Simulation and analysis of solute transport in 2D fracture/pipe networks: The SOLFRAC program

International audienceThe Time Domain Random Walk (TDRW) method has been recently developed by Delay and Bodin (2001) and Bodin et al. (2003c) for simulating solute transport in discrete fracture networks. It is assumed that the fracture network can reasonably be represented by a network of interconnected one-dimensional pipes (i.e. flow channels). Processes accounted for are: (1) advection and hydrodynamic dispersion in the channels, (2) matrix diffusion, (3) diffusion into stagnant zones within the fracture planes, (4) sorption reactions onto the fracture walls and in the matrix, (5) linear decay, and (6) mass sharing at fracture intersections. The TDRW method is handy and very efficient in terms of computation costs since it allows for the one-step calculation of the particle residence time in each bond of the network. This method has been programmed in C++, and efforts have been made to develop an efficient and user-friendly software, called SOLFRAC. This program is freely downloadable at the URL http://labo.univ-poitiers.fr/hydrasa/intranet/telechargement.htm. It calculates solute transport into 2D pipe networks, while considering different types of injections and different concepts of local dispersion within each flow channel. Post-simulation analyses are also available, such as the mean velocity or the macroscopic dispersion at the scale of the entire network. The program may be used to evaluate how a given transport mechanism influences the macroscopic transport behaviour of fracture networks. It may also be used, as is the case, e.g., with analytical solutions, to interpret laboratory or field tracer test experiments performed in single fractures

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

Construction d'un modèle réservoir 3D par méthodes sismiques. Exemple d'un aquifère proche de la surface.

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Simulation of solute transport in discrete fracture networks using the time domain random walk method

International audienceThe time domain random walk (TDRW) method has been developed for simulating solute transport in discrete fracture networks. The following transport processes have been considered: advective transport in fractures, hydrodynamic dispersion along the fracture axis, sorption reactions on the fracture walls and decay reactions. The TDRW method takes advantage of both random walk and particle-tracking methods. It allows for the one-step calculation of the particle residence time in each bond of the network while avoiding mass balance problems at fracture intersections with contrasted dispersion coefficients. The accuracy of the TDRW method has been addressed by means of synthetic test problems into single fractures and into a 2D discrete fracture network. In each case, simulated and theoretical results compare very well

A dual flowing continuum approach to model denitrification experiments in porous media colonized by biofilms

International audienceWe present a modeling exercise of solute transport and biodegradation in a coarse porous medium widely colonized by a biofilm phase. Tracer tests in large laboratory columns using both conservative (fluorescein) and biodegradable (nitrate) solutes are simulated by means of a dual flowing continuum approach. The latter clearly distinguishes concentrations in a flowing porous phase from concentrations conveyed in the biofilm. With this conceptual setting, it becomes possible to simulate the sharp front of concentrations at early times and the flat tail of low concentrations at late times observed on the experimental breakthrough curves. Thanks to the separation of flow in two phases at different velocities, dispersion coefficients in both flowing phases keep reasonable values with some physical meaning. This is not the case with simpler models based on a single continuum (eventually concealing dead-ends), for which inferred dispersivity may reach the unphysical value of twice the size of the columns. We also show that the behavior of the dual flowing continuum is mainly controlled by the relative fractions of flow passing in each phase and the rate of mass transfer between phases. These parameters also condition the efficiency of nitrate degradation, the degradation rate in a well-seeded medium being a weakly sensitive parameter. Even though the concept of dual flowing continuum appears promising for simulating transport in complex porous media, its inversion onto experimental data really benefits from attempts with simpler models providing a rough pre-evaluation of parameters such as porosity and mean fluid velocity in the syste

Assessment of Groundwater Quality in the Dogger Aquifer of Poitiers, Poitou-Charentes Region, France

International audienceThe Poitou-Charentes, located in the Center-West of France, is a region where economy is based mainly on agriculture. This region, made up of 4 Departments (Vienne, Charente, Charente Maritime, Deux-Sèvres), is supplied largely by groundwater exploitation, both for consumption and for irrigation. This resource is thus vital to the region and its preservation is a major issue. The objective of this study is the determination of the groundwater quality in the Dogger aquifer of Poitiers (Vienne Department), which is the main water resource for this area and to achieve a better understanding of the factors influencing groundwater mineralization. Sixty-six wells, [...

