Prediction of Water Breakthrough Time in Low Permeability Gas Reservoirs with Bottom Water in Consideration of Threshold Pressure

Because of the threshold pressure,the performance of water coning in low permeability gas reservoir with bottom water is quite different from that of conventional gas reservoir with bottom water.Therefore,there is a big gap between the actual performance and the calculated water breakthrough time based on conventional prediction formula.This paper established a water conning model for gas reservoir with bottom water:the gas layer was opened partly in gas wells,gas flows in radial flow form inside the perforation section,and combined with radial flow and semispherical flow below the perforation section.According to this model,it is assumed that the reservoir has horizontal,homogeneous,isotropic layers with equal thickness,water displaces gas in a way of piston-type,the viscosity and density of water and gas are constant,the pressure gradient at the interface of water and gas is the same,and the gravity and the capillary pressure can be neglected.Based on the assumption,the prediction formula of the water breakthrough time has been derived by considering threshold pressure gradient in low permeability gas reservoir with bottom water.Comparing the result calculated by this formula with that of Sobocinski-Cornelius formula,the formula calculation results more close to the real value,and we conclude that the water breakthrough time is reduced with the increase of threshold pressure gradient.