Statistical methodology to predict reaction time of water-rock experiments under quasi-steady state conditions

Abstract

Water-rock interaction experiments have provided important results in mineral dissolution studies although the steady state conditions are rarely attained. These experiments are usually costly, tedious, and time consuming. Among the controlling factors, the reaction time has been identified as a key parameter for designing optimized water-rock experiments. Moreover large discrepancies actually exist defining the reaction time needed to reach steady state conditions. We propose a statistical methodology to predict the reaction time needed to approach a quasi-steady state in water-rock experiments. The method was successfully applied for the cation exchange reaction in feldspars using experimental data already published (granite-water interaction at 200 °C, 50 MPa, fluid/rock ratio=2). Logarithmic regression models were used for fitting fluid composition [log(Na/K)] and reaction time (t). For the most probable fluid composition under unknown steady-state conditions, the log(Na/K) value inferred from Na/K geothermometers was assumed. We found that a reaction time between 3520 and 4996 hours would be required to approach the quasi-steady state in the example experiment.