ABSTRACT

An empirical equation of quartz solubility in H₂O at elevated pressures and temperatures (up to 10 kb and 900°C) is obtained based on the published experimental results. The resultant solubility equation is as follows: lnS_water = –1.51914 + 1.99970lnρ_water – 2636.78T^–1 + 0.00387766T + 0.0198582PT^–1, where S_water, P_water, T, and P stand for quartz solubility (mol/kg H₂O), density (g/cm³) of pure water, temperature (Kelvin), and pressure (bar), respectively. Densities of pure water are calculated by the equation of Haar et al. (1984). The solubility equation is then modified for describing the solubility behavior of quartz in H₂O + CO₂ or H₂O + NaCl fluid. For the modification, this study uses the weight fraction of water in the solvent (F), density of the aqueous fluid (ρ_aq), and an empirical constant (Θ) as the additional terms. The ratio of quartz solubility in aqueous fluid (S_aq) to that in pure water (S_water) at the same pressure-temperature condition is then expressed as ln(S_aq/S_water) = Θln(ρ_aqF/ρ_water). Densities of H₂O + CO₂ and H₂O + NaCl fluids are computed with the equations of Kerrick and Jacobs (1981) and Anderko and Pitzer (1993a), respectively. The empirical constant for quartz solubility in H₂O + CO₂ fluid is 1.49063. For obtaining the constant, this study considers the published experimental results done at 1.5 kb to 5 kb, 600° to 700°C, and up to 71 wt% CO₂ concentration (mole fraction of CO₂ = 0.505). The O value for quartz solubility in H₂O + NaCl fluid is 3.24740, which gives the calculated solubilities consistent with the experimental results done at 0.2 kb to 2 kb, 200° to 700°C, and up to 33.6 wt% NaCl concentration.