Simulation du transport reactif dans un milieu poreux discretement fracture (French text)

Numerical models that couple advective-dispersive transport and chemical interactions of several dissolved species are becoming increasingly popular in hydrogeology. Several such reactive transport models have been presented in the past, with varying degrees of complexity in the representation of the geological material and the chemical reactions. However, there are few examples of reactive transport models that explicitly consider fractures present in geological materials.; Recognizing that fractures can play a crucial role in controlling flow and transport in geological materials, a new reactive transport model has been developed and is presented here. This model, which is an extension of an existing model, solves the 3D flow and multispecies transport equations for discretely-fractured porous media using the control volume finite element technique. The porous geological matrix is discretized with thee-dimensional elements, and discrete fractures are represented by two-dimensional planar elements. The nodes forming the fracture element are superimposed onto the matrix nodes, thus allowing a fully there-dimensional description of the fracture network connectivity and permitting to explicitly simulate diffusion into the porous matrix. Any given chemical system can be considered, with several chemical species that are interacting and undergoing either chemical equilibrium reactions or kinetic reactions. The chemical system is solved by applying the law of mass action to each chemical reaction and the mass conservation law to each basic chemical component. The nonlinear algebraic system of equations arising from the chemical speciation model is solved using the Newton-Raphson technique, and a sequential iterative procedure couples the physical and chemical transport equations.; The model was verified with respect to multi-equilibrium reactions, kinetic reactions and dissolution reactions in a fracture. Verification results show that the mode] achieves a reliable quantification of the reactive processes in a fractured aquifer. An illustrative example is presented to evaluate the dissolution/precipitation fronts in a discretely-fractured carbonate aquifer. The results reveal that chemical reaction fronts are also controlled by fractures and that the topology of the reaction fronts geometry presented by Steefel and Lichtner (1998) for a single dicrete fracture is also applicable for a fracture network. Active fractures have great influence on mineral and species distribution in fractured media. This shows the necessity of explicitly considering both discrete fractures and chemical reactions in the modelling strategy.; The model constitutes a powerful and flexible simulation tool, which allows the user to specify a wide range of possible chemical reactions. It is targeted towards environmental hydrogeology, but also applicable to a full range of geochemical processes.