Study On Injection-production Migration And Dynamic Prediction Of The Storage Capacity In Depleted Gas Reservoir

Most of the underground gas storage reservoirs are built up in depleted gas reservoir inthe world. With the advantages of less investment, quick working effects and reclaiminginvestment quickly, the underground gas storage reservoirs built in depleted gas reservoir areapplied widely. According to the characteristics of the great depth, water invasion, reservoirheterogeneity and mid-low permeability reservoirs, the main motive of the paper is to solveseveral key problems to improve the safety of the underground gas storage in depleted gasreservoir. The theory analysis and numerical simulation are adopted in the dissertation. Manydetailed problems are contained, such as study on the maximum operating pressure of gasstorage, equivalent seepage model of natural gas migration, dynamic migration law of inertcushion gas during injection-production process, dynamic prediction of the storage capacity,leakage model of natural gas through an abandoned well and, etc. Basing on the study results,the detailed studies are depicted as follows.(1) The analytical model and numerical model of the maximum operating pressure of gasstorage in depleted natural gas reservoir are presented in this paper, and the maximumoperating pressure of gas storage is calculated based on the proposed model. The calculatingresults are compared with the analytical and numerical models to verify the precision of themodels. The influences of reservoir thickness, permeability, injection rates and draindischarge on injection point pressure are studied. The examples show that the results of theanalytical model are consistent with that of the numerical model for a compressional stressfield. The biggest relative error is about8％. For an extensional stress field, the biggestrelative error is about31%~500%, the results of the analytical model are overly conservative.The injection point pressure decreases with reservoir thickness, permeability, and draindischarge increasing, whereas it increases with the increase of injection rate.(2) According to the characteristics that a very small ratio of reservoir thickness to areain depleted gas reservoir, an equivalent seepage model is established by the vertical integralaverage for the natural gas migration in the underground gas storage reservoir． Thethree-dimensional seepage problem of complex type underground gas storage reservoir can be simplified to the plane problem taking integral average．The numerical simulation of under-ground gas storage reservoir in aquifer was carried out based on the equivalent seepagemodel．The variarions of gas saturation and pressure of the reservoir by gas injection werestudied．The results were compared with the three-dimensional seepage model to verify theprecision of the proposed model． The influences of reservoir thickness， reservoirpermeability， porosity and injection rate on saturation and reservoir pressure werestudied．The examples show the equivalent seepage model is precise and correct，and canmeet the actual engineering demands，which includes less parameters，higher calculationspeed and facility of nu-merical simulation．The reservoir pressure increases non-linearly withthe increase of injection rate，but decreases non-linearly with the increase of reservoirthickness， reservoir permeability， porosity increasing． The gas saturation increasesnon-linearly with the increase of reservoir permeability and p injection rate，but decreasesnon-linearly with the increase of reservoir thickness and porosity increasing．（3）According to the elastic-plastic deformation characteristics of underground gasstorage reservoir under the alternative loads, the gas-solid coupling mathematical model ofmulticomponent gas diffusion and seepage is established for the operation of the undergroundgas storage reservoir. The dynamic change laws of nitrogen concentration in withdrawn gasare calculated based on the proposed model. On this basis, reservoir permeability, porosity,elastic modulus, poisson ratio and in-situ stress on the nitrogen concentration in withdrawngas are studied. The examples show that the coupling effect is very significant for gas storagein the underground gas storage reservoir. The nitrogen concentration in withdrawn gas isequidirectional with the increase of reservoir permeability and porosity, reservoir permeabilityand porosity have only minor impacts on degree of mixing when they are more than300mdand0.2respectively, whereas it decreases with in-situ stress. Elastic modulus and poissonratio have only minor impacts on degree of mixing.In order not to affect quality of withdrawngas, it is suitable for the injection in the margin of the reservoir and the optimum percentageof cushion gas that can be replaced by inert substitute nitrogen is less than20.（4）According to the finite element and the optimal method, the multi-well constrainedoptimization back-analysis model is proposed. Optimizing and approximating the objectivefunction through the variation of injection and production rate, eventually the real storagecapacity of depleted gas reservoir is obtained. Finally, the presented model is applied to asimple example of water driving reservoir in order to validate the correctness and reliability. （5）The analytical solution of leakage model for the natural gas through an abandonedwell in depleted natural gas reservoir is presented. On this basis, the mathematical model offluid flow underground the reservoir is established. The fully implicit method is used todiscretize the mathematical model and generate a numerical model. The influences ofoperating pressure of gas storage, permeability of leaky well, natural gas viscosity, and holesize on leakage rate of natural gas are studied. The examples show that the leakage rate ofnatural gas through an abandoned well increase with operating pressure,permeability of leakywell and hole size, whereas it decrease with natural gas viscosity.（6）The statics mechanic model of the casing-cement-formation is established in thispaper. Effects of formation and cement characteristics on casing strength are studied underinjection-production condition. The examples show that the maximum stress occurs in theinternal wall of the casing, along the orientation of minimum horizontal stress. The casingmaximum strength increase with non-uniform in-situ stress coefficient and formation poissonratio, whereas it decrease with formation elastic modulus. Howerer, The casing maximumstrength is equidirectional with the increase of cement elastic modulus and poisson ratio,whereas it decreases with cement elastic modulus,poisson ratio and cement thickness, theyhave only minor impacts on casing maximum strength when they are more than30GPa,0.3and30mm respectively.