HR: 0800h
AN: V31A-1403    [Abstracts]
TI: Calcite Dissolution Kinetics and Solubility in NaCl-CaCl$_{2}$-MgCl$_{2}$ Brines up to 1 bar pCO$_{2}$ and 80\deg C
AU: * Gledhill, D K
EM: dgledhill@ocean.tamu.edu
AU: Morse, J W
EM: morse@ocean.tamu.edu
AB: Sedimentary basins can contain close to 20% by volume pore fluids that are commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of migration, dispersive mixing or by anthropogenic injection of CO$_{2}$. Both the solubility of calcite and its dissolution kinetics are currently poorly constrained in these concentrated solutions. This study measured calcite solubility and dissolution rates in geologically relevant NaCl-CaCl$_{2}$-MgCl$_{2}$ synthetic brines (50,000 to 200,000 mg L$^{-1}$ TDS). The EQPITZER calculated calcium carbonate ion activity product at steady-state was in reasonable agreement (\pm10%) with K$_{cal}$ in low concentration brines but systematically exceeded K$_{cal}$ with increasing brine concentration. The deviation was most strongly correlated with calcium activity and was independent of magnesium concentration. This has been interpreted as an uncertainty in the carbonate ion activity coefficient perhaps related to errors in the calcium interaction parameters with the carbonic acid system under these conditions. The dissolution rate dependency on brine concentration, pCO$_{2}$ (0.1 to 1 bar) and temperature (25\deg C to 80\deg C) was modeled using the empirical rate equation R=k(1-\Omega) $^{n}$. Relative to an apparent kinetic solubility it was found that n could be assumed to be first order over the range of degree of disequilibrium investigated (\Omega = 0.2 to 1.0). Rates increased with increasing pCO$_{2}$ as did the sensitivity to brine concentration. At 0.1 bar rates were independent of concentration (k = 9.0\pm1.0\times10$^{-3}$ moles m$^{-2}$ hr$^{-1}$). However, at higher partial pressures rates were linearly correlated to TDS. At 1 bar pCO$_{2}$ and 25\deg C the rate constant can be described by the linear regression k(moles m$^{-2}$ hr$^{-1}$) = 0.053 - 1.7\times10$^{-7}$(TDS), R$^{2}$ = 0.996. The specific effects of Ca$^{2+}$, Mg$^{2+}$ and ionic strength were tested in addition to the inhibitory effect of the presence of 1000 mg L$^{-1}$ SO$_{4}$$^{2-}$. A roughly three fold increase in rate was measured at 80\deg C relative to 25\deg C. The relatively high activation energy (E$_{a}$ = 20 kJ mol$^{-1}$) along with a stirring rate independence suggest the dissolution is dominated by surface controlled processes. These findings may offer important implications to reaction-transport models in carbonate bearing saline reservoirs.
DE: 4825 Geochemistry
DE: 3699 General or miscellaneous
DE: 1099 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting