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Technical Report NTB 92-09

Time-space continuum description of fluid rock interaction in permeable media

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MPATH (Multiple Reaction Path Model) was developed for modelling coupled processes in geological and engineered systems. Key applications of MPATH include modelling the effect of cement in a repository on the surrounding host rock or oxidation of pyrite in the host rock during the operational phase of the repository. These applications will be dealt with in separate reports, while the present report discusses the basic principles of MPATH.

The feasibility of solving multi-component mass transport equations incorporating mineral reactions in a homogeneous porous medium over geologic time spans is considered for the case of pure advection. Solution of the mass transport equations is based on a quasi-stationary state approximation in which the time evolution of the geochemical system and the chemical reactions in the multi-component system are represented by a sequence of stationary states or reaction paths. The method is implemented in the computer code MPATH which solves mass transport equations in a single spatial dimension taking into account (irreversible) mineral precipitation/dissolution reactions and reversible and irreversible aqueous complexing reactions. An adaptable discretisation enables the positions of reaction zones, with widths which vary over many orders of magnitude and which move with greatly differing velocities, to be tracked over geologic time spans. For the code MPATH, there appears to be virtually no practical limitation to the number of chemical species that can be considered, given the computational capacity of present-day, high-performance workstations. The numerical accuracy of the solution to the mass transport equations can be verified through global mass conservation conditions and, in certain cases, by comparing the asymptotic kinetic solution with the corresponding solution to algebraic equations representing local equilibrium conditions for pure advective transport. In this report, the code MPATH is applied to several examples including migration of redox fronts, weathering of alaskite and the hydrothermal alteration of alaskite in a spatially varying temperature field.

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