Optimization of Safety Height of Casing Shoe in Salt Cavern Gas Storage: A Case Study of Pingdingshan, China

Abstract

Abstract Salt cavern gas storage typically involves frequent multi-cycle injection and production, resulting in cyclic loading of the salt cavern system. The casing shoe, as one of the most vulnerable components, exhibits fatigue failure under cyclic loading, posing the risk of engineering accidents, including bond failure between the cement sheath and the formation, salt cavern collapse, and gas leakage. However, there has been limited research on the design of casing shoe heights for salt cavern gas storage. Therefore, in this paper, the casing-cement sheath-strata assembly was determined based on the Drucker-Prager criterion. The stability analysis of salt caverns under different interlayer distributions was conducted, and the distribution characteristics of plastic damage zones in salt caverns under different interlayer position distributions were compared. Considering the feasibility and stability of the salt cavern, the optimal range of the height of the casing shoe that meets the safety sealing requirements, the plastic zones and other indicators is proposed. The results of this paper are as follows (1) (1) Under unclamped conditions, the degree of influence of the stress concentration zone decreases as the height of the casing shoe increases. (2) When the interlayer is located above the casing shoe, the reservoir Von Mises stress becomes obviously smaller as the distance between the interlayer and the casing shoe increases. At the same time, the high strain region at the contact between the wellbore and the top of the cavity also decreases significantly, and the surrounding rock displacement at the casing shoe decreases most obviously. (3) When the interlayer is located below the casing shoe, the average value of surrounding rock displacement at the casing shoe is small, and the stress concentration area of the gas reservoir is insensitive to changes in the distribution of the interlayer. Therefore, in engineering practice, it is recommended to establish the casing shoe above the interlayer, which is more conducive to the stability of the gas storage reservoir. However, it should be noted that when the number of interlayers below the casing shoe increases, the surrounding rock displacement at the casing shoe will increase substantially, so it is recommended to have only a single interlayer below the casing shoe as much as possible. (4) No matter where the interlayer is distributed, the point of contact between the open well section and the cavity is the point of maximum equivalent stress, and there is a layer of 0.18~0.25m thick stress concentration zone on the inner side wall of the open well section. In order to avoid the local damage of the open well section leading to tensile damage at the casing shoe, the casing shoe height should be kept high enough to form a safety buffer zone. It is recommended that the casing shoe height be greater than 10m.