A seminar by Modeste Irakarama, Gautier Laurent and Julien Renaudeau at ENSG, Nancy, room G201

On Friday 1st of December, 1:00 pm.

We introduce a new method for implicit structural modeling. The method exploits the highly symmetric nature of the Cartesian grid to implement efficient finite difference operators. The proposed method has the advantage of being much more simple to implement, and arguably numerically more efficient, than existing methods.


A seminar by Agathe Carbonie at ENSG, Nancy, room G201

On Friday 24th of November, 1:00 pm.

Geological structural models (geomodels) representing the subsurface, individualize structures such as rock units or fault compartments.
In order to use these models for mesh based numerical simulations, their meshes must integrate these structures. Indeed, it allows to store consistent physical properties in each compartments and differentiate locally the rock mechanical properties.

We propose a method to update a geomodel by cutting its mesh elements. We implemented for this a version of the marching tetrahedra algorithm that keeps the mesh tetrahedral. This work has been developed with an implicit modeling framework where geological objects are represented as iso-values of a 3D scalar
field. Therefore, expliciting iso-surfaces within an implicit geological model can be useful for visualization and giving a boundary representation to the model.

The method starts with a purely tetrahedral mesh model. The mesh elements are first cut by an iso-value of an implicit property. Then the geomodel is updated,  replacing the cut components by the individualized ones.
This work opens new perspectives for automatic mesh simplification and for incremental structural modeling. The module has been implemented in SCAR library using RINGMesh geomodels.

A seminar by Pauline Collon at ENSG, Nancy, room G201

On Friday 17th of November, 1:00 pm.

Ce séminaire sera l'occasion de vous présenter le système karstique de Furfooz (région de Dinant, Belgique) sur lequel Lorraine Dewaide, que je co-encadre, effectue sa thèse à l'Université de Namur. Dans ce système, développé dans les mudmounds Waulsortiens, différentes investigations de terrain ont été réalisées afin d'en comprendre le fonctionnement. Parmi celles-ci, plusieurs essais de traçage ont été caractérisés par l'apparition d'un "double-pic" assorti d'une "traînée". Plusieurs interprétations sont possibles pour expliquer ce double-pic. Pour mieux comprendre les phénomènes potentiellement en jeu, un analogue expérimental a été réalisé. En parallèle, un code numérique a été développé pour simuler le transport 1D de soluté dans des "conduits" parallèles échangeant entre eux.

A seminar by Nicolas Mastio at ENSG, Nancy, room G201

On Friday 10th of November, 1:00 pm.

A geomechanical model is usually contained inside a rectangular box shape. Displacements on the edges are fixed in some direction in order to impose the model to keep his shape. This kind of boundary conditions are called Dirichlet boundary conditions. Nevertheless, this kind of boundary conditions create boundary effects. Rather than imposing the ground to be contained in a rectangular box, a geomechanical model could be better represented by a semi infinite volume. Such a representation obviously cannot be solved by the finite element method, but Neumann boundary conditions can be found in the rectangular box which mimic the semi infinite extention of the model.

I will present the bibliography and the method I intend to use and I hope to present the results at the end of the year.

A seminar by Antoine Mazuyer at ENSG, Nancy, room G201

On Friday 13 of Octobre, 1:00 pm.

Estimating the stress in reservoirs and their surroundings prior to the production is a key issue for reservoir management planning. In this study, we propose an integrated inverse method to estimate such initial stress state. The 3D stress state is constructed with the displacement-based finite element method assuming linear isotropic elasticity and small perturbations in the current geometry of the geological structures. The Neumann boundary conditions are defined as piecewise linear functions of depth. The discontinuous functions are determined with the CMA-ES (Covariance Matrix Adaptation Evolution Strategy) optimization algorithm to fit wellbore stress data deduced from leak-off tests and breakouts. The disregard of the geological history and the simplified rheological assumptions means that only the stress field, statically admissible and matching the wellbore data should be exploited. The spatial domain of validity of this statement is assessed by comparing the stress estimations for a synthetic folded structure of finite amplitude with a history constructed assuming a viscous response. 

