Speaker: Ahmad Mostafa

Date: Thursday 10^th of November 2022, 1:15 pm.

Abstract:
Coal is fractured by nature; it is a dual porosity/permeability system made up of a porous matrix surrounded by fractures known as (figure (1), scale II). The orientation of this quasi-orthogonal cleat network including tensile fractures or face cleats, and compressive and strike-slip fractures or butt cleats depends on the principal stress’s direction (Laubach et al., 2017) and provides pathways for fluid flow with apertures up to 100 microns. In contrast, methane gas is stored within the low porosity coal matrix with pore sizes generally varying from a few to several dozens of nanometers (Li et al., 2017) (Wang et al., 2018). The model implemented in the open-source software Yade DEM (Smilauer et al., 2015) is based on the hydro-mechanical model proposed by (Catalano et al., 2014). The coal matrix is treated as an assembly of bonded particles interacting one with another through elastic-brittle contact laws as initially proposed by (Scholtes and Donze, 2013). The pore space is discretized into tetrahedra, generated from a regular triangulation of the particle’s centers. Knudsen and surface diffusions as well as sorption processes are modeled considering the coal matrix as a microporous material. The method is hydro-mechanically coupled in the sense that changes in pore pressure produce hydrostatic forces that deform the solid skeleton, while deformation of the pore space induces pore pressure changes that promote interpore flow. In addition, sorption induced deformations are taken into account by considering an additional pressure term related to the concentration of gas within the medium (the so-called solvation pressure) (Vandamme et al., 2010). The cleat system is considered as a discrete fracture network where a single-phase advection flow takes place. The ultimate goal of my PhD thesis is to combine the models describing the different transport processes at stake in both the coal matrix and the cleat network to provide a multi-scale model able to simulate the hydro-mechanical behavior of coal in the context of coal bed methane recovery.

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Category: Seminar

Speaker: Mariam Joundi

Date: Thursday 20^th of October 2022, 1:15 pm.

Abstract:
Les joints rocheux sont des discontinuités dans les massifs rocheux représentant des sources d'instabilités mécaniques. La formation de ces joints varie d’un massif à un autre en fonction de l’intensité des contraintes, de la rhéologie des matériaux, de la présence ou non de fluides, de l’histoire géologique et le chemin de chargement suivi par les matériaux, etc. La morphologie des joints rocheux et le comportement mécanique de la matrice sont reconnus comme des éléments clés contrôlant le comportement mécanique du joint, notamment en cisaillement, incluant ses phases pré-pic et post-pic ainsi que sa dilatance. Ce travail étudie le comportement en cisaillement et les processus d'endommagement des joints rocheux sollicités mécaniquement à l’aide du logiciel UDEC. Des joints de morphologies différentes et de matrices intactes variées sont explicitement simulés dans des essais de cisaillement direct sous plusieurs valeurs de contrainte normale appliquée. L'effet de la magnitude de la contrainte normale appliquée, de la morphologie du joint et des propriétés mécaniques de la matrice intacte sur le comportement au cisaillement du joint sont évalué. L'analyse numérique a montré que sous faibles contraintes normales, le comportement est contrôlé par le glissement de la surface du joint. Dans ce cas, le déplacement normal reflète la géométrie de la surface du joint. Alors que sous contraintes normales plus élevées, un déplacement normal relativement limité est observé en raison du processus de dégradation des aspérités. Il a été constaté que les valeurs de la résistance au cisaillement maximale et résiduelle augmentent lorsque la contrainte normale appliquée augmente. La résistance au cisaillement augmente également pour une matrice plus rigide. En outre, il a été prouvé que la présence des aspérités augmente la résistance au cisaillement globale des joints.

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Category: Seminar

Speaker: Coffi Gbewade

Date: Thursday 13^th of October 2022, 1:15 pm.

