The Seepage Mechanism And Productivity Analysis In Low-permeability Gas Reservoir

There are many low permeability gas reservoirs widely distributed in China,which reserves occupy a large proportion of entire natural gas reserves.For low permeability gas reservoir,its seepage mechanism is very complex because of its tiny pore throats,large surface area,low permeability.As a result,productivity analysis results of low permeability gas reservoirs tend to be too optimistic by using routine methods applied for middle & high permeability gas reservoirs.In this paper,the research objective is productivity of low permeability gas reservoir.Researched literature systematically and analyzed its seepage mechanism in-depth,including low velocity non Darcy effect,slippage effect,threshold pressure gradient,stress sensitivity and high velocity non Darcy.Current study of low permeability gas reservoir seepage mechanism is developed reference to low permeability oil reservoir,which is not a very good combination of fluid properties and percolation law for low permeability gas reservoir.Low permeability gas reservoir has great different percolation mechanism with low permeability oil reservoirs.Next,this paper put the above flow mechanism into the model,and built productivity equation of vertical wells,vertical fracturing wells and horizontal wells.For vertical wells,stable productivity and no stable productivity were respectively established considering Darcy,high velocity non-Darcy,threshold pressure gradient.slippage effect,stress sensitivity.In final,a stable productivity equation was established taking various factors into consideration comprehensively.For vertical fracturing wells,the productivity equation was built respectively on linear flow+radial flow model and elliptic flow model,and results of the two models were compared.For horizontal wells,this paper built a productivity equation considering various factors comprehensively by using elliptic flow model,and then made a sensitivity analysis of different parameters.Finally,this paper analyzed the abnormity productivity curve caused by unclear understanding of mechanism,containing threshold pressure gradient,slippage effect,stress sensitivity and so on,and gave corresponding solutions.Given low permeability gas reservoirs' special seepage mechanism,a single point method productivity formula meeting low permeability gas reservoir characteristics was derived.