Dissolution kinetics of calcite in NaCl-CaCl2-MgCl2 brines at 25 °C and 1 bar pCO2

Calcite dissolution rates were measured as a function of saturation state in NaCl-CaCl2-MgCl2 solutions at 1 bar (0.1 MPa) pCO(2) and 25degreesC. 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 Omega is saturation state. For rates measured in the pseudo-seawater, n=1.5 and k=4.7x10(-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+/-7x10(-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.