SYNTHETIC FLUID INCLUSIONS: XIII. EXPERIMENTAL DETERMINATION OF PVT PROPERTIES IN THE SYSTEM H2O + 40 WT% NACL + 5 MOL% CO2 AT ELEVATED TEMPERATURE AND PRESSURE

Abstract

The location of the liquid + vapor -> liquid phase boundary and the P-T slopes of iso-Th lines were determined for a constant composition of 40 +/- 0.1 wt% NaCl and 5 +/- 0.15 mol% CO2 (both relative to H2O) at high density. Synthetic fluid inclusions with this composition were formed in cold-seal pressure vessels at pressures of 2 and 4 kbar and temperatures between 350° and 700°C. The inclusions were analyzed on a gas-flow heating/cooling stage to determine the temperatures of halite dissolution [Tm(H+L+V->L+V)] and total homogenization [Th(L+V->L)].Addition of 40 wt% NaCl to an aqueous solution containing 5 mol% CO2 causes a significant shift of the liquid + vapor -> liquid boundary towards higher pressures. The slopes of the iso-Th lines decrease from 29.5 bars/°C for Th(L+V->L) of 400°C, to 6.4 bars/°C for Th(L+V->L) = 600°C. Addition of 5 mol% CO2 to an aqueous solution containing 40 wt% NaCl results in halite dissolution temperatures that are slightly higher (Tm(H+L+V->L+V) ~ 332°C) than the literature value of 323°C for the vapor-saturated liquidus of an H2O-40 wt% NaCl mixture.Calculated molar volumes for 40 wt% NaCl + 5 mol% CO2 solutions at 2 and 4 kbar show trends that are similar to those of other compositions in the ternary system H2O-CO2-NaCl at the same pressures and temperatures. In the P-T range of this study, all excess volumes are negative and lie between the values for the compositions H2O-5 mol% CO2 and H2O-40 wt% NaCl.