Dissolution and Precipitation Kinetics of Kaolinite<subtitle>Initial Results at 80°C with Application to Porosity Evolution in a Sandstone</subtitle>

Abstract

Dissolution and precipitation rates of kaolinite were determined at steady-state in 80°C solutions near pH 3 as a function of deviation from equilibrium using flow-through reaction cells. The dependence of dissolution rate (mol/m2/s) on solution saturation state can be described equally well by either of the empirical relations: [EQUATION] or [EQUATION] The present data on dissolution rates indicate linearity of the rates near equilibrium. The dissolution and precipitation rates near equilibrium were used to bracket the solubility of the kaolinite, providing a self-consistent thermodynamic reference point for the calculation of solution saturation state. To investigate the effects of near equilibrium kinetics during diagenesis in a sandstone we have carried out a numerical simulation in which the reactions are driven by fluid flow. Full dissolution and crystal growth laws were obtained from the rates measured for kaolinite and from published dissolution rates far from equilibrium for other minerals, together with the principle of detailed balancing. The results indicate that for a flow velocity of 10 m/yr, the assumption of kaolinite formation under local equilibrium conditions may not be justified. The simulation can account for the timing of diagenetic mineralization inferred from observed textural relationships between quartz, feldspar, and kaolinite. The results emphasize the need for accurate kinetic data on silicate-aqueous reac ions and the importance of flow of chemically-reactive fluids in producing disequilibrium mineral assemblages and in controlling the spatial and temporal evolution of porosity in subsurface rocks.