Underlined names are links to current or past GEG members
REFEREED PUBLICATIONS IN JOURNALS
Huang, P.-W., B. Flemisch, C.-Z. Qin, M.O. Saar, and A. Ebigbo, Validating the Nernst–Planck transport model under reaction-driven flow conditions using RetroPy v1.0, Geoscientific Model Development, 16, pp. 4767-4791, 2023. https://doi.org/10.5194/gmd-16-4767-2023 [Download] [View Abstract]Reactive transport processes in natural environments often involve many ionic species. The diffusivities of ionic species vary. Since assigning different diffusivities in the advection-diffusion equation leads to charge imbalance, a single diffusivity is usually used for all species. In this work, we apply the Nernst–Planck equation, which resolves unequal diffusivities of the species in an electroneutral manner, to model reactive transport. To demonstrate the advantages of the Nernst–Planck model, we compare the simulation results of transport under reaction-driven flow conditions using the Nernst–Planck model with those of the commonly used single-diffusivity model. All simulations are also compared to well-defined experiments on the scale of centimeters. Our results show that the Nernst–Planck model is valid and particularly relevant for modeling reactive transport processes with an intricate interplay among diffusion, reaction, electromigration, and density-driven convection.
Huang, P.-W., B. Flemisch, C.-Z. Qin, M.O. Saar, and A. Ebigbo, Relating Darcy-scale chemical reaction order to pore-scale spatial heterogeneity, Transport in Porous Media, 2022. https://doi.org/10.1007/s11242-022-01817-0 [Download] [View Abstract]Due to spatial scaling effects, there is a discrepancy in mineral dissolution rates measured at different spatial scales. Many reasons for this spatial scaling effect can be given. We investigate one such reason, i.e., how pore-scale spatial heterogeneity in porous media affects overall mineral dissolution rates. Using the bundle-of-tubes model as an analogy for porous media, we show that the Darcy-scale reaction order increases as the statistical similarity between the pore sizes and the effective-surface-area ratio of the porous sample decreases. The analytical results quantify mineral spatial heterogeneity using the Darcy-scale reaction order and give a mechanistic explanation to the usage of reaction order in Darcy-scale modeling. The relation is used as a constitutive relation of reactive transport at the Darcy scale. We test the constitutive relation by simulating flow-through experiments. The proposed constitutive relation is able to model the solute breakthrough curve of the simulations. Our results imply that we can infer mineral spatial heterogeneity of a porous media using measured solute concentration over time in a flow-through dissolution experiment.
Naets, I., M. Ahkami, P.-W. Huang, M. O. Saar, and X.-Z. Kong, Shear induced fluid flow path evolution in rough-wall fractures: A particle image velocimetry examination, Journal of Hydrology, 610/127793, 2022. https://doi.org/10.1016/j.jhydrol.2022.127793 [Download] [View Abstract]Rough-walled fractures in rock masses, as preferential pathways, largely influence fluid flow, solute and energy transport. Previous studies indicate that fracture aperture fields could be significantly modified due to shear displacement along fractures. We report experimental observations and quantitative analyses of flow path evolution within a single fracture, induced by shear displacement. Particle image velocimetry and refractive index matching techniques were utilized to determine fluid velocity fields inside a transparent 3D-printed shear-able rough fracture. Our analysis indicate that aperture variability and correlation length increase with the increasing shear displacement, and they are the two key parameters, which govern the increases in velocity variability, velocity longitudinal correlation length, streamline tortuosity, and variability of streamline spacing. The increase in aperture heterogeneity significantly impacts fluid flow behaviors, whilst changes in aperture correlation length further refine these impacts. To our best knowledge, our study is the first direct measurements of fluid velocity fields and provides insights into the impact of fracture shear on flow behavior.
Huang, P.W., and F. Wellmann, An Explanation to the Nusselt–Rayleigh Discrepancy in Naturally Convected Porous Media, Transport in Porous Media, 2021. https://doi.org/10.1007/s11242-021-01556-8 [Download] [View Abstract]We model hydrothermal convection using a partial differential equation formed by Darcy velocity and temperature—the velocity formulation. Using the Elder problem as a benchmark, we found that the velocity formulation is a valid model of hydrothermal convection. By performing simulations with Rayleigh numbers in the non-oscillatory regime, we show that multiple quasi-steady-state solutions can be one of the reasons that caused the Nusselt–Rayleigh discrepancy found in previous experiments. The results reveal more understandings about the nature of uncertainty of convection modes in porous media.
