Quartz dissolution kinetics from 100--200 C as a function of pH and ionic strength

Of importance to geothermal energy development, oil, gas and mineral recovery, and waste storage is the characterization of the dissolution rate of host reservoir rock as a function of temperature, pressure and liquid-phase composition. As a major constitutive mineral in natural geologic systems, quartz was selected for study. Dissolution experiments were carried out in a continuous-flow, titanium autoclave reactor system at 100--200 C in various chemical environments. Acidification to pH 1.1 using nitric acid showed very little effect on the quartz dissolution rate. The effect of hydroxide ion concentration and ionic strength were evaluated in NaOH, NaOH/NaCl and NaOH/Na2SO4 solutions. The fractional-order dependency of the quartz dissolution rate on hydroxide ion and sodium ion (or ionic strength) concentration was determined in NaOH/NaCl solutions. The results that extend the available range of kinetic data for quartz generally agree with previous work. The observed fractional-order kinetics were qualitatively described using classical adsorption isotherms. No significant variation in the apparent reaction order of the hydroxide ion with increasing temperature could be determined due to the scatter in the data. Quartz dissolution rates were slower by about 40% in NaOH/Na2SO4 solutions than in NaOH/NaCl solutions at sodium concentrations higher than 0.01 molal. The apparent activation energy from 100 to 200 C in NaOH/NaCl solutions up to 0.01 molal hydroxide ion and 0.1 molal sodium ion was estimated to be 72 (±6) kJ/mol