Time-dependent subsidence prediction model and influence factor analysis for underground gas storages in bedded salt formations

• A model is proposed to predict the time-dependent subsidence above storage caverns in rock salt.
• The model accounts for creep and impurity of the salt in bedded salt formations.
• Formulae are developed for the volume and volume reduction of the cavern over time.
• Formulae are derived for the time-dependent surface subsidence, radial tilt and radial curvature.
• Parametric analyses are carried out to study the influences of them on the surface subsidence.

Taking into account the impurity and creep of rock salt, a new model is proposed to predict the time-dependent subsidence of the surface above underground gas storage caverns in bedded salt formations. In this model, the shape of the storage cavern is assumed to be a sphere. Formulae are developed for the volume and volume reduction of the cavern over time during both the construction and the operating periods. A probability integral method is adopted to determine the shape of the subsidence regions, and corresponding formulae are derived for the time-dependent surface subsidence, radial tilt and radial curvature. FLAC3D was used to verify the proposed prediction model. Using the proposed model, parametric analyses are carried out to study the influences of draw angle, volume adjusting coefficient, brine withdrawal rate, internal pressure, cavern depth, material properties of the rock salt, cavern diameter, and operating time on the surface subsidence. The results show that: the subsidence increases with an increase of the draw angle, volume adjusting coefficient, brine withdrawal rate, cavern diameter, material properties of the rock salt, and time, but decreases with an increase of internal pressure. Subsidence increases first and then decreases with increasing cavern depth. Draw angle, volume adjusting coefficient, cavern depth, cavern diameter, material properties of the rock salt, time and internal pressure have a large influence on the subsidence, but there is little influence from the brine withdrawal rate and salt impurity.