Ground Subsidence Prediction Model and Parameter Analysis for Underground Gas Storage in Horizontal Salt Caverns

Due to a great demand of natural gas or oil storage in these years, horizontal caverns were proposed to fully use bedded salt formations of China. Under the same geological and operating conditions, the horizontal cavern would shrink more than traditional pear-shaped cavern, which might bring larger ground subsidence and affect the safety of storage facilities. A new prediction model was proposed in this paper for the time-dependent ground subsidence above horizontal caverns. The proposed model considered the impurity of bedded salt formations and simplified the horizontal cavern to an ideal cylinder. The shape of the subsidence trough was determined by the probabilistic integration method, and corresponding calculation formulas for the tilt, curvature, horizontal displacement, and horizontal strain were derived. Based on the assumption that the subsidence volume at the ground was proportional to the reduced volume of horizontal cavern, a formula for the reduced volume over time was established. FLAC3D was introduced to simulate the ground subsidence, and the results show that the proposed prediction model agreed well with the simulation results. Finally, the proposed prediction model was used to analyze the impacts of different stratigraphic parameters and design parameters. The results mainly show that, as the draw angle increases, the subsidence trough becomes deeper and narrower; as the depth of the cavern increases, the maximum subsidence first increases and then decreases, and the subsidence trough gradually becomes round; with the increase of the purity, the subsidence gradually decreases; with the increase of the creep properties and the stress exponential constant, the maximum subsidence first increases rapidly and then slowly approaches the limit; increasing the brine extraction velocity can shorten the cavern construction period and then reduce excessive ground subsidence; the subsidence decreases nonlinearly with the increase of internal pressure; with the increase of the cross section diameter and length of the horizontal cavern, the subsidence presents a significant nonlinear increase. In addition, unlike the traditional pear-shaped cavern, under the same conditions, the ground subsidence above the horizontal cavern according to this newly proposed model is much larger, and the ground subsidence contour line is no longer a standard circle. The findings of this study can help for better understanding of the prediction of ground subsidence above salt caverns and also provide a reference for the design and construction. However, the proposed prediction method is ideal and theoretical and should be further improved by engineering practice in the future.