Influence Of Movable Water On Gas Well Productivity In Tight Sandstone Reservoir

There is a great potential for the development of tight sandstone gas reservoir in China,and it is expected to continue to act as the "leader" for the development of unconventional gas in our country for a long time.However,as the development process continues to advance,gas wells will produce water.The quantitative evaluation method of the influence of movable water on productivity is not yet mature.On the basis of deep understanding of the mechanism of microcosmic percolation mechanism,the method of calculating steady productivity of both gas and water with vertical and horizontal well is reestablished,and the effect of movable water on the productivity is quantitatively evaluated based on the gas water two-phase permeability curve.However,there are some shortcomings in the the experimental methods for measuring the relative permeability of tight sandstone gas reservoirs.Therefore,a new theoretical model for calculating the relative permeability of gas and water based on fractal theory is established.The following studies have been carried out in this paper.Based on fully investigating the theoretical calculation method of core relative permeability in tight sandstone gas reservoir,this paper analyzes the deficiency of the current theoretical calculation method,and puts forward an improved model for the theoretical calculation of core relative permeability in tight sandstone gas reservoir based on the Mo model.On the basis of the model proposed by Mo and others,the microcosmic distribution of movable water is considered,and the microscopic equivalent seepage model is established based on the fractal theory.The expression of bound water saturation is rededuced.Then the expression of the relative permeability of gas and water is derived from the Hagene Poiseulle equation and the extended Darcy equation.The moedel modifies the error of Mo et al when using the formula of water film thickness of Tian et al.Besides,the influence of confining pressure on capillary radius is also considered in the model.By comparing the error of theory and experiment on the the bound water saturation and the relative permeability curve of the gas and water,it is found that the calculated bound water saturation is small and the relative permeability curve is shifted to the left,and the main reason is the effect of the heterogeneity.By analyzing the influence of various factors on the bound water saturation and the relative permeability curve,it is found that the displacement pressure gradient and the surface tension have an obvious influence on the irreducible water saturation and the relative permeability curve,so the experimental measurement process should be considered reasonably.Through this model,it is found that with the increase of the movable water saturation in the layer,the thickness of the water film in the capillary will increase continuously and the effective flow channel of the gas phase will be smaller,thus the effective permeability of the gas phase will be reduced continuously.On the basis of full analysis of reservoir percolation mechanism and water production mechanism,the productivity equations of vertical and horizontal wells in tight sandstone gas reservoir with stress sensitivity,slippage effect,turbulence effect and the effect of movable water are derived.It is necessary to redefine the generalized pseudo pressure of gas and water two phase,and then deduce the productivity equation of the water production well.Then,using the Joshi horizontal well productivity analysis method,combined with the conformal transformation method and the equivalent percolation resistance method,the steady-state productivity formula of the water producing horizontal gas well is derived.In the calculation model,it points out the deficiency in the method of solving the productivity of the well produced in the low permeability and tight sandstone gas reservoir,and puts forward a solution method based on the relative permeability curve.The accuracy of the the productivity formula is verified by an example,and the influence of various factors on the productivity of vertical and horizontal wells is analyzed.It is found that the movable water saturation and the stress sensitive effect have great influence on the single well produvtivity,and the effect of gas slippage and high speed non Darcy effect can be ignored.It will provide theoretical analysis and assistance for productivity prediction of gas production wells in tight sandstone gas reservoirs.