Abstract
Calcite solubility was determined experimentally in NaCl solutions from 25° to 89°C and in Na-Ca-Cl solutions at 25°C. The results are important for understanding the subsurface diagenesis of calcite cements. Solution compositions and ionic strengths spanned those observed in brines from sedimentary basins. The maximum ration of calcium to sodium molality examined was ~ 0.17 ([almost equal to] 30,000 ppm Ca). The stoichiometric solubility products (K[sp]*) in NaCl solutions at 25°C were adequately predicted by Pitzer's equations using published interaction parameters. There was little variation in pK[sp]* (= -log K[sp]*) from 0.5 to 6 m ionic strength. The maximum in solubility occurred at 2 to 3 m. In Na-Ca-Cl solutions, the values of K[sp]* varied systematically from the calculated using Pitzer's equations and published interaction parameters. The variation was attributed to error in the determination of the activity coefficient of free carbonate ion (γCo3[2-]). As the ratio of calcium to ionic strength (Ca/I) increased, γCO3[2-] first increased to ~ 40 times the value calculated from the Pitzer model γCO3[2-] decreased until Ca/I [about equal to] 0.1 at which point the model prediction was again correct. At higher Ca/I values γCO3[2-] decreased by as much as 0.025 times the value predicted by the model. Correspondingly, the K[sp]* of calcite in these Na-Ca-Cl brines was 40 times higher than predicted using Pitzer's equations. The K[sp]* of calcite in NaCl solutions from 0.5 to 6 m ionic strength and 25° to 89°C decreased with increasing temperature. The trend is similar to that obtained from published data for calcite solubility in pure water over the same temperature range. The reduction in solubility decreases with increasing ionic strength. For example, at 0.5 to 2.0 m NaCl pK[sp]* decreased by ~ 0.4 log units from 25° to 39°C, whereas at 6 m NaCl the decrease was < 0.1 log unit. An Arrhenius plot of the equilibration rate in supersaturated 2 m NaCl solution from 25° to 89°C yielded an activation energy (Ea) of 15.5 kcal/mole. From 25° to 60°C, Ea = 11.2 kcal/mole. A small temperature dependence to Ea was indicated. The latter value of Ea is the same as previously published values for calcite precipitation rates from 10° to 40°C.
Nagy, Kathryn Louise (1988). The solubility of calcite in NaC1 and Na-Ca-C1 brines. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -771520.