Illite dissolution kinetics from 100 to 280 °C and pH 3 to 9

Some geothermal reservoirs may require permeability enhancements (either by reactivating existing fracture networks or creating new ones) to allow sufficient fluid and heat transfer throughout the system. In these engineered geothermal systems (EGS), fluid/mineral reactivity is of interest because mineral reaction can affect fracture apertures, wall rock stability, and potential downstream precipitation. In this study we focus on the reactivity and dissolution of illite, an aluminosilicate sheet mineral common in geothermal systems, measured in flow-through experiments carried out at temperatures of 100-280 °C and pH 2.7-9.7. Under these conditions, illite dissolves readily according to:Rillite=1×10-2·e-58/RT·aH+0.55+2×10-5·e-54/RT+1.5·e77/RT·aOH-0.35·f(ΔGr),where Rillite represents the surface area-normalized dissolution rate. Illite dissolves congruently under alkaline pH conditions at temperatures of 100 °C and above, but as pH decreases below 5 illite dissolution is accompanied by precipitation of an aluminum oxy(hydroxide) phase identified as boehmite by X-ray diffraction of bulk samples. Precipitation of AlO(OH)(s) appears to proceed rapidly and is limited by the rate of illite dissolution in our experiments.