Roof Stability Of Horizontal Cavern In Bedded Salt Formations For Compressed Air Energy Storage

The construction of compressed air energy storage power stations in salt caverns is one of the important means to solve the problem of wind/solar energy generation grid connection and grid peak regulation.Based on the practices in the United States and Germany,the current compressed air energy storage were all built in pear-shaped single caverns,which were solution mined in salt domes with large thicknesses.Due to the special geological and sedimentary environment of China's salt formations deposit,the salt formations are mostly bedded,and the thickness of a single salt layer is much lower than those salt domes.In order to make full use of our country's bedded salt formations,in recent years,the option of building horizontal caverns has been proposed.However,horizontal caverns bring us not only more cavern space but also larger dimensions and larger roofs,which are easily suffered from unbalanced deformation,failure and even cracking to be the channel of air leakage.To this end,a series of theoretical analyses and numerical simulations were carried out to investigate the roof stability of horizontal cavern in bedded salt formations for compressed air energy storage and the influence factors.The main studies include: first,the argillaceous anhydrite layer has been identified as the key roof in the selection of the horizontal cavern for energy storage.Secondly,based on the linear planning method,a feasible range of internal pressures and the depths of the cavern for compressed air energy storage were determined by combining the theory of plate stability and engineering practices.Finally,various factors affecting the horizontal cavern in bedded salt formations for compressed air energy storage were analyzed in detail by numerical simulation,including the length of the long side,the thickness of the protective salt layer,the maximum and minimum operating pressures and the rate of gas extraction and injection during the operation of the reservoir.Some useful conclusions reached are as follows:(1)The maximum operating internal pressure is the main determinant of the stability of the key roof above horizontal cavern and the viability of the horizontal cavern.Higher maximum internal pressure are good for the stability of the roof,but too high of the internal pressure may create tensile damage in the roof.Taken together,the maximum internal pressure should be less than 75% of the original gravitational stress.(2)The horizontal cavern is more sensitive to changes in the maximum internal pressure than a typical pear-shaped single cavern,and a limit for the maximum internal pressure should be set.Under the conditions presented in this paper,the limit for the maximum internal pressure of the circulation of the cavern should not be less than 60% of the original gravitational stress.(3)The cavern dimensions mainly affect the displacement,plastic zone expansion and expansion damage factor SFvs of the surrounding rocks and the key roof of the horizontal cavern.Meanwhile,it has little effect on the volume shrinkage of the cavern and the stress distribution in the roof.When the length of the long axis of the horizontal cavern is about 2 times of its width,its volume shrinkage has minimum value.(4)Since the setting of the air injection rate needs to take into account the peak and valley distribution of the grid,changes of the air injection rate in a limited range have less impact on the roof stability of the cavern.(5)The effect of the thickness of protective salt layer on the displacement,stress and plasticity zones of the argillaceous anhydrite roof shows a two-sided nature.The increase of the thickness of the protective salt layer reduces the displacement and plastic zone volume of the argillaceous anhydrite roof,but increases the final volume shrinkage and the time to stabilization of vertical directional stresses in the roof and their extremes in the process.Moreover,too thick of the protective salt layer is also not conducive to the full utilization of bedded salt formations.Therefore,the thickness of the retained salt layer needs to be reasonably determined.There are 59 figures,7 tables and 113 references in this paper.