Research On Injection-production And Reliability Of Aquifer Underground Gas Storage

Due to the advantages of aquifer underground gas storage,such as high permeability,high porosity,good hydraulic conductivity,large reserves and low cost,aquifer underground gas storage can significantly increase natural gas injection and production rate,and is considered as one of the best place to storage natural gas.After depleted reservoir underground gas storage,aquifer underground gas storage is the second in gas storage application all over the world.But China has not yet put aquifer underground gas storage into production at present.In the process of aquifer underground gas storage operation,injection,production and security issues have always been the hot issues.As results,the research on injection-production analysis,casing safty,cap rock and faults reliability are the keys with aquifer underground gas storage.The main motive of this paper is to solve several key problems to improve the safty of aquifer underground gas storage.Well logging data,ground stress field observation are adopted in the dissertation.And fractal models,reliability models are established in this paper.Reservoir pressure,casing safety,cap rock and faults reliability can be predicted with those models.Ultimately,the safety and reliability models of aquifer gas storage forecasting are established.The detailed studies are depicted as follows.(1)The seepage analysis of aquifer underground gas storage is divided into two mediums: one is fracture medium;the other is porous medium.Considering the influence of fractal characteristics of fractured media and critical migration water saturation,the relationship among fracture size,representative elementary volume,capillary pressure is obtained based on two-phase flow theory and fractal theory.Seepage law and bottomhole pressure of aquifer underground gas storage is analyzed.Considering the fractal feature of porous medium and the influence of reservoir pressure change on seepage parameters in depleted reservoir gas storage,the fractal seepage model was built up to calculate the reservoir pressure based on fractal theory and unstable gas seepage flow.The results show that,in fracture medium,the bottomhole pressure is equidirectional with the increase of fractal dimension,reservoir thickness,drainage rate,but reverse to the increase of representative elementary volume,Lmin/Lmax,injection rate;in porous medium,The reservoir pressure of aquifer gas storage is equidirectional with the increase of tortuosity,gas viscosity,injection rate and time,but reverse to the increase of fractal dimension and reservoir thickness.The fractal dimension,reservoir thickness,injection rate and time have great effects on the reservoir pressure,whereas the tortuosity and gas viscosity have little influences on it.(2)Considering the influence of rock parameters uncertainty on the failure of potential fault,the confidence interval of risk coefficient is derived to analyze the dead fault stability based on the logging data and interval theory.The maximum operating pressure is given considering the rock parameters uncertainty.Using the above method,the confidence interval of risk coefficient and maximum operating pressure are studied for the underground gas storage in certain oilfield.The research results show that the fault risk coefficient first increases and then decreases with the fault dip,and the fault is in the most dangerous situation when the relationship between fault dip angle ? and friction angle ? is(10)?(28)??/245.In order to ensure the safety of underground gas storage,the lower limit value of maximum operating pressure confidence interval is chosen to be the maximum operating pressure of underground gas storage.(3)By means of the convex model,the non-probabilistic reliability model was built up to calculate the fault slip reliability,and the limit state function of dead fault was derived based on shear failure.Linear approximation of the limit state function was done to calculate the fault slip reliability.Using the above method,the reliability of dead fault slip was studied for the block in certain oilfield.The reliability results by the new method were compared with the results by Monte-Carlo method.In addition,parameters were studied to investigate reliability as a function of pore pressure,fault dip,non-uniformity coefficient of crustal stress.The results show the model doesn’t need a large number of parameter samples and probability distribution function,and can meet the actual engineering demands.The fault slip reliability of depleted reservoir gas storage is reverse to the increase of pore pressure and non-uniformity coefficient of crustal stress.With the increase of fault dip,the fault slip reliability decreases first,then increases.The reliability of the fault is minimal when [(10)?(10)?????/2)45max(/2),45min(].By means of interval theory and non-probabilistic reliability theory,the confidence interval of cap rock parameters are derived based on the logging data and rock failure criterions,and the confidence interval of safety factor and reliability index under different failure criterions are given.The research results show that the minimum confidence intervals of safety factor based on Hoek-Brown criterion and Mohr-Coulomb criterion are [1.715,1.458] and [2.037,1.706],and the minimum reliability indexs are-3.213 and-0.777.The reliability index calculated by Hoek-Brown criterion is more secure.Compared with the traditional method of probabilistic reliability analysis,the non-probabilistic reliability method needs less rock parameters,and this advantage makes it much easier to forecast the cap rock reliability index in underground gas storage.(4)Considering the thinning of casing and expansion of outside diameter with aquifer underground gas storage,the residual strength of casing with underground gas storage sleeves was analyzed with several different methods.In the analysis of residual internal pressure strength of the casing,the improved equations of residual internal pressure strength were established based on inner diameter.Using this method,the residual internal pressure strengths of casing in underground gas storage were analyzed.The results by the improved equations were compared with traditional method.The results show the improved method can meet the actual engineering demands.The improved method has very good application in casing engineering,and provides a new method of casing in underground gas storage.