# Entrevista a Albert Folch Sancho

Entrevista a Albert Folch Sancho sobre el peligro de residuos contaminantes como son los plástico a ríos y mar.

Entrevista a Albert Folch Sancho sobre el peligro de residuos contaminantes como son los plástico a ríos y mar.

a cargo de

**Sonia Gumbau PhD Student **

Jueves 22 de Marzo a las 12:15 h

en

Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

Recently there have been a number of approaches to interpret the column experiments of Gramling et al. (2002) involving the formation and transport of CuEDTA^{2-} in an irreversible reaction. The experimental data could not be properly fitted with an advection-dispersion equation assuming instantaneous reaction. Alternative approaches have been used to model the experiments using upscaled models, where instantaneous reactions are substituted by models based on incomplete mixing or in non-instantaneous reactions including some upscaled coefficients. Here we explore another possibility for an upscaled model, where reaction is again assumed as instantaneous, and incomplete mixing is modeled by means of a single-rate mass-transfer model where the transfer coefficient varies in time (t-SRMT). This model is the extension to reactive transport of the model presented recently by *Fernàndez-Garcia and Sanchez-Vila* [2015], being a proxy for a general multi-rate mass-transfer model. The evaluation of the model performance allows getting insight about the physical meaning of the parameters in the t-SRMT model.

**Keywords:** multi-rate mass transfer, bimolecular reaction, column experiment

a cargo de

(Helmholtz Centre for Environmental Research - UFZ)

Jueves 15 de marzo 2018 a las 12:15 h

en

Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

In the last decade, the possibility of extracting energy from supercritical geothermal systems (SCGS) has drawn increasing attention from the scientific community and is strongly emerging as a new technology. Albeit their definition is not yet agreed upon universally (sometimes called superhot geothermal systems), supercritical geothermal systems are geothermal systems in which the circulating fluid is close to or above the supercritical conditions (374°C and 22.1 MPa for pure water). Although power production from supercritical fluids could be enhanced by up to one order of magnitude compared to more traditional geothermal systems, drilling and operating under such high temperature conditions pose great technological challenges, several of which are yet to be overcome. Current research projects are located in active volcanic areas such as Kakkonda in Japan, Taupo Volcanic Area in New Zealand, Larderello in Italy, Krafla and Reykjanes in Iceland, The Geysers and Salton Sea in USA and Los Humeros in Mexico. Research activity at Los Humeros site is inserted in the GEMex international research project, a cooperation project between European Union and Mexico. A geothermal power plant is currently operated in Los Humeros Volcanic Complex, in which temperatures exceeding 400°C have been registered in wells at 2.5 km depths. Such depth is, at the time speaking, the deepest wellbores have been drilled. One of the goals of the GEMex project is to evaluate the possibility of extending the current field and reach the roots of the geothermal system, possibly at 4 or 5km depth. At such depth, permeability of the host rock might not be high enough to allow for hydrothermal fluid circulation: the host rock is most probably beyond the brittle-ductile transition conditions. In this case, stimulation techniques to create a network of more permeable fractures are necessary: this leads to a new kind of technology named Enhanced Supercritical Geothermal Systems (ESCGS). This seminar aims at presenting the latest research challenges and results relative to mechanical stimulation in supercritical geothermal reservoirs, with rocks beyond the brittle ductile transition. To tackle said challenges, we have developed models of fracture propagation (damage, phase-field and lower interface elements) and models for the rheology of rocks that are extended beyond the brittle-ductile transition conditions. The seminar is divided into three parts: in part I, a non-local plastic damage model is presented and validated against a set of fracture mechanics experiments on granite; in part II, three different approaches to simulate hydraulic fracture propagation have been compared and their performance

assessed against available analytical solutions; in part III, a rheological model for brittle-ductile transition of rocks has been developed and validated against observations. Finally, the talk is concluded with a brief discussion on future research perspectives and outlooks.

