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+34 93 401 18 60This email address is being protected from spambots. You need JavaScript enabled to view it.
UPC: C/ Jordi Girona 31, (08034 - Barcelona) - IDAEA: C/ Jordi Girona 18-26, (08034 - Barcelona)

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Training courses, events and seminars

Is diffusion the same as mixing?

Joaquim Soler

14 de Diciembre a las 12:15h

 Módulo D2 Planta Baja Aula CIHS

 

Is diffusion the same as mixing?

 ABSTRACT:

"Advection dispersion equation is the formulation most widely used to model solute transport in porous media. The dispersion term covers both: spreading of the plumes and mixing of the solute. In order to define the dispersion flux, concentration gradient is used. Because the two processes produce different effects, it is desired a distinction in the formulation. We propose a discussion about the very fundamental concepts of these processes (especially mixing and/or diffusion) which leads to a new formulation. A case study and results will be also presented."

Shear and Tensile failure in fragile rocks: a numerical and an analytical method

a cargo de
 

a cargo de: BERTA GÓMEZ (UPC-CSIC)


Jueves 18 de enero a las 12:15 h
en
Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja

Abstract:

Fracture failure is usually simulated by means of complex constitutive laws (i.e., plasticity, viscoelasticity, viscoplasticity, creep, etc.) which do not adequately represent the behavior of stiff, fragile rocks when their elastic limit is reached. The interest in the failure mechanism of this type of rocks has increased in the last years due to the progress of geological activities involving injection of fluids at depth (i.e., geological storage of CO2, enhanced geothermal energy or hydraulic fracturing operations). These applications usually provoke the creation of new fractures and/or the propagation of the pre-existing ones. Moreover, they may induce seismicity even after the shut-in, which is still not completely understood.

In the framework of the EU - FracRisk project, we want to improve the understanding of the failure process in fragile rocks and develop new methods to correctly simulate and predict the failure area in real fracking sites. In this regard, two methods to solve shear and tensile failure have been developed in the hydromechanical-application of the finite element framework Kratos. The first method is based on the analytical solution of Okada (1992), which is included in an iterative process to allow the simulation of the domino effect due to consecutive failure events that may take place. This is a straightforward method which avoids numerical issues but the underlying assumptions impose some restrictions to the modeling of real problems. In order to overcome these drawbacks, a novel numerical method has been developed. This method entails the construction of a Failure Matrix, which consists of the stress state variations in each fracture element due to a perturbation applied to each of them. Therefore, the superposition of the contribution to failure of each fracture element is considered. This Failure Matrix is specific for each model and it is built in a step previous to the beginning of the simulation.

The application of these methods allows us to correctly simulate failure in fragile rocks and to predict failure area and occurrence of seismicity during hydraulic fracturing operations.

Use of multi-element (C, Cl and H) isotope analysis to identify (bio)degradation pathways of halogenated contaminants in groundwater

a cargo de: Jordi Palau (CSIC-IDAEA)

Jueves 11 de Enero a las 12:15 h
en
Departamento de Ingeniería del Terreno, Aula CIHS, Planta Baja


Abstract:
Groundwater contamination by halogenated hydrocarbons like chlorinated ethanes is a major environmental problem and it has an adverse impact on water resources. In order to evaluate the fate and long-term impact of halogenated hydrocarbons in the aquifer, understanding of in situ contaminant transformation reactions is essential. Contaminant remediation schemes designed to enhance in situ degradation need to consider the reaction pathways by which contaminant transformation occurs and the rate of those reactions. However, the high susceptibility of chlorinated ethanes to be transformed via distinct degradation pathways complicates the assessment of their fate in the subsurface. This study investigates for the first time the use of a multi-element (C, Cl and H) isotope approach to identify the degradation pathways of 1,2-dichlorethane using five microbial cultures. In addition, this approach was tested in two contaminated field sites. The results demonstrated the potential of a multi-element isotope approach to identify 1,2-dichlorethane degradation pathways in groundwater, which opens further possibilities for pathway identification in future field studies.

Modeling of reactive transport with particle tracking and kernel density estimators

LECTURA DE TESIS

Maryam Rahbaralam

 
Thesis advisors: Dr. Daniel Fernández García (UPC) / Dr. Xavier Sànchez-Vila (UPC) 

The defense will take place:
Friday, February 23rd 2018, 12:00

Place:
UPC, Campus Nord
Building C1. Classroom: 002
C/Jordi  Girona, 1-3

08034 Barcelona
 
Abstract:

