MODELING COUPLED VARIABLE-DENSITY FLOW AND REACTIVE SOLUTE TRANSPORT IN DISCRETELY-FRACTURED POROUS MEDIA

A three-dimensional numerical model is developed that couples reactive transport of silica with variable-density, variable-viscosity flow in fractured porous media. The new model also solves for heat transfer in fractured porous media. The fluid properties density and viscosity as well as chemistry constants (dissolution rate constant, equilibrium constant and activity coefficient) are calculated as a function of the concentrations of major ions and of temperature. Reaction and flow parameters, such as mineral surface area and permeability, are updated at the end of each time step with explicitly calculated reaction rates. Adaptive time stepping is used to accelerate and slow down the simulation process in order to prevent physically unrealistic results. New time increments depend on maximum changes in matrix porosity and/or fracture aperture. The code is verified against existing analytical solutions of heat transfer and reactive transport in fractured porous media. The complexity of the model formulation allows studying chemical reactions and variable-density flow in a more realistic way than done previously.

The developed model was applied to simulate some examples of coupled density-driven flow and reactive transport in fractured media. Thermohaline (double-diffusive) transport impacts both buoyancy-driven flow and chemical reactions. Free convective flow depends on the density contrast between the fluid (hot saltwater or cooler freshwater) and the reference fluid. However, density contrasts are generally small and fractures do not act like preferential pathways but contribute to transverse dispersion of the plume. Hot zones correspond to areas of quartz dissolution while in cooler zones, precipitation of imported silica prevails. The silica concentration is inversely proportional to salinity in high-salinity regions and proportional to temperature in low-salinity regions. The model was also used to carry out a sensitivity analysis. The system is the most sensitive to temperature inaccuracy. This is because temperature impacts both the dissolution kinetics (Arrhenius equation) and the quartz solubility (Rimstidt, 1997, GCA).