In the last decade, the possibility of extracting energy from supercritical geothermal systems (SCGS) has drawn increasing attention from the scientific community and is strongly emerging as a new technology. Albeit their definition is not yet agreed upon universally (sometimes called superhot geothermal systems), supercritical geothermal systems are geothermal systems in which the circulating fluid is close to or above the supercritical conditions (374°C and 22.1 MPa for pure water). Although power production from supercritical fluids could be enhanced by up to one order of magnitude compared to more traditional geothermal systems, drilling and operating under such high temperature conditions pose great technological challenges, several of which are yet to be overcome. Current research projects are located in active volcanic areas such as Kakkonda in Japan, Taupo Volcanic Area in New Zealand, Larderello in Italy, Krafla and Reykjanes in Iceland, The Geysers and Salton Sea in USA and Los Humeros in Mexico. Research activity at Los Humeros site is inserted in the GEMex international research project, a cooperation project between European Union and Mexico. A geothermal power plant is currently operated in Los Humeros Volcanic Complex, in which temperatures exceeding 400°C have been registered in wells at 2.5 km depths. Such depth is, at the time speaking, the deepest wellbores have been drilled. One of the goals of the GEMex project is to evaluate the possibility of extending the current field and reach the roots of the geothermal system, possibly at 4 or 5km depth. At such depth, permeability of the host rock might not be high enough to allow for hydrothermal fluid circulation: the host rock is most probably beyond the brittle-ductile transition conditions. In this case, stimulation techniques to create a network of more permeable fractures are necessary: this leads to a new kind of technology named Enhanced Supercritical Geothermal Systems (ESCGS). This seminar aims at presenting the latest research challenges and results relative to mechanical stimulation in supercritical geothermal reservoirs, with rocks beyond the brittle ductile transition. To tackle said challenges, we have developed models of fracture propagation (damage, phase-field and lower interface elements) and models for the rheology of rocks that are extended beyond the brittle-ductile transition conditions. The seminar is divided into three parts: in part I, a non-local plastic damage model is presented and validated against a set of fracture mechanics experiments on granite; in part II, three different approaches to simulate hydraulic fracture propagation have been compared and their performance

assessed against available analytical solutions; in part III, a rheological model for brittle-ductile transition of rocks has been developed and validated against observations. Finally, the talk is concluded with a brief discussion on future research perspectives and outlooks.

a cargo de

Estudiante PhD

Jueves 08 de Marzo a las 12:15 h

en

Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

In recent years, a number of Lagrangian approaches [e.g. 1,2,3,4,5] have been proposed for the numerical solution of nonlinear reactive transport problems. In these approaches, reactions are driven by the interaction between numerical particles that represent either a volume of fluid or a mass of solute. This interaction is a function of the relative position of particles and can be derived mathematically by equipping each particle with a kernel function, sometimes referred to as the smoothing function. In some of these approaches, the choice of the kernel shape and size does not follow objective criteria, but is instead rather heuristic. On the other hand, some authors have proposed that the smoothing function should be derived from the dispersive process [e.g. 2,6].

Recently, Fernàndez-Garcia and Sànchez-Vila [7] proposed that the kernel should represent the probability density distribution of the actual particle location, and should be globally optimized by minimizing the Mean Integrated Squared Error (MISE) of the density estimation. By using the existing algorithms for the determination of a time-dependent globally optimal Kernel Density Estimator (KDE), it is possible to compute probabilities (or rates) of reaction of numerical particles to model simple bimolecular reactions [8] or kinetic reactions of any complexity involving two reactants [9].

Most research on KDE focuses on 1D distributions of limited complexity. However, solute concentration distributions in heterogeneous media exhibit complex features and variations in space and time. For this reason, the globally optimal kernel is often far from being locally optimal, in particular, in those parts of the domain with relative small particle densities, and also in those with particularly abrupt concentration changes (such as mixing fronts). A novel approach is presented to define an adaptive locally optimal kernel function in 1-, 2- or 3-D that is not only time-dependent but also space-dependent. The presented local approaches are compared against the existing global ones, and also against the simple binning method, in the context of a Random Walk Particle Tracking (RWPT) model of reactive transport through a heterogeneous porous medium. The results show that the use of a locally adaptive kernel has a considerably positive impact on the accuracy of concentration estimations and chemical reaction simulations.

References:

[1] Tartakovsky, A. M., Meakin, P., Scheibe, T. D., & Eichler West, R. M. (2007). Simulations of reactive transport and precipitation with smoothed particle hydrodynamics. Journal of Computational Physics, 222(2), 654–672.

[2] Benson, D. A., & Meerschaert, M. M. (2008). Simulation of chemical reaction via particle tracking: Diffusion-limited versus thermodynamic rate-limited regimes. Water Resources Research, 44(12).

[3] Herrera, P. A., Massabó, M., & Beckie, R. D. (2009). A meshless method to simulate solute transport in heterogeneous porous media. Advances in Water Resources, 32(3), 413–429.

[4] Ding, D., & Benson, D. A. (2015). Simulating biodegradation under mixing-limited conditions using Michaelis-Menten (Monod) kinetic expressions in a particle tracking model. Advances in Water Resources, 76, 109–119.

