| Underground natural gas storage(UGS)has been the main way to ensure the continuous gas supply and meet the peak shaving demand of the natural gas market.The best choice for rebuilding UGS is to use depleted reservoir with single pore sandstone structure.However,the existing depleted oil and gas reservoirs in China are mostly distributed in the northwest and other remote areas.Unconventional reservoirs with low permeability are mainly distributed in the central and eastern areas where gas consumption is urgently needed.So far,there is no research and engineering practice on the reconstruction of low permeability gas reservoir into UGS.To meet the peak shaving demand of the natural gas market in the central and eastern regions of China,it is imperative to carry out theoretical and experimental research on the feasibility analysis,injection-production characteristics and optimized operation measures of the reconstruction of low permeability gas reservoir into UGS.This paper studies the feasibility of transforming low permeability gas reservoir into UGS,as well as the existing technical problems,through analyzi ng the geological characteristics of low permeability gas reservoir.It provides a basis for the theoretical and Experimental Research on the transformation of low permeability gas reservoir into UGS.Firstly,the permeability and starting pressure gradient of matrix system and fracture system in low permeability gas reservoir are tested by experiments.The results of permeability test show that the pressure-sensitivity of matrix system is obviously higher than that of fracture system.The permeability variation of fracture system and matrix system are 52.97% and 65.21% respectively,when the pore pressure increases from 2MPa to 10.8MPa.The pressure sensitivity of matrix system is higher than that of fracture system.The pressure sensitivity permeability of fracture system and matrix system should be considered separately.The results of starting pressure gradient test show that the starting pressure gradient of matrix system is much larger than that of fracture system,and the maximum difference can be 14.28 times.When establishing the seepage differential equation of low permeability UGS,the starting pressure gradient of matrix system and fracture system should be different.According to the variation of porosity and permeability of low permeability reservoir during the injection-production process,the physical parameters are inversely solved.the inversion identification problem of physical parameters is transformed into the optimization problem by comparing the difference between the measured and calculated values of formation pressure.The inversion of physical parameters is realized by solving thevariation of porosity and permeability caused by formation pressure.The results show that the physical parameters of UGS change with the increase of formation pressure.When the formation pressure increases from10 MPa to 38.2 MPa,the porosity of the gas reservoir increases by 20%,while the permeability increases by 68.64%.When establishing the seepage differential equation of low permeability gas reservoir,the changes of reservoir permeability and porosity can not be ignored.This paper establishes the injection-production model of low permeability UGS via introducing Warren Rutter model.The pressure sensitivity of permeability and starting pressure gradient are considered in both matrix and fracture systems.Diffusion and slippage effects are also considered in the matrix system.The injection production model of UGS is solved by using the pseudo pressure method.The injection-production model is verified by well test data of low permeability gas reservoir.Based on the geological data of a low permeability gas reservoir in China,the injection-production mathematical model is used to simulate and analyze the gas injection process of single well and multi well,respectively.The average formation pressure of single well continuous gas injection increases with the time of gas injection,and the average formation pressure increases faster in the early stage than in the later stage.The pressure distribution in the reservoir is not uniform,and the maximum pressure difference can reach 5.224 MPa of single well continuous gas injection process.The maximum formation pressure point in the reservoir occurs at the injection production well point.On the 170 th day of gas injection,the pressure has reached 39.822 mpa,exceeding the maximum allowable pressure.The formation pressure inhomogeneity of single well intermittent gas injection is improved,but in the whole formation,there is still a large pressure gradien t,and the maximum pressure difference can still reach 3.372 MPa.This shows that even if intermittent gas injection is adopted,the formation pressure at the well point may still exceed the maximum allowable formation pressure,thus affecting the continuous gas injection.When multiple wells inject gas,under the same gas injection rate,the formation pressure change of each injection-production well location is different.The pressure of 3rd injection well increases fastest,and the final formation pressu re is also the highest,which is 38.51 MPa.The comparison between continuous gas injection and intermittent gas injection shows that intermittent gas injection can effectively improve the heterogeneity of formation pressure.To meet the peak shaving demand of natural gas,the optimal operation models and constraints of single well and multi well of low permeability gas reservoir are proposed,and the sequential solution method is used to solve the established double objective function.Compared with the optimized gas injection with the single objective of minimizing the standard deviation of injection-production pressure,the power consumption is reduced by 5.41% and the standard deviation of injection-production pressure is increased by 0.064 MPa.Through the coupling solution of double objective optimization,the optimal gas injection scheme of low permeability gas reservoir is given.In the future,with the development of China's natural gas industry,the peak shaving demand of urban natural gas will increase year by year.To meet the growing peak shaving demand of natural gas,China needs to increase the pace of underground natural gas storage.The research results of this topic can provide theoretical help for the construction of underground gas storage in the future. |
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