Stability analysis of U-shaped horizontal salt cavern for underground natural gas storage

Abstract

More and more attentions are being paid to horizontal salt caverns for natural gas storage because of their large working gas capacity and favorable stability. To accelerate the construction of gas storage underground salt caverns, stability analysis of this type of cavern is necessary to assure the safety of such caverns. In this paper, the stability is investigated of a U-shaped horizontal salt cavern under different constant and cyclic internal gas pressures. A 3D geomechanical model is established based on sonar scanning in the field and predicted cavern shape. The stability is analyzed of the rock masses around the cavern under different internal gas pressures and cycle frequencies. Five evaluation criteria are proposed to predict the feasibility and stability of such caverns, including deformation, dilatancy safety factor, volume shrinkage, plastic zone, and equivalent strain. The stability of the rock mass around the cavern under different internal gas pressures is compared to that under different cycle frequencies. The results show that the cavern has a good stability under the constant internal gas pressure of 8 MPa and cyclic internal gas pressures ranging from 8 ~ 18 MPa. The five evaluation indexes of the rock masses around the cavern improve with increasing internal gas pressure. It is proposed that the corner, horizontal-roof center, and external waist positions of the cavern are to be highlighted in the design and construction phases. The cycle frequency has appreciable impact on the stability of the rock mass around the cavern. The difference between the volume shrinkage under cycling compared to constant internal gas pressure is that the cycling simulated results show a rising wave-like curve of creep time. The plastic zone ratio increases with creep time and has a flat peak and a sharp bottom with oscillations. This study provides the design parameters for U-shaped salt cavern in the Huaian salt district and can also be a reference for horizontal salt caverns.