Synthetic fluid inclusions: IX. Critical PVTX properties of NaCl-H2O solutions

Abstract

Critical PVTX properties of NaCl-H2O fluids having compositions of 0–30 wt% NaCl have been determined using the synthetic fluid inclusion technique. Critical temperatures and pressures increase from 374.1°C and 220 bars for pure H2O, to 820°C and 1574 bars for 30.0 wt% NaCl. Critical specific volumes decrease from 3.11 to 1.44 cm3/g over this same salinity range. Critical temperatures of NaCl-H2O fluids over the range 0–30 wt% NaCl are described by the equation T = 374.1 + 8.800φ + 0.1771φ2 − 0.02113φ3 + 7.334·10−4φ4 where T is the critical temperature in degrees Celsius (±4.0°C) and φ is fluid composition in weight percent NaCl. The critical pressure, determined from the intersection of the critical isochore with the critical temperature in P-T space, is expressed as a function of the critical temperature according to the relationship P = 2094 − 20.56T + 0.06896T2 − 8.903 × 10−5T3 + 4.214·10−8T4 where P is the critical pressure in bars (±21 bars) and T is the critical temperature in degrees Celsius. The critical specific volume is given as SV = 3.106 − 0.5967ψ − 0.01066ψ2 + 0.01267ψ3 where SV is the critical specific volume in cm3/g (± 0.02 cm3/g) and ψ is related to the salinity as ψ = ln (wt% NaCl + 1) The slopes (ΔPΔT) of P−T projections of the critical isochores increase regularly with increasing salinity, from ~2.8 bars/°C for pure H2O to ~6.8 bars/°C for a 30 wt% NaCl solution.