Abstract: The upper section of Ordovician Yingshan Formation on Tazhong Northern Slope of Shunnan Block in the Tahe Oilfield belongs to a type of porous fractured carbonate reservoir.in this type reservoir,gas cut is the most serious during drilling,and this gas cut is often treated by increasing the density of drilling fluid and circulating the drilling fluid to exhaust the gas in the field.In order to solve the problems of high density drilling fluid,including its tendency to leak,and long treatment time required for exhaust circulation,researchers studied the factors that most influenced the gas cut time of gas-liquid displacement in the fractured reservoir.The viscosity and density of the drilling fluid on time required for gas cut were analyzed by building the leaking rate model and a gas cut model. Thus it was possible to evaluate effects of width,height and length of fracture,formation pressure and pressure difference..It was found that gas cut is slower under conditions of smaller width,greater height and length of fracture,greater viscosity and smaller density of drilling fluid,as well as lower pressure and greater pressure difference of formation.Therefore,proper drilling fluid density and pressure difference control measure are necessary for field operation to effectively inhibit and reduce the displacement gas cut rate.