Chlorite dissolution kinetics at pH 3-10 and temperature to 275 °C

Sheet silicates and clays are ubiquitous in geothermal environments. Their dissolution is of interest because this process contributes to scaling reactions along fluid pathways and alteration of fracture surfaces which could affect reservoir permeability. In order to better predict the geochemical impacts on long-term performance of engineered geothermal systems, we have measured chlorite dissolution and developed a generalized kinetic rate law applicable over an expanded range of solution pH and temperature. Chlorite, (Mg,Al,Te)(12)(Si,Al)(8)O-20(OH)(16), commonly occurs in many geothermal host rocks as either a primary mineral or alteration product. A combination of new rate data from this study (collected using an Mg-rich chlorite variety, at 100-275 degrees C and pH > 5.5) as well as all available published chlorite dissolution datasets results in a kinetic rate equation that is valid over temperatures of 25-275 degrees C and 3 <= pH <= 10: R = ([1 . 10(-4) . e(-30) / (RT) . a(H+)(0.74)] + [4.7 . 10(-11) . e(-13) / (RT)] + [1.5 . 10(-9) . e(-15) / (RT) . a(OH-)(0.43)]) . (1-e(Delta Gr) / (RT)) The form of this equation, which includes a reaction affinity term to slow reaction as equilibrium is approached, can be incorporated into most existing reactive transport codes for use in prediction of rock-water interactions in engineered geothermal systems. (C) 2015 Elsevier B.V. All rights reserved.