Estimation of reactive mineral surface area during water–rock interaction using fluid chemical data

Abstract

Mineral dissolution and precipitation reactions actively participate to control fluid chemistry during water–rock interaction. However, it is difficult to estimate and normalize bulk reaction rates if the mineral surface area effectively participating in the reactions is unknown. In this study, we evaluated the changing of the reactive mineral surface area during the interaction between CO 2-rich fluids and albitite rock reacting under flow-through conditions. Our methodology, adopting an inverse modelling approach, is based on the measured chemical fluid composition as raw data. We estimated the rates of dissolution and the reactive surface areas of the different minerals by reconstructing the chemical evolution of the interacting fluids. This was done by a reaction process schema that was defined by a fractional degree of advance of the irreversible mass-transfer process and by attaining the continuum limit during the water–rock interaction. Calculations were carried out for albite, microcline, biotite and calcite assuming that the ion activity of dissolved silica and aluminium ions was limited by the equilibrium with quartz and kaolinite. We found that the absolute dissolution rate of albite, microcline, biotite and calcite remains essentially constant as a function of time, and the calcite dissolution rate is orders of magnitude higher than silicate minerals. On the contrary, the reactive surface area of the parent minerals varied by more than two orders of magnitude during the observed reaction time, especially for albite. We propose that the reactive surface area depends mainly on the stability of the secondary mineral coating that may passivate the effective reactive surface area of the parent minerals.