[5] Engdahl, N. B., Benson, D. A., & Bolster, D. (2017). Lagrangian simulation of mixing and reactions in complex geochemical systems. Water Resources Research, 53(4), 3513–3522.

[6] Paster, A., Bolster, D., & Benson, D. A. (2013). Particle tracking and the diffusion-reaction equation. Water Resources Research, 49(1), 1–6.

[7] Fernàndez-Garcia, D., & Sanchez-Vila, X. (2011). Optimal reconstruction of concentrations, gradients and reaction rates from particle distributions. Journal of Contaminant Hydrology, 120–121(C), 99–114.

[8] Rahbaralam, M., Fernàndez-Garcia, D., & Sanchez-Vila, X. (2015). Do we really need a large number of particles to simulate bimolecular reactive transport with random walk methods? A kernel density estimation approach. Journal of Computational Physics, 303, 95–104.

[9] Sole-Mari, G., Fernàndez-Garcia, D., Rodríguez-Escales, P., & Sanchez-Vila, X. (2017). A KDE-Based Random Walk Method for Modeling Reactive Transport with Complex Kinetics in Porous Media. Water Resources Research.

In recent years, a number of Lagrangian approaches [e.g. 1,2,3,4,5] have been proposed for the numerical solution of nonlinear reactive transport problems. In these approaches, reactions are driven by the interaction between numerical particles that represent either a volume of fluid or a mass of solute. This interaction is a function of the relative position of particles and can be derived mathematically by equipping each particle with a kernel function, sometimes referred to as the smoothing function. In some of these approaches, the choice of the kernel shape and size does not follow objective criteria, but is instead rather heuristic. On the other hand, some authors have proposed that the smoothing function should be derived from the dispersive process [e.g. 2,6].

Recently, Fernàndez-Garcia and Sànchez-Vila [7] proposed that the kernel should represent the probability density distribution of the actual particle location, and should be globally optimized by minimizing the Mean Integrated Squared Error (MISE) of the density estimation. By using the existing algorithms for the determination of a time-dependent globally optimal Kernel Density Estimator (KDE), it is possible to compute probabilities (or rates) of reaction of numerical particles to model simple bimolecular reactions [8] or kinetic reactions of any complexity involving two reactants [9].

Most research on KDE focuses on 1D distributions of limited complexity. However, solute concentration distributions in heterogeneous media exhibit complex features and variations in space and time. For this reason, the globally optimal kernel is often far from being locally optimal, in particular, in those parts of the domain with relative small particle densities, and also in those with particularly abrupt concentration changes (such as mixing fronts). A novel approach is presented to define an adaptive locally optimal kernel function in 1-, 2- or 3-D that is not only time-dependent but also space-dependent. The presented local approaches are compared against the existing global ones, and also against the simple binning method, in the context of a Random Walk Particle Tracking (RWPT) model of reactive transport through a heterogeneous porous medium. The results show that the use of a locally adaptive kernel has a considerably positive impact on the accuracy of concentration estimations and chemical reaction simulations.

References:

[1] Tartakovsky, A. M., Meakin, P., Scheibe, T. D., & Eichler West, R. M. (2007). Simulations of reactive transport and precipitation with smoothed particle hydrodynamics. Journal of Computational Physics, 222(2), 654–672.

[2] Benson, D. A., & Meerschaert, M. M. (2008). Simulation of chemical reaction via particle tracking: Diffusion-limited versus thermodynamic rate-limited regimes. Water Resources Research, 44(12).

[3] Herrera, P. A., Massabó, M., & Beckie, R. D. (2009). A meshless method to simulate solute transport in heterogeneous porous media. Advances in Water Resources, 32(3), 413–429.

[4] Ding, D., & Benson, D. A. (2015). Simulating biodegradation under mixing-limited conditions using Michaelis-Menten (Monod) kinetic expressions in a particle tracking model. Advances in Water Resources, 76, 109–119.

[5] Engdahl, N. B., Benson, D. A., & Bolster, D. (2017). Lagrangian simulation of mixing and reactions in complex geochemical systems. Water Resources Research, 53(4), 3513–3522.

[6] Paster, A., Bolster, D., & Benson, D. A. (2013). Particle tracking and the diffusion-reaction equation. Water Resources Research, 49(1), 1–6.

[7] Fernàndez-Garcia, D., & Sanchez-Vila, X. (2011). Optimal reconstruction of concentrations, gradients and reaction rates from particle distributions. Journal of Contaminant Hydrology, 120–121(C), 99–114.

[8] Rahbaralam, M., Fernàndez-Garcia, D., & Sanchez-Vila, X. (2015). Do we really need a large number of particles to simulate bimolecular reactive transport with random walk methods? A kernel density estimation approach. Journal of Computational Physics, 303, 95–104.