Abstract:
The Callovo-Oxfordian (COx) claystone in Meuse/Haute-Marne (France) is considered as a potential host rock in the French concept of high-level radioactive waste disposal at great depth. The excavation of galleries or micro tunnels in this host rock located at a depth of 500 meters in Bure (in the Meuse department), creates a fractured zone around them called the Excavation Damaged Zone (EDZ). The heat generated by waste could affect the favorable properties for containment of this host rock, especially its transport properties. The overpressure generated by the difference between thermal expansion coefficient of pore water and the solid rock skeleton could induce fractures opening or propagation in the near field (i.e., in the EDZ), altering the permeability. Then, laboratory experiments need to be performed to study the effects of increased temperature on the mechanical behavior of the COx claystone. These effects could be related to the evolution of the effective stress due to the temperature increase and to the modification of the physical properties of rock-forming minerals. Deviatoric (uniaxial and triaxial) compression tests are carried out in laboratory on cylindrical samples of the COx Claystone with a diameter of 20 mm and a heigh of 40 mm. Tests are performed under different temperatures from 20°C to 100°C to study the impact of temperature increase on the mechanical parameters of the host rock. The samples are cored in two directions, parallel and perpendicular to the bedding plane. The tests are carried out in a conventional triaxial cell equipped with two heating collars connected to a heat generator to apply the temperature imposed on the system. Triaxial tests are performed at low confining pressures (0 and 4 MPa) representative of the near field conditions (i.e, in the EDZ). Confining pressure is generated by a syringe pump while the axial stress is applied using a hydraulic press. Axial and lateral deformations are continuously measured during the mechanical loading using strain gauges. The deviatoric compression tests are performed under controlled displacement in order to characterize the post-peak behavior. Unloading-reloading cycles are performed at different stress levels in order to estimate the evolution of elastic properties. Uniaxial tests performed at temperatures ranging from 20 to 100°C, for both directions (parallel and perpendicular to the bedding) showed a significant decrease in the peak resistance due to the temperature increase. This decrease in the peak resistance is almost non-existent on triaxial tests (4 MPa confining pressure) in the parallel orientation, thus showing the impact of confining pressure. Nevertheless, an increase in the peak resistance is observed for a temperature of 150 °C for both cases. Further tests will be necessary by adjusting the thermal loading rate and the time between the thermal and mechanical loading, in order to see their impact on the mechanical properties of the COx claystone.

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Category: Seminar

Speaker: Paul Marchal

Date: Thursday 29^th of September 2022, 1:20 pm.

Abstract:
Large volume of scientific and technical data have been acquired within research & development projects carried out by Orano and the Université de Lorraine. Throughout this long-term collaboration, top-ranked articles have been published based on genuine datasets resulting from specialized analyses, in particular mineralogy, isotope geochemistry and fluid composition. Samples are stored in a well referenced lithothèque and regularly used for new batches of analysis. These results have largely contributed to the knowledge of the unconformity related deposits and are considered as key parameters for defining new projects, constraining any modelling and defining machine learning analysis strategies. In order to provide access to these numerical datasets through Otelo and to connect to the Orano standards under acQuire, a joint-project for data valorization has been set up in order to develop semi-automated methods for historical data integration and design of database templates. This project aims first to provide standards methods and templates to help new data integration into Otelo’s data repository and secondly to build tools to format and integrate historical datasets. The application of these methods and standards contribute to making datasets easier to find, accessible, interoperable and reusable (FAIR Principles). For dataset, we focused especially on drillhole data analysis, i.e. data associated with sampling on cores. Concerning new data integration, initial work was the redaction of good practice guidelines1 for data management based with focus on: Explicit & coherent data naming; access & centralization of all the necessary data; use of international standards; minimization of useless duplicates & blanks. To implement, we elaborated a standardized data template containing all required metadata data fields, in order to generalize for each new project, the realization of an integrable data file collecting all samples analysis data. Then, based on established models and templates, we designed a JavaScript web-application, on the one hand to help new file edition & also to assess historical data files quality and on the other hand extract information from sparse files. We had also implemented a geographical visualization tool based on drillholes. It allows to compare data from different projects & to extract samples lists according to available attributes, either it’s about project metadata or analysis data values.

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Category: Seminar

Speaker: Nathan Amrofel

Date: Thursday 15^th of September 2022, 1:15 pm.

Abstract:
A full understanding of the migration behavior of corrosion gases in clay rock is of fundamental importance for the reliability of scenarios predicting the long-term evolution of geological repositories. Due to the low permeability of host clay rock, the produced gas will accumulate as a distinct phase until the pressure becomes large enough. The high pressure generated will desaturate the surrounding clay rock by displacement of pore water far along gas paths, but also by the diffusion of water vapor through the gas. In order to better understand the impact of key transport processes occurring in gas flow in clay material, a pore-scale numerical study taking into account the capillary-dominated two-phase flow, the diffusion of water vapor in the gas phase and specific features of nanoporous materials such as kelvin effect is proposed. The work has been carried out using the Smoothed Particle Hydrodynamics (SPH) method, a Lagrangian and meshless method which has emerged as an efficient and reliable tool for simulating complex fluid flows. A drying algorithm with Kelvin effect, which drives the thermodynamic equilibrium between the fluid phase and the gas phase at nanoscale, has been implemented in a two-phase flow SPH code, initially developed at IRSN.

