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

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

A new method for generating a pipe network to handle channelled flow in fractured rocks

International audienceThe pipe network generation proposed here assumes that flow in a 3D fracture field is highly channelled, and that there exist invariant crux nodes corresponding to crossroads connecting preferential flow paths whatever the mean head gradient's direction. The method allows preferentially for these crux nodes by conditioning the simulated network first on a ‘seeding' of nodes and second, in generating 1D pipes between these nodes. Note that usually, nodes are the consequence (the intersection) of the pipe generation. Pipes can be also conditioned on their length and other features, but this conditioning is not strict, since all pipes must pass by the nodes. In the end, the network is simple without dead ends; each cluster is flowing and connected to the boundaries of the system. This type of network is expected to facilitate further calculations, for instance solving inverse problems in deforming the network by simply moving the crux node locations