Simulation and analysis of force on connection of casing thread under alternating load of gas storage

Underground gas storage as the main system of natural gas peak regulation, its construction demand continues to grow in the world. Different from other common oil and gas well strings, the service time of the gas storage string is longer and the service environment is worse, which poses a greater challenge to the integrity design of the string joint. In this paper, starting from the most important connection and sealing performance of the pipe string joint, aiming at the special threaded joint suitable for the gas storage pipe string, based on the analysis of the force and deformation of the gas storage pipe string, the joint connection strength and sealing performance of the pipe string joint under the axial load, internal and external pressure and longterm alternating load are considered. Based on the FINITE element model of VAM TOP special threaded joint, the deformation and sealing behavior of the joint under different composite loads were numerically simulated, and the variation law of joint connection stress and sealing contact pressure with load was obtained, and the most vulnerable failure position and the most dangerous working condition were analyzed.


Introduction
With the continuous growth of the demand for natural gas, the phenomenon of insufficient natural gas supply often occurs. In order to adjust the balance of natural gas supply and demand, it is necessary to use natural gas reserve system. At present, underground gas storage has become the most important way of natural gas storage and peak regulation in the world because of its advantages of high safety factor and large storage capacity [1][2][3][4][5]. Different from the common oil and gas well pipe string, the underground gas storage pipe string has been in service for more than 30 years, and the service environment is very bad. The alternating load caused by the long-term gas injection and production process poses a severe challenge to the design of pipe string, especially the design of pipe string joint. As the most critical part of the whole string, the joint of the string is also the weakest link, which can easily cause gas leakage due to failure [6][7][8]. There are two main reasons for the failure of underground gas storage pipe string joint: connection failure and sealing failure. Connection failure mainly includes the following forms :(1) trip failure (2) fracture failure (3) shear failure (4) over torsion failure. Seal failure refers to the gas leakage phenomenon of threaded joint due to various reasons such as machining accuracy, operation mode and working condition. In view of the above failures, it is necessary to study the connection and sealing integrity of threaded joint in order to maintain the operation safety of gas well in gas storage. The research methods of mechanical properties of threaded joints mainly include analytic method, full-size test method and finite element method.

Theoretical Basis
(1) Axial force calculation In the early stage of production, the injection-production string is set inside the production tubing by a packer, and changes in the external extrusion pressure and internal pressure can be translated into axial force by the bulging effect or the piston effect. In the production process, the internal pressure in the string fluctuates and alternates with the frequent alternation of injection-production cycle, resulting in the change of external extrusion pressure and axial force. During gas injection and recovery, the formation volume expands or compacts the string in an axial direction. At the same time, the inner diameter of the pipe string varies, and the temperature, density, and flow rate of the pipe string fluctuate, which will impose additional axial load on the pipe string. Therefore, the axial force of the gas storage pipe string is the sum of the superimposed axial force caused by dead weight, temperature effect, expanding diameter effect and ground stress during gas injection and production. The axial tension generated by the dead weight of the tube body can be calculated by: During gas recovery, axial pressure will be exerted on the tubing due to the decline of the inner layer of the well. At this time, the tubing compressive strength is:

Modeling process
The working environment of tubing is bad and the alternating load is large. Because the joint part itself is relatively weak, coupled with its sectional shape change, there is a high alternating stress, stress concentration is serious, is the most dangerous part of the force. The model established in this paper is shown in Figure 1 and Figure   2. V-038r, taper 1:4, elastic modulus E = 200GPa, Poisson's ratio = 0.3, density 7850kg/m3.

Discussion
Stress concentration occurs at the last thread of female thread, the maximum deformation is 3.0mm, the maximum equivalent stress is 584.8mpa, the contact compressive stress is very large, there is a risk of fatigue damage.

Figure 4. Stress analysis of thread connection
The stress concentration occurs at the last thread of female thread, the maximum deformation is 2.9mm, the maximum equivalent stress is 565.8MPa, the maximum deformation increases, but the maximum equivalent stress decreases. Under different internal compressive strength, the maximum strain is 3.41mm, the minimum strain is 2.51mm, the maximum stress is 668.62MPa, and the minimum stress is 488.23MPa. The maximum equivalent stress of 666.8mpa exceeds the thread connection limit stress, and the thread fatigue failure occurs.Two-dimensional binomial fitting was carried out, the fitting error R2=0.9947, and the fitting effect was relatively good.

Conclusion
(1) Under the same external pressure, with the increase of internal pressure, the maximum strain increases gradually, the maximum stress increases gradually, and the limit life decreases.
(2) The thread connection needs to be considered, the lower part needs to check the compressive strength and extrusion strength. The upper part needs to be checked for axial tension.