During the manufacturing process of tubing and casing, manufacturing deviations can occur in quality parameters, such as yield strength, outer diameter, and wall thickness, which can lead to uncertainty in the actual strength of the tubing string. Specifically, tubing string failures periodically occur in high-temperature, high-pressure, and ultra-deep gas wells owing to the uncertainty in string strength, attenuation of high-temperature material properties, and influence of complex load fluctuations. Therefore, in this study, Monte Carlo method is adopted to simulate the local fluctuation of downhole actual load, and the load variation rule of tubing string in high-temperature and high-pressure ultra-deep well is obtained. By considering the effect of manufacturing tolerance and corrosion, variation law of string strength is obtained. Additionally, the influence of load, temperature, manufacturing tolerance, and other uncertainties on the strength and stress of the tubing string is analyzed, and a quantitative evaluation method for the mechanical integrity of the tubing string is established. The calculation results show that the uncertainty of complex loads and influence of high temperature increase the failure probability of the mechanical integrity of tubing string. Furthermore, a failure risk can potentially exist under conditions wherein conventional verification results denote no failure. It is determined via sensitivity analysis that temperature, corrosion rate, and steel grade are the main factors for the failure of the tubing string mechanical integrity. By considering the influence of high temperature, the failure probability of the tubing string is increased by 25.33% when compared to the original basis. However, considering the manufacturing tolerance of tubing string, the actual yield strength of tubing string is higher than the nominal yield strength, which reduces the failure probability of the tubing string. This implies that reduction of the subjective influence in the risk assessment process can lead to precise and quantitative analysis of the failure
Pitting corrosion often occurs due to the presence of various corrosive substances, such as CO 2 and H 2 S, in the pipe service environment. As a result of this process, the residual strength of oil pipes is reduced and this can compromise the integrity of the entire pipe string. In the present work, a model is introduced on the basis of the API579 standard to determine the so-called stress concentration coefficient. The model accounts for pitting corrosion shapes such as shallow semi-circles, semi-circles, and deep semi-circles. The relationship between the corrosion pit depth and opening diameter and the residual strength of the oil casing is obtained. The results show that the influence of the pit opening diameter on the stress concentration coefficient is smaller than that of the pit depth. For a constant pit opening diameter, the coefficient increases gradually with increasing the pit depth. The compressive strength and internal pressure strength of the carbon steel oil casing decrease accordingly. When the depth of the corrosion pit is relatively small, the growth of the coefficient is slower; when the depth of the corrosion pit increases to a certain value, the increase in stress concentration coefficient becomes obvious.
The trapped annular pressure (TAP) caused by thermal expansion is one of the serious challenges for the safe production of a deep gas well. Therefore, this article proposes a model to calculate the temperature profile of the deep gas well based on the heat transfer process and the gas properties. With the help of the temperature model, the TAP in the tubing–casing annulus is analyzed according to the annular fluid distribution and the volume consistence law. The results indicate that the temperature inside the tubing string decreases faster under higher bottom hole pressure. When the tubing–casing annulus is totally filled with the annular protection liquid, the TAP continues increasing with the production rate. Considering the high production rate, the TAP is inevitable and high enough to damage the integrity of the deep gas well. The nitrogen gas mitigates the TAP by reducing the annular liquid volume and providing the extra space to accommodate the thermal-expanded annular liquid. A good mitigation performance can be achieved no matter how large the production rate is. The mitigation performance can be divided into the fast-decreasing stage, the efficient control stage, and the stable stage. These three stages occur as the nitrogen gas column length increases. The compression of the nitrogen gas volume plays a major role in the fast decrease stage while the reduction of the annular liquid plays a major role in the stable stage. For the best cost-effectiveness, the nitrogen gas column is recommended in the efficient control stage and should not exceed 15%.
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