Abstract: At the beginning of production of development wells in low permeability reservoirs of Xihu sag,there was different degree of water production and the water production remained stable for a long time.Aiming at the reason of water breakthrough and its characteristics,through movable water experiment and'dual water'curve evaluation method,two kinds of water occurrence state in low permeability reservoirs of Xihu sag were analyzed.Through discriminant engraving method of water production and numerical simulation of gas reservoir,combined with comprehensive analysis,one of the characteristics was water breakthrough at the beginning of production,and there was no fingering and serious waterflooding in short period.