Calcite dissolution kinetics and solubility in Na-Ca-Mg-Cl brines of geologically relevant composition at 0.1 to 1 bar pCO2 and 25 to 80??C

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Title: Calcite dissolution kinetics and solubility in Na-Ca-Mg-Cl brines of geologically relevant composition at 0.1 to 1 bar pCO2 and 25 to 80??C
Author: Gledhill, Dwight Kuehl
Abstract: 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 processes such as migration , dispersive mixing , or anthropogenic injection of CO2 . This study measured calcite solubility and dissolution rates in geologically relevant Na -Ca -Mg -Cl synthetic brines (35 to 200 g L -1 TDS ) . In brines < 50 g L -1 TDS , the EQPITZER calculated calcium carbonate ion activity product (IAP ) at steady -state was in reasonable agreement ( ? ?10 % ) with the thermodynamic solubility constant for calcite (Kc ) . However , the IAP systematically exceeded Kc in more concentrated brines . The deviation was strongly correlated with calcium concentration and also was observed in magnesium -free solutions . This is interpreted as an uncertainty in the carbonate ion activity coefficient , and minor adjustment in stoichiometric association constants (K*M2+CO30 ) for the CaCO30 or MgCo30 ion pairs would correct for the error . The dissolution rate dependency on brine composition , pCO2 (0 .1 to 1 bar ) , and temperature (25 .0 to 82 .5 ? ?C ) was modeled using the empirical rate equation ( )nkR & #8486 ; & #8722 ;=1 where R is the rate , k and n are empirical fitting terms , and & #8486 ; the degree of disequilibrium with respect to calcite . When & #8486 ; was defined relative to an apparent kinetic solubility , n could be assumed first -order over the range of & #8486 ; investigated ( & #8486 ; = 0 .2 to 1 .0 ) . Rates increased with increasing pCO2 as did the sensitivity to brine concentration . At 0 .1 bar , rates were nearly independent of concentration (k = 13 .0 ? ?2 .0 x 10 -3 moles m -1 hr -1 ) . However , at higher CO2 partial pressures rates became composition dependent and the rate constant , k , was shown to be a function of temperature , pCO2 , ionic strength , and calcium and magnesium activity . The rate constant (k ) can be estimated from a multiple regression (MR ) model of the form k = B0 + B1 (T ) + B2 (pCo2 ) + B4 (aCa2+ ) + B5 (aMg2+ ) . A relatively high activation energy (Ea = 20 kJ mol -1 ) was measured , along with a stirring rate independence suggesting the dissolution is dominated by surface controlled processes at saturation states & #8486 ; > 0 .2 in these calcium -rich brines . These findings offer important implications to reaction -transport models in carbonate -bearing saline reservoirs .
URI: http : / /hdl .handle .net /1969 .1 /3810
Date: 2006-08-16

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Calcite dissolution kinetics and solubility in Na-Ca-Mg-Cl brines of geologically relevant composition at 0.1 to 1 bar pCO2 and 25 to 80??C. Available electronically from http : / /hdl .handle .net /1969 .1 /3810 .

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