| Underground gas storage(UGS)is one of the effective solutions to improve the gas supply capacity,mitigate seasonal and regional contradictions of natural gas supply.Currently,more than 90%of UGS in China are reconstructed by using complex structures or depleted gas reservoirs.Due to the influence of continental deposition and tectonic movement,these gas reservoirs have the characteristics of poor physical properties and development of fractures.These factors bring theoretical and technical challenges to the rapid circulation,full utilization and safe operation of UGS.To meet the demand of natural gas peak regulation,the UGS has the characteristics of large-scale gas injection and production.The gas injection and production rate can reach more than 10 times that of conventional gas reservoir development.This paper aims at the problems such as the lack of flow theory of high-rate injection-production,dynamic parameter diagnosis and leakage analysis method for gas injection,and the difficult on-site monitoring and deployment.The systematic research on W gas storage was conducted by combining physical simulation experiments,porous flow theory derivation and geological leakage risk analysis.The main work is as follows:(1)In terms of flow theory,based on the characteristics of high-rate injectionproduction.irregular outer boundaries and natural fractures,a mathematical model for high-rate injection-production in gas reservoir-type UGS with developed fractures was established.The mathematical model is solved by combining point source function,line source function integration.Laplace transformation,boundary element and Stehfest numerical inversion methods.The reliability of the model was verified by using general numerical simulation software.The sensitivity analysis method was used to study the influencing factors of pressure and pressure derivative curve.(2)In terms of diagnostic methods,the diagnostic chart under the condition of high-rate injection-production of UGS was established by using the derivative analysis method of pressure.The high-rate injection-production diagnostic method for gas reservoir-type UGS with developed fractures was formed.The pressure derivative diagnostic curve shows that its flow characteristics can be divided into ten flow stages.It includes wellbore storage effect,skin effect,vertical radial flow and linear flow(for horizontal wells),pseudo-radial flow in the inner zone,quasisteady state interporosity flow in the inner zone,transition flow,quasi-steady state interporosity flow in the outer zone,system pseudo-radial flow,and boundary dominant flow stages.(3)The diagnostic analysis of W gas storage shows that in the early stage of operation,the rising degree of the pressure derivative curve gradually increases in the later stage,the mobility ratio and composite radius gradually increase.The radial composite characteristics brought by high-rate injection-production become more and more obvious.After the fourth cycle,the mobility ratio and composite radius gradually decrease.The skin factor gradually decreases with the increase of injection-production cycle and eventually tends to 0.The control area of the injection well can reach 2.5 times that of the production,the cumulative pore volume for gas injection and production are 0.18×108m3 and 0.14×108m3 respectively.(4)In terms of leakage risk analysis,it is clarified that the geological leakage risk factors of UGS includes four parts:reservoir,cap,shelter,and well facilities.A geological leakage risk analysis method suitable for gas reservoir-type UGS with developed fractures was proposed.The application analysis of W gas storage shows that the leakage risk order of gas storage is the first permeable layer P1q,the second permeable layer Plm,the third permeable layer P2ch and the shallow surface layer.(5)In terms of monitoring and early warning,the UGS monitoring system was established by combining reservoir monitoring wells,caprock monitoring wells,fault monitoring wells,trap monitoring wells,shallow monitoring wells and drilling test.The monitoring system of W gas storage was designed and applied.The application results show that the pressure increase of monitoring well 3 in the second permeable layer is due to the pressure recovery caused by the continuous accumulation of gas at the boundary to the bottom of the well after shut-in.It is predicted that the pressure abnormal layer will tend to stabilize in 2026.Through the above research,the flow theory of high-rate injection-production,monitoring methods,and application technology in gas reservoir-type UGS with developed fractures were formed.The relevant content has been successfully applied to the W gas storage in China.These research results will provide theoretical basis and technical support for the design of UGS in Bohai Rim,southwest and Changqing,as well as the adjustment and safe operation of the active UGS such as Hutubi.Dazhangtuo and Shuangliu. |
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