THE KINETICS OF CALCITE DISSOLUTION IN NACL-CACL2-MGCL2 BRINES AT 1 BAR PCO2 AND 25 °C

Free drift dissolution rates of powdered calcite were determined in concentrated NaCl-CaCl₂-MgCl₂ solutions at 1 bar pCO₂ and 25 °C.  Calcium and magnesium were covaried along with ionic strength to yield solutions approximating that of subsurface brines (I=0.7; 1.5; 2.9; 4.2m).  The Ca²⁺:Mg²⁺ ratio ranged from 0.2 (seawater) to 7.4 with a maximum calcium concentration of 0.9 molal. Dissolution was followed to near equilibrium (W > 0.95) starting from an initial saturation state (W) of 0.2.  Rate data fit using the empirical rate equation R=k(1-W)ⁿ shows a switch in reaction order for the least concentrated solution (I=0.7 m) where n₁=4.8 for W < 0.5 and n₂=2.4 for W > 0.5.  With increasing concentration the discontinuity moves to higher saturation states, becomes less pronounced and is not observed for I > 2.9 m.  In cases where the split is observed, the corresponding pH (5.755 ± 0.250) is consistent with a change in reaction mechanism from diffusion to surface controlled kinetics.  The rate constant obtained from the region preceding the switch (k₁) is a linear function of the Ca²⁺:Mg²⁺ ratio (e) and is described by the equation k(moles m^-2 hr^-1)=0.097873 – 0.00041467(e) (R²=0.99932) .  In all solutions where I > 0.7, the reaction order is less then 2.5 and in general trends towards first order with increasing brine concentration. These findings have important application to reaction-transport modeling in carbonate bearing saline reservoirs which may serve as potential repositories for CO₂ sequestration.