As a new type of cavern, small-spacing twin well (SSTW) salt cavern has an extensive application prospect in underground gas storage (UGS) due to its good performance such as rapid construction rate, adaptability of thin salt layer, and high injection-production efficiency. While it has not been found to be used in ultra deep formation. Based on the geological condition and rock mechanical parameters of Pingdingshan Salt Mine, Henan province, China, a 3D geomechanical model considering viscoelastic plasticity of target stratum rock is established to numerically investigate the feasibility of SSTW salt cavern gas storage with the upper height of 70 m, well spacing of 25 m and cavern height of 235 m in ultra deep formation (more than 1700 m). A set of assessment index is introduced to evaluate the safety and stability of salt cavern UGS. The results show that the displacement, plastic zone volume and volume loss rate of the salt cavity increase with the decreasing of internal gas pressure while the stress-strength ratio has a decreasing trend, showing the cavern tends to be unstable. The operation pressure has a large influence on the stability of the UGS. Properly increasing the minimum operating pressure can still maintain the stability of salt cavity in extra deep formation. The deformation in long axis direction is smaller than that in short axis direction, which indicates the pillar needs to be lengthened in short axis direction when the cavern groups are planned. The cavern shows good stability under cycle internal gas pressure as well. All the evaluation indexes present that it is feasible to build the SSTW cavern gas storage in such ultra-deep formation when the proper internal gas pressure is chosen. This research provides a theoretical reference for the application of SSTW salt cavern UGS in similar geological condition.

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