Abstract: A gasliquidgas phase equilibrium theoretical calculation model suitable for hydrocarbon mixtures contained polar material water is presented based on PR equation of state and excess energy HV mixing rule. Moreover, a functional relation between formation water evaporation, water saturation at near well bore zone and salinity is set up using this equilibrium theory, the change of phase volume of hydrocarbon and water with temperature and pressure is studied. Through combining with single well radial numerical simulator, the change of vaporous water content Xw, water saturation Sw and water salinity versus radius and production time are investigated. The results show that at near well bore zone, the pressure drop gradient decreases considerablly, formation water vaporizes prominently, and the Xw increases exponentially which can even exceed 7 percent at low pressure. It also proved that the formation water saturation Sw may decrease by 50 percent after vaporization, which is in favor of production. In addition, formation water vaporization would result in salinity increasing, which may lead to salt out and the salt out generally occurs about 5 meters range around well bore.

Key words: high temperature, gas reservoir, formation water, vaporization, salt out