A seminar by Pierre Martz at ENSG, Nancy, room G201

On Friday 23d of June, 1:00 pm.

Multiple ages from uranium oxides spanning over more than 1 Ga have been proposed as markers of several U-mineralizing events in the Athabasca basin (Saskatchewan, Canada) but the understanding of the nature and significance of the potential distinct U-mineralizing events is difficult. To unravel this potential complex sequence of fluid-flow events and to assess the importance of each events in the formation and evolution of the deposit as it is currently depicted, an integrated and systematic study has been carried out in 2D by coupling mineralogy, in situ U-Pb dating, and trace element geochemistry of UO2 from samples coming from two crossections crosscutting the Cigar Lake orebody, a world-class sandstone-hosted archetype of unconformity-related U deposit. This study shows that a first and major event of U deposition occurred at ~1430 followed by one poorly constrained non mineralizing major brecciation event occurring between 1000 and 700 Ma and three much younger events of U remobilization and deposition at ~340, ~225, and ~205 Ma. Each mineralization stage is characterized by a specific fingerprint including distinct mineralogical associations and geochemical signatures in trace elements. Using geochemical data from drillholes, a 3D scale modelization has been built showing the spatial repartition of the distinct U oxide types. It reveals that the U stock of the deposit was deposited during the first mineralizing stage, currently preserved in the core of the deposit, right at the unconformity and along the E-W shear structurally controlling the deposit. However younger fluid flow events have greatly remobilized the primary U stock so that the present mineable volume is now in fact formed by young mineralization (<350 Ma).

A seminar by Paul Cupillard at ENSG, Nancy, room G201

On Friday 2nd of June, 1:00 pm.

La rétropropagation de signaux sismiques enregistrés à un ensemble de récepteurs permet de focaliser l'énergie sismique à son origine avec une précision plus ou moins grande. Ce phénomène est permis par la réversibilité en temps et en espace de l'équation des ondes. Dans cette présentation, je jetterai les bases théoriques du phénomène puis discuterai de sa possible utilisation dans la caractérisation d'évènements microsismiques en m'appuyant sur la bibliographie récente.

A seminar by Julien Renaudeau at ENSG, Nancy, room G201

On Monday 22nd of May, 1:00 pm.

There exists two different approaches to solve the implicit structural modeling problem that have been proven sufficiently efficient to be widely used. The first one is a mesh based approach consisting in solving both smoothness and data constraints, while the second one is a meshless approach consisting in solving only data constraints. The presence or not of the mesh radically changes the problem at hand. In the first case, data information is reverberated to its surroundings by the mesh connectivity and the smoothness constraint, resulting in a sparse system. In the second
case, data information is perceived everywhere in the studied domain thanks to globally defined shape functions, and which generates a dense system. Therefore, while avoiding the mesh creation in the second approach, we still have to deal with the problematic dense system, making a strong argument of efficiency for the first approach.
In this article, we suggest a third alternative: a meshless approach consisting in solving both smoothness and data constraints, but with locally defined shape functions. Our goal is not to demonstrate a perfect combination but to show that the local meshless concept in structural modeling enables to avoid the creation of a complex mesh while solving a sparse system and still obtaining comparable results. Moreover, we use the variational minimization of the bending energy in order to constrain the smoothness, which is new in structural modeling.

A seminar by Francois Bonneau at ENSG, Nancy, room G201

On Friday 12th of May, 1:00 pm.

This paper proposes to improve the natural fracture network characterisation by analysis microseismic data. We use a 3D Hough transform to build fracture orientation and density maps describing discontinuities that cannot be described by other measurements. The stochastic Discrete fracture simulator combines geostatistical and pseudo-genetic approaches to generate models that honour both measures from field characterisation and hierarchical organisation that result from the fracturing process itself. We suggest that using realistic fracture network characterisation and introducing mechanical concepts in the DFN simulation process affect connectivity of DFNs, hence is essential for predictivity of reservoir flow models.