Abstract
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.
Highlights
In China, most of the salt formations were deposited bedded with various interlayers between salt rocks [1, 2]
Considering that the mechanical properties of bedded salt rocks are different from the almost pure rock salt in the salt domes, a time-dependent prediction model was proposed by Zhang et al [11] for the ground subsidence of the underground gas storage cavern in impure salt formations
Q/Rm T, (13) C(α) Cs · α + Ci · (1 − α), T 273.15 + 20 + 0.02 · H, where k stands for k1 and k2 and indicates the shrinkage rate of unit volume of a cylindrical salt cavern, m3/m3, P represents the brine pressure P1 during the building period or the gas pressure P2 during the operation period, MPa, P0 represents the original in situ pressure, MPa, Cs and Ci represent the creep properties of the rock salts and interlayers, 1/month, and H is the depth of the cavern center, m
Summary
In China, most of the salt formations were deposited bedded with various interlayers between salt rocks [1, 2]. Fokker and Visser [22] successfully predicted the ground subsidence caused by salt cavern shrinkage in the Netherlands by a theoretical model In their model, surface movement matches the influence functions derived from geomechanics, and these influence functions may change over time. Considering that the mechanical properties of bedded salt rocks are different from the almost pure rock salt in the salt domes, a time-dependent prediction model was proposed by Zhang et al [11] for the ground subsidence of the underground gas storage cavern in impure salt formations. Is paper proposed a new prediction model for the time-dependent ground subsidence above the horizontal cavern in impure bedded salt formations, considering both the construction period and the storage period. The influence of some factors on the ground subsidence was carried out with this model, such as the creep time, the draw angle, the subsidence adjusting coefficient, the cavern depth, the purity, the creep properties of the rock salt, the brine extraction rate, the internal pressure, the cross-sectional diameter, and the length of the horizontal cavern. e work of this paper is helpful to establish the prediction theory of ground subsidence above horizontal salt caverns and provides a reference for the design and construction
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