Abstract: There is little the research on the influence of evaporation of formation water on gas reservoirs,the authors of this paper indicate the law of the change of water vapor content in gas includes three stages,i.e.water-gas two phases equilibrium stage near well area;the under saturated stage adjacent bottom hole and the non-equilibrium over saturated stage from a certain point in well bore to surface,that is,the generation of water condensate.Meanwhile,it is considered that the reservoir pressure nearby the bottom hole drops and the re-evaporation of formation water may form salt block to the reservoir rocks near the bottom hole,as to decrease the permeability and impact production.The higher the compositional content of gases hydrocarbon is,the more the saturated water vapor is while equilibrium.Considering existence of water vapor in formation water,the dew point of condensate gas,checked from the results of experimental test or phase equilibrium calculation,will drops.

Key words: formation water, evaporation, gas reservoir, fluid, law