Dissolution Kinetics of Calcite in NaCl–CaCl2–MgCl2Brines at 25 °C and 1 bar pCO2

Abstract

Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl2–MgCl2 solutions at 1 bar (0.1 MPa) pCO2 and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca2+:Mg2+= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca2+:Mg2+= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 − Omega;) n , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10−2 mol m−2 hr−1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10−2 mol m−2 hr−1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca2+:Mg2+ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO2 sequestration.