Brine chemistry effects in calcite dissolution kinetics at reservoir conditions

Abstract

Understanding the chemical interactions between CO2-saturated brine systems and reservoir rocks is essential for predicting the fate of CO2 following injection into a geological reservoir. In this work, the dissolution rates of calcite (CaCO3) in CO2-saturated brines were measured at temperatures between 325 K and 373 K and at pressures up to 10 MPa. The experiments were performed in batch reactors implementing the rotating disk technique in order to eliminate the influence of fluid-surface mass transport resistance and obtain surface reaction rates. Three aqueous brine systems were investigated in this study: NaCl at a molality m = 2.5 mol·kg−1, NaHCO3 with m ranging from (0.005 to 1) mol·kg−1 and a multicomponent Na-Mg-K-Cl-SO4-HCO3 brine system with an ionic strength of 1.8 mol·kg−1. Measured dissolution rates were compared with predictions from previously published models. Activity calculations were performed according to the Pitzer model as implemented in the PHREEQC geochemical simulator. Calcite dissolution rates in NaCl and the multicomponent brine system showed minor increases when compared to the (CO2 + H2O) system at identical conditions, despite the lower concentration of dissolved CO2. These trends are consistent with the expected minor decreases in solution pH. In NaHCO3 systems, consistent with increase in solution pH, significant decreases in dissolution rates were observed. In addition, these systems significantly deviated from model predictions at higher salt molalities. Vertical scanning interferometry (VSI) was used to examine the mineral surfaces before and after dissolution experiments to provide qualitative information on saturation states and dissolution mechanism.