A model for the kinetic control of quartz dissolution and precipitation in porous media flow with spatially variable permeability: Formulation and examples of thermal convection

We present the formulation and model results of kinetically controlled quartz dissolution and precipitation in a two-dimensional heterogeneous permeable medium. The quartz matrix is modeled as a partially occluded spherical close pack, with dissolution and precipitation occurring on the exposed faces of the grains. This formulation yields larger permeabilities and lower surface area to fluid volume ratios in regions of larger grain radii, and thus allows us to investigate the influence of crack-like regions on the flow and silica exchange. We use the kinetic data for quartz dissolution of Rimstidt and Barnes [1980]. Thermal convection results indicate that channelizing of fluid flow in high-permeability zones is enhanced in the transient regime by buoyancy effects arising from the advection of heat. The highly permeable zones are most out of chemical equilibrium, owing to their lower surface areas and to more rapid advection. Porosity changes are most pronounced in regions of high surface area often downstream of high-permeability zones. Oscillatory convection is observed, accompanied by saturation state reversals peripheral to the high-permeability zones. Such internally generated oscillatory regimes provide a mechanism for quartz zonation down to the nanometer scale. When the thermal forcing is strong enough, recurrent plumes emanate from the thermal boundary layers and plunge through the high-permeability zones. When departures from equilibrium become significant, we observe some regions of undersaturated upwelling fluid moving down temperature, and regions of oversaturated downwelling fluid moving up temperature, both cases opposing the conventional wisdom based on equilibrium.