Chenalisation de l'écoulement et du transport dans les milieux fracturés (approche discrète par réseaux de liens)

La complexité des réservoirs fracturés fait qu'aujourd'hui, aucune approche conceptuelle n est capable de proposer un modèle à la fois simple et précis. L'approche continue simple milieu est certainement la plus facile à appréhender mais reste imprécise en raison de son incapacité à homogénéiser toute l'information locale. Les approches multi-continuums et les approches discrètes du réseau de fractures s'avèrent plus judicieuses mais supposent des efforts numériques conséquents et une paramétrisation importante souvent non conditionnable sur les données disponibles. Le travail consigné dans ce manuscrit emprunte une modalité de représentation discrète du milieu avec ajout éventuel de continuums "stagnants" pour le transport de solutés. Le modèle se veut pour autant simple mais cependant moins précis. La perte de précision est le fait d'une prise en compte uniquement des flux majeurs (chenaux). La simplification est le fait d'une homogénéisation sur un lien 1D de l'hétérogénéité locale d'un chenal d'écoulement. Un réseau 3D de liens est créé dynamiquement sur la base préliminaire d'un semis de noeuds invariants puis en prenant en compte à la fois la direction du gradient hydraulique général et la géométrie des principales familles de fractures simulées. Le modèle peut ensuite calculer les écoulements en régime permanent et transitoire ainsi que le transport de soluté avec quelques effets réactifs. Plus spécifiquement, le transport utilise une méthode Lagrangienne dans le domaine des temps qui se révèle rapide et efficace sur un réseau de liens 1D. Au final, le modèle proposé s'avère intéressant car il génère un réseau simplifié et évolutif (déformable sur des points d'appui fixes, les noeuds) en fonction des conditions d'écoulement tout en préservant le comportement moyen d'un réseau de fractures. Les capacités de déformation du réseau de liens et la relative facilité de manipulation devraient, à terme, permettre d'aborder l'inversion de scénarios de transport à la fois sur les paramètres locaux des liens et la géomètrie du réseauThe complexity of fractured reservoirs makes that for the moment there is not any conceptual approach to these media either simple and accurate. The continuous simple-medium approach is probably the easier-one to handle but remains imprecise because unable to homogenize the local information. Multicontinuum approaches and discrete approaches to the fracture network are more relevant but induce some numerical efforts as well as a huge parameterization often unaffordable in terms of conditioning on available data. The work in this manuscript is on the side of a discrete representation of the medium with the eventual addition of stagnant continuums for simulating solute transport. The model claims to be but simple and thus slightly less accurate. The loss of precision is the consequence of accounting for main water fluxes (channels) only. The simplification comes from a 1D single bond homogenized representation of the local heterogeneity within each flowing channel. A 3D network of 1D bonds can be built dynamically by accounting for both the general head gradient and the geometry of the principal families of fractures. This network of bonds rests however on an invariant bombing of nodes representing bond intersections. The model can then calculate steady-state and transient flow as well as solute transport with a few additional retention and reaction mechanisms. Incidentally, solving transport is based on a Time Domain Random Walk method (TDRW) which is worth and rapid when handled over a network of 1D bonds. Finally, the model reveals interesting since it generates an evolutionary simplified network (the network can be deformed, or more exactly redistributed, while keeping invariant seed nodes) according to flow conditions and it is able to mimic correctly the mean behavior of a fracture network. The deformation capacity of the bond network and its relative ease of handling should allow in the end to tackle with the inversion of transport scenarios by optimizing both the local parameters of the bonds and the network geometry.POITIERS-BU Sciences (861942102) / SudocSudocFranceF