition, we find out that the quadratic expansion TLM is more accurate and efficient than the linear expansion TLM. In the second part, we utilize the substructure method and the paraxial approximation for the purpose of analyzing the semi-infinite and infinite multilayered media by means of the TLM. The substructure method is applied to the TLM formulation in the time-domain, while the paraxial approximation is used for the TLM formulation in the frequency-domain. In addition, for the application of the substructure method, we derive new closed-form Green's functions in the wavenumber-time domain for a homogeneous half-space by means of contour integration. We extensively investigate the characteristics of the both formulations to improve their stability and accuracy. For stable and effective calculation, we propose the use of an artificial buffer layer and an adaptive buffer layer for the time-domain and frequency-domain formulations, respectively. Furthermore, we also derive the exact analytical Green's functions in the space-frequency domain ...by Joonsang Park.Ph.D

Vertically integrated models for coupled two‐phase flow and geomechanics in porous media

Models of reduced dimensionality have been found to be particularly attractive in simulating the fate of injected CO2 in supercritical state in the context of carbon capture and storage. This is motivated by the confluence of three aspects: the strong buoyant segregation of the lighter CO2 phase above water, the relatively long time scales associated with storage, and finally the large aspect ratios that characterize the geometry of typical storage aquifers. However, to date, these models have been confined to considering only the flow problem, as the coupling between reduced dimensionality models for flow and models for geomechanical response has previously not been developed. Herein, we develop a fully coupled, reduced dimension, model for multiphase flow and geomechanics. It is characterized by the aquifer(s) being of lower dimension(s), while the surrounding overburden and underburden being of full dimension. The model allows for general constitutive functions for fluid flow (relative permeability and capillary pressure) and uses the standard Biot coupling between the flow and mechanical equations. The coupled model retains all the simplicities of reduced‐dimensional models for flow, including less stiff nonlinear systems of equations (since the upscaled constitutive functions are closer to linear), longer time steps (since the high grid resolution in the vertical direction can be avoided), and less degrees of freedom. We illustrate the applicability of the new coupled model through both a validation study and a practical computational example.publishedVersio

Effect of brine-CO2 fracture flow on velocity and electrical resistivity of naturally fractured tight sandstones

Fracture networks inside geological CO2 storage reservoirs can serve as primary fluid flow conduit, particularly in low-permeability formations. While some experiments focused on the geophysical properties of brine- and CO2-saturated rocks during matrix flow, geophysical monitoring of fracture flow when CO2 displaces brine inside the fracture seems to be overlooked. We have conducted laboratory geophysical monitoring of fluid flow in a naturally fractured tight sandstone during brine and liquid CO2 injection. For the experiment, the low-porosity, low-permeability naturally fractured core sample from the Triassic De Geerdalen Formation was acquired from the Longyearbyen CO2 storage pilot at Svalbard, Norway. Stress-dependence, hysteresis and the influence of fluid-rock interactions on fracture permeability were investigated. The results suggest that in addition to stress level and pore pressure, mobility and fluid type can affect fracture permeability during loading and unloading cycles. Moreover, the fluid-rock interaction may impact volumetric strain and consequently fracture permeability through swelling and dry out during water and CO2 injection, respectively. Acoustic velocity and electrical resistivity were measured continuously in the axial direction and three radial levels. Geophysical monitoring of fracture flow revealed that the axial P-wave velocity and axial electrical resistivity are more sensitive to saturation change than the axial S-wave, radial P-wave, and radial resistivity measurements when CO2 was displacing brine, and the matrix flow was negligible. The marginal decreases of acoustic velocity (maximum 1.6% for axial Vp) compared to 11% increase in axial electrical resistivity suggest that in the case of dominant fracture flow within the fractured tight reservoirs, the use of electrical resistivity methods have a clear advantage compared to seismic methods to monitor CO2 plume. The knowledge learned from such experiments can be useful for monitoring geological CO2 storage where the primary fluid flow conduit is fracture network.acceptedVersio

A Study on the Lateral Load Capacity of a Novel Hybrid Monopile via a Centrifuge Model Test

publishedVersio

Enabling Hard Constraints in Differentiable Neural Network and Accelerator Co-Exploration