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Category: Seminar

Speaker: Narges Dashtbesh

Date: Thursday 23^rd of June 2022, 1:15 pm.

Abstract:

Understanding and modelling contaminant transport is necessary to assess the lifetime of pollution sources and their severity and optimize the remediation strategies. The transfer of contaminants from the NAPL (Non-Aqueous Phase Liquid) phase to the aquifer is a multi-scale problem driven by mass transfer between both phases and is generally described by local non-equilibrium models. While the mass exchange coefficient, an important property in such models, play a key role in the fate of the pollution source, it is usually approximated by a constant value estimated from empirical correlations. However, it generally shows a transient behaviour and can evolve with NAPL phase composition and relative solubilities, which remains poorly studied. In this work, we upscale numerically this effective property from 2D pore-scale numerical simulations. We study the impact of different factors on the form and behaviour of the mass exchange coefficient in the presence of a multi-component NAPL source depending on whether the internal mass transfer is limiting. The potential implications of replacing this time-and-space-dependent mass transfer coefficient with a constant and unique value are discussed.

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Category: Seminar

Speaker: Julien Herrero

Date: Thursday 16^th of June 2022, 1:15 pm.

Abstract:

Evaluation of georesources involves to appropriately manage the level of detail needed in geomodels for subsurface porous flow and transport problems. The non-uniqueness of the problems considered in aquifer or reservoir modeling calls for using methods such as stochastic Bayesian inversion to appropriately estimate rock property parameters and quantify uncertainty. These approaches often consider a fixed number of model parameters. In geomodeling, however, some model components are discrete at the scale of concern (e.g., minerals, facies, fractures, layers), hence lead to unknown number of parameters in the inverse problem. To address this issue, we propose to use transdimensional Monte Carlo methods (also known as reversible jump Markov chain Monte Carlo) which are a way to solve the inverse problem with a suitable geological parameterization when the number of model parameters is an unknown. We consider as a first application example the 2D case of implicit layer interfaces defined by a level-set in a porous reservoir model. Using a Voronoi diagram parameterization, the number of discrete layers becomes an unknown parameter. Rather than randomly perturbing the petrophysical field of interest, we build the prior model by an averaging process of log data located inside layers. Hence, each layer is defined by thickness and interface depth information, and a constant petrophysical value. Through a history matching problem, a set of flow simulations is performed to generate production data used in the Markov chain as an acceptance criterion. These numerical simulations can only be solved on a model discretization conformal to discontinuities. To address this challenge, we capitalize on the local mesh updating strategy presented by Legentil et al. (2022). First results on a set of horizontal layers demonstrate that the algorithm is capable to capture the main geological discontinuities from a prior permeability model and flow data, suggesting that this transdimensional tool could be applied for more complex geometries such as anticlines or faulted reservoir models.

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Category: Seminar

Speaker: Gloria Arienti

Date: Thursday 19^th of May 2022, 1:15 pm.

Abstract:

Introduction to the geomodelling workflow that is being used to build the new 3D structural model of the Italian North-Western Alps. Input data are represented by structural surveys and detailed geological mapping, defining a truly 3D dataset that compensates for the absence of subsurface data thanks to important differences in elevation of 3-4 km, from valley floors to mountain summits. Our modelling workflow is based on a first step of conceptual modelling in vertical cross-sections, based on classical and sound structural concepts, followed by interpolation with implicit and explicit surface algorithms. In our area, geological complexity is given by several finite faults, internal to the model, that intersect each other as the result of multiple brittle deformation events. The basement units are also interested by polyphase ductile tectonics: isoclinal and cuspate-lobate folds, polyphasic folds, and other structures that produce great thickness variations of the lithological bodies.

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Category: Seminar

Speaker: Luyu Wang

Date: Thursday 12^th of May 2022, 1:15 pm.

Abstract:

In a geological repository, percolation of gas through clay rock may generate an expansion of the conductive pathways, which will lead to a localized displacement of water away from these paths. To this end, this work investigates the hydromechanical coupling during gas migration in saturated porous media. A mesoscopic model based on the multiple relaxation time Lattice Boltzmann Method (MRT-LBM) and the spring model is proposed, in which the interpolated bounce-back (IBB) boundary is applied to capture the exact displacement of the deformable pore walls with irregular shapes. We propose a numerical assessment of this non-conforming method, coupling IBB and LBM, by comparing with the reference solution obtained on the boundary-conforming grid. The impact of grid resolution on accuracy is discussed for single- and two-phase flow conditions.

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Category: Seminar