Browsing by Author "Lee, Dongwon"

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Open Access
Advancing XRCT techniques : from enhanced segmentation to improved temporal resolution and advanced micromodel fabrication for pore-scale studies
(Stuttgart : Institute of Applied Mechanics, 2024) Lee, Dongwon; Steeb, Holger (Prof. Dr.-Ing)
This study explores innovative strategies to overcome spatial and temporal constraints inherent in lab-based X-ray computed tomography (XRCT) for pore-scale studies of porous media. The research focuses on three main aspects: enhancement of segmentation techniques, improvement of temporal resolution, and integration of advanced micromodel fabrication with XRCT imaging. Firstly, an exploration is conducted into the refinement of segmentation workflows tailored for micro-fracture networks within Carrara marble XRCT datasets, which are often characterized by low-contrast imaging and ambiguous boundaries due to apertures below the spatial resolution limit. Through a meticulous examination of various methodologies, including machine learning-based algorithms, significant advancements in computation time and accuracy are demonstrated compared to conventional segmentation workflows. Notably, machine learning methods exhibit superior performance, even in scenarios where images are contaminated with noise, showcasing their potential for enhancing segmentation outcomes. Secondly, the challenge of temporal limitations in XRCT imaging, especially during the study of dynamic processes within porous media, is addressed. Conventional XRCT technologies often encounter a trade-off between image quality, including spatial resolution, and scanning time. To mitigate this constraint, innovative workflows leveraging machine learning algorithms are proposed to augment temporal resolution. By capturing pore space alterations during phenomena such as Enzyme Induced Calcite Precipitation (EICP) with heightened fidelity, these approaches offer invaluable insights into the dynamic fluid flow dynamics that govern porous media behavior. Thirdly, an integrated methodology that combines 3D micromodel fabrication with XRCT imaging techniques is introduced. This comprehensive approach enables the design, fabrication, and validation of 3D micromodels that faithfully replicate the pore-scale characteristics of natural porous media. Leveraging stochastic reconstruction algorithms and advanced 3D printing technologies, highly detailed micromodels with unprecedented spatial resolutions are created. These micromodels serve as invaluable tools for validating numerical simulations, elucidating pore-scale phenomena, and advancing the understanding of fluid dynamics in complex yet well-controlled porous media systems. The outlook suggests further advancements through the integration of multi-modal techniques, machine learning, and expansion of training datasets to overcome current limitations, offering unprecedented insights into complex fluid flow phenomena within porous media.
Open Access
Experimental methods and imaging for enzymatically induced calcite precipitation in a microfluidic cell
(2021) Weinhardt, Felix; Class, Holger; Vahid Dastjerdi, Samaneh; Karadimitriou, Nikolaos; Lee, Dongwon; Steeb, Holger
Enzymatically induced calcite precipitation (EICP) in porous media can be used as an engineering option to achieve precipitation in the pore space, for example, aiming at a targeted sealing of existing flow paths. This is accomplished through a porosity and consequent permeability alteration. A major source of uncertainty in modeling EICP is in the quantitative description of permeability alteration due to precipitation. This report presents methods for investigating experimentally the time‐resolved effects of growing precipitates on porosity and permeability on the pore scale, in a poly‐di‐methyl‐siloxane microfluidic flow cell. These methods include the design and production of the microfluidic cells, the preparation and usage of the chemical solutions, the injection strategy, and the monitoring of pressure drops for given fluxes for the determination of permeability. EICP imaging methods are explained, including optical microscopy and X‐ray microcomputed tomography (XRCT), and the corresponding image processing and analysis. We present and discuss a new experimental procedure using a microfluidic cell, as well as the general perspectives for further experimental and numerical simulation studies on induced calcite precipitation. The results of this study show the enormous benefits and insights achieved by combining both light microscopy and XRCT with hydraulic measurements in microfluidic chips. This allows for a quantitative analysis of the evolution of precipitates with respect to their size and shape, while monitoring their influence on permeability. We consider this to be an improvement of the existing methods in the literature regarding the interpretation of recorded data (pressure, flux, and visualization) during pore morphology alteration.
Open Access
Machine learning assists in increasing the time resolution of X-ray computed tomography applied to mineral precipitation in porous media
(2023) Lee, Dongwon; Weinhardt, Felix; Hommel, Johannes; Piotrowski, Joseph; Class, Holger; Steeb, Holger
Many subsurface engineering technologies or natural processes cause porous medium properties, such as porosity or permeability, to evolve in time. Studying and understanding such processes on the pore scale is strongly aided by visualizing the details of geometric and morphological changes in the pores. For realistic 3D porous media, X-Ray Computed Tomography (XRCT) is the method of choice for visualization. However, the necessary high spatial resolution requires either access to limited high-energy synchrotron facilities or data acquisition times which are considerably longer (e.g. hours) than the time scales of the processes causing the pore geometry change (e.g. minutes). Thus, so far, conventional benchtop XRCT technologies are often too slow to allow for studying dynamic processes. Interrupting experiments for performing XRCT scans is also in many instances no viable approach. We propose a novel workflow for investigating dynamic precipitation processes in porous media systems in 3D using a conventional XRCT technology. Our workflow is based on limiting the data acquisition time by reducing the number of projections and enhancing the lower-quality reconstructed images using machine-learning algorithms trained on images reconstructed from high-quality initial- and final-stage scans. We apply the proposed workflow to induced carbonate precipitation within a porous-media sample of sintered glass-beads. So we were able to increase the temporal resolution sufficiently to study the temporal evolution of the precipitate accumulation using an available benchtop XRCT device.
Open Access
Modelling and simulation of natural hydraulic fracturing applied to experiments on natural sandstone cores
(2024) Wang, Junxiang; Sonntag, Alixa; Lee, Dongwon; Xotta, Giovanna; Salomoni, Valentina A.; Steeb, Holger; Wagner, Arndt; Ehlers, Wolfgang
Under in-situ conditions, natural hydraulic fractures (NHF) can occur in permeable rock structures as a result of a rapid decrease of pore water accompanied by a local pressure regression. Obviously, these phenomena are of great interest for the geo-engineering community, as for instance in the framework of mining technologies. Compared to induced hydraulic fractures, NHF do not evolve under an increasing pore pressure resulting from pressing a fracking fluid in the underground but occur and evolve under local pore-pressure reductions resulting in tensile stresses in the rock material. The present contribution concerns the question under what quantitative circumstances NHF emerge and evolve. By this means, the novelty of this article results from the combination of numerical investigations based on the Theory of Porous Media with a tailored experimental protocol applied to saturated porous sandstone cylinders. The numerical investigations include both pre-existing and evolving fractures described by use of an embedded phase-field fracture model. Based on this procedure, representative mechanical and hydraulic loading scenarios are simulated that are in line with experimental investigations on low-permeable sandstone cylinders accomplished in the Porous Media Lab of the University of Stuttgart. The values of two parameters, the hydraulic conductivity of the sandstone and the critical energy release rate of the fracture model, have turned out essential for the occurrence of tensile fractures in the sandstone cores, where the latter is quantitatively estimated by a comparison of experimental and numerical results. This parameter can be taken as reference for further studies of in-situ NHF phenomena and experimental results.