Abstract:

During leaching for salt cavern underground energy storage, leaching string accidents for solution mining, excessive bending or breakage, etc., often occur. They seriously affect project progress and shape control of the cavern construction. Preliminary theoretical and experimental researches on the dynamic characteristics of leaching strings were carried out. It is shown that the main reason why the overbending or breakage of leaching strings occurs is self-excited fluid-structure coupling vibration of leaching strings in confined space, well. The critical flow velocity expression of leaching strings is obtained under idealized condition, revealing that leaching bending stiffness of leaching strings, overhang length and overflowing area are the main influence factors of tubing dynamic stability. The phenomena of water hammer, static buckling and chaotic motion were observed in the model experiment, accounting for the main influence factors. Finally, some research advices on the overall analysis of dynamic characteristics of the leaching string are proposed.

Introduction

Deep salt rocks formation is an ideal place of underground storage for oil, natural gas and high-level radioactive nuclear waste [1]. The salt rock storage is usually built by the method of solution mining through fresh water injection and brine removal [2]. Fig. 1 shows the process of dissolving salt rock: fresh water or weak brine is introduced into salt cavern through the inner leaching tubing (or the annulus between the inner leaching tubing and the outer leaching tubing), and brine in the cavern is discharged to the surface through the annulus (or the inner leaching tubing). The volume of the cavern gradually expands due to dissolution and removal of salt rock. The annulus between the outer leaching tubing and the production casing is full of diesel oil or other liquid forming an oil cushion at the top of the cavern. The salt rock at the top of the cavern and weak brine are separated by the oil cushion to suppress upward leaching of the cavern and promote lateral leaching. Conversely, if the oil cushion is removed, salt rock of the top of the cavern will be contacted byweak brine, resulting salt cavern developing upward. Reasonable adjustment of the oil cushion depth can achieve the controlling of the shape of salt cavern.

During salt cavern leaching, some engineering accidents of leaching strings may occur, such as wear and corrosion of leaching strings, bending deformation, fracture of tubing and damage of casing coupling. These accidents affect the project schedule, and increase the construction cost. Fig. 2 shows that the bending deformation of inner leaching tubing is serious. When the inner leaching tubing is lifted to the surface for repair or replacement during well workover, the outer leaching tubing may be damaged if the excessively bendy inner tubing is lifted forcibly in confined cylindrical space. Sometimes the inner tubing with serious bending deformation has to be cut off, abandoned in the cavern permanently.

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