High-Temperature Illite Dissolution Kinetics

Abstract

Fluids cycled through producing or engineered geothermal reservoirs are likely to be out of equilibrium with surrounding rock and mineral phases in terms of both temperature and chemical composition. Knowledge of the reactivity of common fracture-filling minerals is needed to predict longer-term flow and permeability sustainability in systems where fractures make up a large portion of the fluid pathways within the reservoir. Currently the majority of kinetic reaction data for many common fracture-associated phyllosilicate minerals has been obtained at lower temperatures, and extrapolation from these may or may not accurately predict mineral kinetics at geothermal conditions. Here we present rate data and a preliminary kinetic rate formulation for illite (special clay “IMt-1,” Clay Minerals Society) over temperatures of 100-280 °C and pH levels of 3-9. We find that up to temperatures of 280 °C, the variation of dissolution rate with pH is relatively small (less than 0.5 orders of magnitude) for acid to mildly acidic solutions (3