Experimental Determination of Chlorite Dissolution Kinetics at Geothermal Conditions

Abstract

Phyllosilicates such as clays and other sheet silicates are ubiquitous in geothermal environments, and are often assumed to react quickly when unequilibrated fluids are introduced to these systems. Rapid reaction of these phases may contribute to scaling reactions or alteration along fracture surfaces which can affect resource permeability; thus, knowledge of the reaction kinetics for these minerals is useful in modeling long-term reservoir performance. A suite of experiments was performed to develop a kinetic rate law for the dissolution of chlorite, (Mg,Al,Fe)12(Si,Al)8O20(OH)16, which is commonly noted in many geothermal host rocks as either a primary mineral or alteration product. We find that over temperatures of 100-275 °C, pH levels of 3-10, and even in the presence of dissolved carbon dioxide, the dissolution of chlorite is relatively slow compared to other common silicate minerals for which higher-temperature data are available (e.g., quartz, feldspars). This finding conflicts with a previously reported high activation energy for this reaction (Palandri and Kharaka, 2004), but is corroborated by comparison of our experimental data with two other relatively recent datasets collected at lower temperatures (Lowson et al., 2007; Black and Haese, 2014). The combined treatment of these data results in a kinetic rate equation that should be easily incorporated into most existing reactive transport codes for use in prediction of rock-water interactions in engineered geothermal systems.