Co-exploration of an optimal neural architecture and its hardware accelerator is an approach of rising interest which addresses the computational cost problem, especially in low-profile systems. The large co-exploration space is often handled by adopting the idea of differentiable neural architecture search. However, despite the superior search efficiency of the differentiable co-exploration, it faces a critical challenge of not being able to systematically satisfy hard constraints such as frame rate. To handle the hard constraint problem of differentiable co-exploration, we propose HDX, which searches for hard-constrained solutions without compromising the global design objectives. By manipulating the gradients in the interest of the given hard constraint, high-quality solutions satisfying the constraint can be obtained.Comment: publisehd at DAC'2

Using Prokaryotes for Carbon Capture Storage

Geological storage of CO2 is a fast-developing technology that can mitigate rising carbon emissions. However, there are environmental concerns with long-term storage and implications of a leak from a carbon capture storage (CCS) site. Traditional monitoring lacks clear protocols and relies heavily on physical methods. Here we discuss the potential of biotechnology, focusing on microbes with a natural ability to utilize and assimilate CO2 through different metabolic pathways. We propose the use of natural microbial communities for CCS monitoring and CO2 utilization, and, with examples, demonstrate how synthetic biology may maximize CO2 uptake within and above storage sites. An integrated physical and biological approach, combined with metagenomics data and biotechnological advances, will enhance CO2 sequestration and prevent large-scale leakages

Stochastic Particle Flow for Nonlinear High-Dimensional Filtering Problems

A series of novel filters for probabilistic inference that propose an alternative way of performing Bayesian updates, called particle flow filters, have been attracting recent interest. These filters provide approximate solutions to nonlinear filtering problems. They do so by defining a continuum of densities between the prior probability density and the posterior, i.e. the filtering density. Building on these methods' successes, we propose a novel filter. The new filter aims to address the shortcomings of sequential Monte Carlo methods when applied to important nonlinear high-dimensional filtering problems. The novel filter uses equally weighted samples, each of which is associated with a local solution of the Fokker-Planck equation. This hybrid of Monte Carlo and local parametric approximation gives rise to a global approximation of the filtering density of interest. We show that, when compared with state-of-the-art methods, the Gaussian-mixture implementation of the new filtering technique, which we call Stochastic Particle Flow, has utility in the context of benchmark nonlinear high-dimensional filtering problems. In addition, we extend the original particle flow filters for tackling multi-target multi-sensor tracking problems to enable a comparison with the new filter

Induced-seismicity geomechanics for controlled CO2 storage in the North Sea (IGCCS)

The aim of the current study, IGCCS (2017–2020), is to evaluate the feasibility of micro-seismic (MS) monitoring of CO2 injection into representative storage candidates in the North Sea, based on broad and quantitative characterization of relevant subsurface behavior with respect to geology, geomechanics and seismicity. For this purpose, we first group potential CO2 storage sites in the North Sea into three different depths. Then, advanced triaxial rock mechanical tests are performed together with acoustic emission (AE) acquisition under representative loading for CO2 storage sites in the North Sea and for formations of each depth group, covering shale, mudstone and sandstone cores. Our work focuses particularly on quantifying the effects of injected fluid type and temperature on mechanical behavior and associated MS response of subsurface sediments. The experiment results show that each depth group may behave differently in responses to CO2 injection. Particularly, the occurrence of detectable MS events is expected to increase with depth, as the combined effects of rock stiffness and temperature contrast between the host rock and injected CO2 are increasing. In addition, lithology plays an important role in terms of the MS response, i.e. high AE event rate is observed in sandstones, while aseismicity in shale and mudstone. The test results are then scaled up and applied to advanced coupled flow-geomechanics simulations and a synthetic field-scale MS data study to understand micro-seismicity at fracture, reservoir and regional scales. The numerical simulation of scCO2 injection scenario shows quite different stress-strain changes compared to brine injection, resulting mainly from the thermally-induced behavior. Furthermore, the numerical simulation study via so-called Cohesion Zone Modeling (CZM) approach shows strong potential to improve our understanding of the multiphase-flow-driven fracture propagation. Our synthetic MS data study, focused on slow-earthquake scenario, also suggests that sensors with high sensitivity at low frequency might be necessary for better signal detection and characterization during CO2 injection. This manuscript covers the main findings and insights obtained during the whole study of IGCCS, and refers to relevant publications for more details