Calcite Reaction Kinetics in Saline Waters
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The effect of ionic strength (I), pCO2, and temperature on the reaction kinetics of calcite was investigated in magnesium-free, phosphate-free, low calcium (mCa^2 ≈ 0.01 – 0.02 molal) simple KCl and NaCl solutions from both undersaturated and oversaturated conditions. First order kinetics were found sufficient to describe the dissolution rate data. Dissolution rates decreased with increasing I and were faster in KCl than NaCl solutions at the same I indicating that Na^ interacts more strongly with the calcite surface than K^ or that water is less available in NaCl solutions. Rates increased with increasing pCO2 and temperature, and their influence diminished at high I. Arrhenius plots yielded a relatively high activation energy (Ea ≈ 20 ± 2 kJ mol-1) which indicated that dissolution was dominated by surface controlled processes. These results are consistent with the hypothesis that the mole fraction of "free" solvent plays a significant role in the dissolution kinetics of calcite with a minimum value of ~45-50 percent required for dissolution to proceed in undersaturated solutions at 25-55 degrees C and pCO2 = 0.1 – 1 atm. Precipitation rates were modeled using the general and Davies and Jones rate equations yielding similar results. Reaction orders were found to typically range between 0.8 and 2.5 for both rate equations regardless of electrolyte. For both solutions, rate constants were found to range between 100.8 and 101.7 mmole m-2 hr-1 (general rate equation) and 101.5 and 102.2 mmole m-2 hr-1 (Davies and Jones rate equation). Under the experimental conditions employed and the resultant precision (~20-25 percent), I and pCO2 do not have a significant influence on the precipitation rate of calcite. Precipitation rates increased with temperature although Arrhenius plots yield a broad range of activation energies (Ea ≈ 15 – 28 kJ mol-1, R2 = 0.72). The relatively low calculated activation energies coupled with the precision of the results suggest the possibility of surface nucleation in the present results. Overall, these findings may be useful in understanding and predicting the interaction and reactivity of the host carbonate minerals in subsurface reservoirs to the injection of CO2 although much work needs to be completed at elevated temperatures and pressures.
Finneran, David (2010). Calcite Reaction Kinetics in Saline Waters. Doctoral dissertation, Texas A&M University. Available electronically from