Study On Ground Subsidence Of Cavern Natural Gas Storage In Bedded Salt Rock Based On FLAC^3D

Due to its ability of self-recovery,good creep properties and low permeability;salt rock has been an ideal medium for underground energy storage.The surface subsidence above the salt rock gas storage,which is caused by the creep of the salt karst cavity and the transfer along the overburden,will cause serious damage to both the environment and economy.Meanwhile,China’s layered salt rock is very different from the giant salt domes abroad,therfore one cannot fully draw on the experience of the construction of foreign salt rock gas storage,so then it is necessary to take a scientific method to simulate the specific scenario of the layered distribution of salt rock in China.Based on the numerical simulation using FLAC30 software,this paper will conduct a deep research into the surface subsidence of layered salt rock gas storage.The main research contents and results are as follows:(1)Based on the research results at home and abroad,the mechanism of surface subsidence of bedded salt rock underground gas storage and the main factors affecting surface subsidence were analyzed.The mechanical characteristics,creep characteristics and mechanism of creep constitutive equation of bedded salt rock were studied.A summary was made and the CPOWER creep constitutive equation had been used for numerical simulation.(2)For the single-cavity salt rock underground gas storage,the effects of cavity shape,operating pressure,working mode and gas production rate on surface subsidence were studied.①Under the same pressure,the surface settlement of the same volume of the rotary cavity is smaller than the ellipsoid in different time periods.Therefore it is better to use the rotary cavity in the project.②The surface subsidence of the bedded salt rock underground gas storage reservoir decreases with the increase of the gas storage pressure.Therefore,if the conditions permit,it is beneficial to increase the operating pressure to the surface subsidence and the cavity creep shrinkage of the gas storage.Then 1stOpt 6.0 Software was used to fit the formula of the maximum settlement value of the surface to the operating pressure and operating time of the gas storage.③The traditional operation mode of a gas storage is:Maintain low pressure for three months,then increase the pressure to inject gas for three months,after which the gas is stored at a high pressure for three months,and lastly reduce the pressure to extract gas for three months.In this paper,the pressure reduction and gas recovery rate is optimized to 0.4MPa/day,and the surface subsidence of the gas storage is significantly reduced.(3)For the group gas reservoir underground gas storage,this paper considers the influence of the adjacent cavity spacing and the adjacent cavity pressure difference.①When the distance between adjacent cavities is less than 300m,the surface subsidence of the gas storage reservoir decreases significantly with the increase in distance between adjacent cavities.After the distance between adjacent cavities reaches 300m,the surface settlement is basically stable,so 300m(1.5 times the length of the hole)is a suitable distance.②Under the interval of 300m,the influence of the pressure difference between the adjacent cavities of the gas storage on the surface subsidence is not obvious,and the surface settlement curve has no obvious asymmetry.③The pressure difference between adjacent cavities has a great influence on the shrinkage of the cavity.When the pressure difference between adjacent cavities is too large,it is not conducive to the stability of the cavity.