Chlorite Kinetics and Impact on EGS-CO2

Abstract

Although CO2 has been identified as a possible option for use in enhanced geothermal systems (Pruess, 2006), its effect on mineral alteration reactions and/or scaling in these systems is not currently well-constrained. Model results investigating a CO2-rich natural analog site (the Ohaaki-Broadlands system, New Zealand) suggest that elevated levels of aqueous CO2 cause relatively rapid and extensive dissolution of chlorite (and other sheet silicate minerals), accompanied by recrystallization of carbonates. These simulations were performed using available low-temperature kinetic data for chlorite (Palandri & Kharaka, 2004) which were then extrapolated to the higher temperatures of the geothermal system. However, our experiments have shown that measured chlorite dissolution rates at higher temperatures (100-280°C) and CO2 concentrations are several orders of magnitude slower than the extrapolated rates. A larger experimental dataset is being collected to quantify the exact mechanisms influencing chlorite dissolution rates, but we currently find that the use of low-temperature kinetics data for chlorite (and possibly other sheet silicates for which higher-temperature dissolution data are lacking) may result in significantly higher predictions of mineral alteration.