A coupled model for transport of multiple chemical species and kinetic precipitation/dissolution reactions with application to reactive flow in single phase hydrothermal systems

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Abstract
The essential features of a numerical model for computing coupled multi-component chemical reactions, multi-species chemical transport, hydrodynamic flow, and heat transfer are described. The model employs a new algorithm which solves simultaneously for multicomponent reactions and solute transport in one and two dimensions and which uses kinetic formulations for mineral dissolution and precipitation reactions, making the a priori assumption of equilibrium between water and minerals unnecessary. This feature is then used to assess the validity of the local equilibrium approximation in single phase hydrothermal systems. The code is also used to examine the problem of reaction-induced porosity and permeability changes in a fractured hydrothermal system. The numerical calculations indicate that significant disequilibrium with respect to silicate phases is likely in thermal boundary layers developed at fow temperature, permeable interfaces (for example, at the seafloor). Disequilibrium is less pronounced in the thermal boundary layers of systems with impermeable upper surfaces because of the local reduction in flow velocities near the boundary. The calculations show that the extent of disequilibrium in thermal boundary layers formed in fractured rock depends on the fracture spacing (or fracture aperture) and on the flow rate, which affects both the rate of solute transport and the thickness of the thermal boundary layer. In the high temperature, nearly isothermal inner portions of high Rayleigh number convection cells, disequilibrium is possible only where very rapid flow and/or widely spaced fractures occur. In systems where the flow is sufficiently slow that linear temperature gradients occur, disequilibrium with respect to silicate phases is likely only in the case where the fractures are extremely widely spaced. The calculations also suggest that the presence or absence of metastable phases (for example, amorphous silica) may be used to estimate permeabilities in paleo-hydrothermal systems if fracture spacings can be determined. Two dimensional calculations of reactive flow in a model single phase geothermal field with a porosity-permeability relationship such as that observed in the contact aureole of the Skaergaard intrusion in Greenland (Manning and Bird, 1991) suggest that the local equilibrium approximation is fully justified in such a system. The same model geothermal system and porosity-permeability relationship have also been used to study the effects of reaction-induced porosity and permeability change on the character of the convective regime. The calculations indicate that the rates of permeability change may be sufficiently rapid that the convection cell never attains a hydrodynamic or thermal steady state. Permeability reduction, which tends to occur where upwelling fluids cool, causes the plume to become increasingly diffuse with time because the ascending fluids diverge around the cemented zone. Permeability enhancement, which most commonly occurs where fluids move up temperature, can result in an instability which causes channeling of flow. These effects do not depend on any particular concentration boundary condition but rather are the natural consequence of imposed thermal gradients in hydrothermal systems.