Random walk particle tracking methods are a computationally efficient family of methods to solve reactive transport problems. While the number of particles in most realistic applications is in the order of 106-109, the number of reactive molecules even in diluted systems might be in the order of fractions of the Avogadro number. Thus, each particle actually represents a group of potentially reactive molecules. The use of a low number of particles may result not only in loss of accuracy, but also may lead to an improper reproduction of the mixing process, limited by diffusion. Recent works have used this effect as a proxy to model incomplete mixing in porous media. The main contribution of this thesis is to propose a reactive transport model using a Kernel Density Estimation (KDE) of the concentrations that allows getting the expected results for a well-mixed solution with a limited number of particles. The idea consists of treating each particle as a sample drawn from the pool of molecules that it represents; this way, the actual location of a tracked particle is seen as a sample drawn from the density function of the location of molecules represented by that given particle, rigorously represented by a kernel density function. The probability of reaction can be obtained by combining the kernels associated with two potentially reactive particles. We demonstrate that the observed deviation in the reaction vs time curves in numerical experiments reported in the literature could be attributed to the statistical method used to reconstruct concentrations (fixed particle support) from discrete particle distributions, and not to the occurrence of true incomplete mixing. We further explore the evolution of the kernel size with time, linking it to the diffusion process. Our results show that KDEs are powerful tools to improve computational efficiency and robustness in reactive transport simulations, and indicates that incomplete mixing in diluted systems should be modeled based on alternative mechanistic models and not on a limited number of particles.

Motivated by this potential, we extend the KDE model to simulate nonlinear adsorption which is a relevant process in many fields, such as product manufacturing or pollution remediation in porous materials. We show that the proposed model  is able to reproduce the results of the Langmuir and Freundlich isotherms and to combine the features of these two classical adsorption models. In the Langmuir model, it is enough to add a finite number of sorption sites of homogeneous sorption properties, and to set the process as the combination of the forward and the backward reactions, each one of them with a pre-specified reaction rate. To model the Freundlich isotherm instead, typical of low to intermediate range of solute concentrations, there is a need to assign a different equilibrium constant to each specific sorption site, provided they are all drawn from a truncated power-law distribution. Both nonlinear models can be combined in a single framework to obtain a typical observed behavior for a wide range of concentration values. This approach opens up a new way to predict and control an adsorption-based process using a particle-based method with a finite number of particles. 

Finally, by classifying the particles to mobile and immobile states and employing transition probabilities between these two states, we take into account the porosity of the diluted system in the KDE model.  The state of a particle is an attribute that defines the domain at which the particle is present at a given time within the porous medium. The  transition probabilities are controlled by two parameters which implicitly determine the porosity. Simulations results show a good agreement with the analytical solutions of complete and incomplete mixing solutions, independent of the number of particles. A transition between the complete and incomplete mixing solutions is also obtained, showing a good match with a transition probability function. These results show the potential of our proposed model to simulate reactive transport problems in porous media.

Simulation of Thermo-Hydro-Mechanical Effects of Gas Injection in Carbonate Reservoirs on the Caprock Integrity

SEMINARIO GRUPO DE HIDROLOGÍA SUBTERRANEA (UPC-CSIC)

Día: 30 de noviembre
Hora: 12:15h
Lugar: Aula del CIHS, Modulo D2, UPC

Autor: AHMAD ZAREI
Amirkabir University of Technology - Tehran Polytechnic.

Abstract: To attain the maximum production in hydrocarbon reservoirs, the use of enhanced oil recovery (EOR) methods is required. One EOR method consists in injecting gas, e.g., methane or CO2, in order to compensate the reservoir pressure decline and subsequently improving hydrocarbon production. This method causes stress changes in the injection region and may affect the geomechanical stability of the caprock. Caprocks of the studied carbonate reservoirs present complex structures and are highly layered, with alternating marl, salt, anhydride, shale and sandstone layers. Furthermore, information on geomechanical properties of these formations is scarce. These limitations and complexities highlight the challenges of performing comprehensive geomechanical studies to assess the short- and long-term integrity of the caprock during gas injection treatments. The overall objective of this research is to gain understanding of the geomechanical response of a fractured reservoir and its caprock to gas injection and to identify the regions of the caprock which are prone to undergo tensile or shear failure during the several cycles of production-injection along the reservoir lifecycle. To this end, thermo-hydro-mechanical coupled numerical modeling of gas injection in naturally fractured carbonate reservoirs, simulating the pressure buildup and temperature change in the fractures during the gas injection procedure will be performed

The dissolution kinetics of montmorillonite in acidic pH: experimental data and reactive transport (RT) modeling

a cargo de

Jordi Cama
Investigador Científico del CSIC

Jueves 23 de Noviembre a las 12:15 h
en
Departamento de Ingeniería del Terreno, Aula CIHS, Planta Baja


Abstract:
Well-mixed flow-through experiments are used to study the dissolution kinetics of K-montmorillonite at 25 °C, acidic pH (2-4) and 0.01 M ionic strength. The variations of Si, Al and Mg over time result in high releases of Si and Mg and Al deficit and also in long periods of incongruent dissolution before reaching stoichiometric steady state. 1D RT simulations of the experimental data are performed to quantitatively interpret the evolution of the released cations and to elucidate the stoichiometry of the reaction.
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