Abstract
Solubilities of specially synthesized coarse-grained barite were determined in H2O solutions from 22 to 280°C and 1 to 1400 bars, and in 0.2 and 4 molal NaCl solutions from 100 to 250°C and 1 to 500 bars. Barite solubilities are low, ranging between 10−6 and 10−3 molal. Isobaric solubilities are maximum near 100°C in H2O solutions; the maxima migrate to progressively higher temperatures in solutions of increasing NaCl concentrations. Isothermal solubilities increase with rising NaCl concentration and pressure. The effect of NaCl on increasing solubilities becomes greater with rising temperature.
Changing temperatures cause different responses, depending on the composition of the solution and the temperature range. Initially-saturated dilute natural waters would precipitate barite with either increasing temperature or decreasing temperature, depending on the circumstances. Saline solutions are capable of behaving much like dilute solutions below 1 molal NaCl, but at higher NaCl concentrations the temperature effect on isobaric solubilities is monotonically positive. Barite solubility in sufficiently saline solutions in nature would increase with depth, due to increasing temperature and pressure. Precipitation would tend to occur during migration of solutions toward the surface. Because the low solubility of barite precludes effective transport of BaSO4 in large quantities, precipitation of barite by the reaction of Ba2+ with sulfate derived from the oxidation of S2− may be more important as a depositional mechanism than changes in the temperature-pressure environment.
Thermodynamic equilibrium constants (K) and the free-energy changes for the reaction (ΔG) for the dissolution of barite were calculated from the solubility data. ΔG values in kcal/mole at 1 bar are closely reproduced by the equation ΔG = 1.15046 × 10−4T2 − 4.02952 × 10−2T + 15.386 (T in °K). Enthalpies, entropies, and volumes of reaction obtained from the solubility data are in accord with published thermodynamic data. Activity coefficients calculated using barite solubilities in NaCl solutions and log К data are accurately modeled by an extended form of the Debye-Hückel equation.