[9] Sole-Mari, G., Fernàndez-Garcia, D., Rodríguez-Escales, P., & Sanchez-Vila, X. (2017). A KDE-Based Random Walk Method for Modeling Reactive Transport with Complex Kinetics in Porous Media. Water Resources Research.

a cargo de

Miguel Angel Marazuela

PhD Student

Jueves 1 de Marzo a las 12:15 h

en

Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

Salt flat brines are a major source of minerals and especially lithium. Moreover, valuable wetlands with delicate ecologies are also commonly present at the margins of salt flats. Therefore, the efficient and sustainable exploitation of the brines they contain requires detailed knowledge about the hydrogeology of the system. A critical issue is the freshwater-brine mixing zone, which develops as a result of the mass balance between the recharged freshwater and the evaporating brine.

The complex processes occurring in salt flats require a three-dimensional (3D) approach to assess the mixing zone geometry. In this study, a 3D map of the mixing zone in a salt flat is presented, using the Salar de Atacama as an example. This mapping procedure is proposed as the basis of computationally efficient three-dimensional numerical models, provided that the hydraulic heads of freshwater and mixed waters are corrected based on their density variations to convert them into brine heads. After this correction, the locations of lagoons and wetlands that are characteristic of the marginal zones of the salt flats coincide with the regional minimum water (brine) heads.

The different morphologies of the mixing zone resulting from this 3D mapping have been interpreted using a two-dimensional (2D) flow and transport numerical model of an idealized cross-section of the mixing zone. The result of the model shows a slope of the mixing zone that is lower than expected, similar to that obtained by 3D mapping. Additionally, the 2D model was used to evaluate the effects of heterogeneity in the mixing zone geometry. The higher the permeability of the upper aquifer is, the lower the slope and the shallower the mixing zone become. This occurs because most of the freshwater lateral recharge flows through the upper aquifer due to its much higher transmissivity, thus reducing the freshwater head. Similarly, aquitards further hinder the flow of groundwater to deeper layers and force it to flow through the upper aquifer.

Salt flat brines are a major source of minerals and especially lithium. Moreover, valuable wetlands with delicate ecologies are also commonly present at the margins of salt flats. Therefore, the efficient and sustainable exploitation of the brines they contain requires detailed knowledge about the hydrogeology of the system. A critical issue is the freshwater-brine mixing zone, which develops as a result of the mass balance between the recharged freshwater and the evaporating brine.

The complex processes occurring in salt flats require a three-dimensional (3D) approach to assess the mixing zone geometry. In this study, a 3D map of the mixing zone in a salt flat is presented, using the Salar de Atacama as an example. This mapping procedure is proposed as the basis of computationally efficient three-dimensional numerical models, provided that the hydraulic heads of freshwater and mixed waters are corrected based on their density variations to convert them into brine heads. After this correction, the locations of lagoons and wetlands that are characteristic of the marginal zones of the salt flats coincide with the regional minimum water (brine) heads.

The different morphologies of the mixing zone resulting from this 3D mapping have been interpreted using a two-dimensional (2D) flow and transport numerical model of an idealized cross-section of the mixing zone. The result of the model shows a slope of the mixing zone that is lower than expected, similar to that obtained by 3D mapping. Additionally, the 2D model was used to evaluate the effects of heterogeneity in the mixing zone geometry. The higher the permeability of the upper aquifer is, the lower the slope and the shallower the mixing zone become. This occurs because most of the freshwater lateral recharge flows through the upper aquifer due to its much higher transmissivity, thus reducing the freshwater head. Similarly, aquitards further hinder the flow of groundwater to deeper layers and force it to flow through the upper aquifer.

A cargo de: **Yoar Cabeza **Fecha: Jueves 22 de Febrero a las 12:15 h

Lugar: Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

Viscous fingering and wormhole growth are complex non-linear and unstable phenomena. Although they are two different cases, we propose simplifying both as a competition for water in which the capacity of a instability to grow depends on its ability to rob water from its surroundings. We compute this competition using an empirical model based on the calculation of capture areas, defined as the flow rate through a finger/wormhole divided by the natural flux. We derived empirical solutions to quantify the capture area in single, two, and multiple-finger/wormhole systems. Viscous fingers grow linearly with the computed capture areas and, for the wormholing case, we use an analytical solution to solve reactive transport within each wormhole and we compute the dissolution at the wormhole walls and tip. We generate patterns for viscous fingering and wormhole growth showing that both phenomena are controlled by competition and, for low Damköhler numbers, the results are comparable.