Seepage Displacement And Operating Simulation Of Fractured Underground Natural Gas Storage Reservoir

Underground natural gas storage?UGS?has been commonly recognized as one of the promising measures to stabilize gas peak fluctuation,ensure continuous gas supply and meet the requirement of city's demand peak shifting.It is the most ideal choice to use reconstructed depleted gas reservoir with single pore structure as an underground natural gas storage.However,the approach using fractured carbonate reservoir as storages is yet available for neither research nor engineering practice,especially for developed areas like the North China and East China,where underground natural gas storage is urgently needed.In order to alleviate the extreme imbalance between supply and demand of natural gas,meeting the needs of urban peaking,it is imperative to carry out preliminary theoretical research on underground natural gas storage,and provide the theoretical guidance for engineering practice in contructing fractured underground gas storages in China.The study investigated a series of problems existing in the reconstruction of underground gas storage in the depleted gas reservoirs,which include the mechanism of gas and water seepage in fractured reservoirs,inversion of formation parameters,the establishment and solution of gas injection and production flow model for gas storage,simulation and optimization of injection and production of underground gas storage in fractured depleted gas reservoirs and the key technical problems of using carbon dioxide?CO₂?as gas storage cushion gas.This research was initialized with an experimental study on ten kinds of carbonate fractured reservoir cores.Core testing method was employed to determine the porosity,permeability and relative permeability curve of fractured reservoir cores.It is observed that porosity,permeability and other parameters are changed in different testing pressure.By using an artificial carbonate rock slab model,the gas-drive water and water-drive gas experiments were performed.The experiments successfully obtained an ideal fracture pore dual channel reservoir structure in carbonate rock reservoirs.This experiment also concluded that the gas storage and transport processes are all in the fracture channel.Moreover,it is confirmed that the matrix reservoir is basically no seepage process in the pressure range of injection and production operation of underground gas storage.The study then proceeded at establishing a gas-water two phase flow displacement model for fractured depleted natural gas reservoirs,as well as its corresponding numerical solution method,via introducing the model theory of dual porosity media.This study presented an effective calculation method of permeability and porosity of fractured underground reservoirs based on the geological statistics method,referring to the theory of parameter inversion.Under the premise of the actual data validation,parameters such as permeability and porosity of the gas reservoir were calculated,and thus provided real formation parameters for the simulation of the underground gas storage and the stability analysis of the boundary.The model was then applied to simulate the storage operation process of its peak injection,expansion construction and migration stability of gas water boundary,referring to the geological data of a fractured depleted gas reservoir in the Northern china.Calculation results showed that at the construction stage,the intermittent gas injection capacity building process should be adopted to ensure the maximum design capacity of the reservoir under the condition of reservoir formation constraints,it is necessary to match the peak injection quantity of each single well reservoir in the process of injection and production for the purpose of avoiding the flooding problem of gas channeling and gas recovery in gas injection.Moreover,this research investigated the problems of mixing and dissolution CO₂ injected into the fractured underground gas storage as cushion gas.Taking into account the mixing and dissolution of natural gas and CO₂ during multi cycle injection and production,the gas mixing zones formed via varied injection modes and various injection gas ratios of cushion gas in gas storage,were analyzed through the establishment of CO₂ and natural gas in the fractured underground gas storage mixed gas model.The effects of mixed zone migration on injection and production were also discussed.Through the simulation of gas storage shows that the operation process of city peak shaving is not affected by the mixed of natural gas and CO₂ when the cushion gas ratio of CO₂ determined to reach no more than 32.5 % when the maximum storage capacity is reached in the UGS.If consider the solubility property of CO₂ in water,the maximum loss ratio of cushion gas is 15.5 % when storage capacity reached the maximum of UGS.Considering the gas storage wells in the city to meet peak demand and the need to optimize the distribution of each single well injection and production of each well were calculated under the conditions of varied optimization objectives.Based on the results,a reasonable and practical operation control scheme of fractured underground natural gas storage was proposed.The case study results showed that the optimal allocation of the injection and production of each well will reduce the fluctuation of formation pressure and reduce the power consumption of compressor to some extent,and ultimately ensure the safe and efficient operation of the gas storage.In China,with the increasing development of the natural gas industry and the increasing demand for urban gas,underground gas storage has become increasingly urgent demand.The results of this project help with improving the theory study of different categories of underground gas storage and providing technical supports for building and maintaining fracture type underground natural gas storage.Therefore,the results of this study have a wide perspective of industrial application.