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Huang, P.W., and J.F. Wellmann, Investigating Different Formulations for Hydrothermal Convection in Geothermal Systems, Proceedings World Geothermal Congress 2020+1, 2020. [Download PDF] [View Abstract]Hydrothermal convection in porous media is an essential piece of physics in geothermal reservoirs, and understanding them leads to better development of geothermal energy. We analyze the validity of simulating hydrothermal convection using different formulations of partial differential equations. Using the Elder problem as a benchmark, we found out that the stream function formulation and the velocity formulation are a valid and efficient model of hydrothermal convection. The Nusselt number and entropy production are measurements of the quality of convective heat transfer. The Rayleigh number describes the physical properties of a porous media. We use simulations to investigate further the discrepancy in the Nusselt Rayleigh relationship found in previous experiments. The conclusion is that the multiple steady states of convection pattern in a 3D box are the main reason for the discrepancy found in the Nusselt-Rayleigh relationship.
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Huang, P.W., Reactive transport modeling at the pore scale and upscaling to the Darcy scale, Dissertation, pp., 2022. https://doi.org/10.3929/ethz-b-000585564 [Download] [View Abstract]Reactive transport processes are fundamental for a large number of applications. At the millimeter--centimeter scale, reactive transport controls the electrolysis in rechargeable batteries, the dendritic growth in lithium-ion batteries, separation processes in chromatographic columns, and the corrosion of steel in concrete structures. At the meter--kilometer scale, reactive transport is relevant for geothermal energy extraction, geologic carbon sequestration, radioactive waste disposal, rock weathering, hydraulic stimulation, the remediation of contaminated sites, and the in-situ leaching of minerals. Therefore, developing reactive transport models is beneficial for understanding and predicting the behavior of reactive systems. Using geothermal energy utilization as an example, what would happen after the heat is extracted from the brine? Will the dissolved minerals precipitate and clog up the pipes? When we use supercritical CO2 as the working fluid, what will happen to the subsurface? Will the porosity or permeability of the reservoir change due to mineral reactions? Such questions can be addressed using reactive transport modeling. The main problems in developing a useful model for subsurface reactive transport are the large spatial and time scale differences between our understanding of reaction kinetics established in the lab and the application in the field. The reaction kinetics of minerals at the temperature and pressure conditions of a geothermal reservoir can also be hard to determine. Furthermore, detailed small-scale information relevant to reactive transport processes cannot be realized using geological modeling and geophysical methods. To contribute to our knowledge of field-scale reactive transport processes, the first part of the thesis focuses on the connection between mineral dissolution processes at the pore and the averaged spatial scale. I established a relationship between dissolution kinetics at these two scales using the reaction order. In addition, I developed a method for a flow-through experiment with which the pore-scale heterogeneity of a sample of porous medium can be assessed using the outlet/inlet concentrations of dissolved minerals. The second part of the thesis is dedicated to modeling the coulombic effects among ionic species in aqueous solutions. Such effects are also known as electromigration and are relevant in tight geological formations such as shale or clay, where the pores are on the scale of sub-micrometers. In these tiny pores, the coulombic effects are particularly relevant since the transport is primarily diffusion-controlled. Referring to results of lab-scale reactive flow experiments performed in a Hele-Shaw cell, I show that the coulombic effects can be essential when modeling reactive transport processes. Finally, I conclude the thesis with a summary and with perspectives. I discuss further research opportunities using the validated reactive transport model. One tangible result of this doctoral work is a modelling tool for reactive transport, RetroPy, which is freely available to the research community.
Huang, P.W., Investigating different formulations for hydrothermal convection in geothermal systems, MSc Thesis, RWTH Aachen, pp., 2017. [View Abstract]Hydrothermal convection in porous media is an important piece of physics in geothermal reservoirs, and understanding them leads to better development of geothermal energy. We analyze the validity of simulating hydrothermal convection using different formulations of partial differential equations. Using the Elder problem as a physical benchmark, we found out that the stream function formulation and the velocity formulation are a valid and efficient model of hydrothermal convection. The Nusselt number and entropy production are measurements of the quality of convective heat transfer. The Rayleigh number describes the physical properties of a porous media. We use simulations to further investigate the discrepancy in the Nusselt Rayleigh relationship found in previous experiments. The conclusion is that the multiple steady states of convection pattern in a 3D box are the main reason for the discrepancy in the Nusselt Rayleigh relationship.