Research on the Determination Method of Additional Safety Factor Margin for Ultradeep Well Pipe Strings under Multisource Loads

The fracturing operations in ultradeep oil and gas wells present greater challenges compared to conventional wells, particularly due to the imprecise safety factor in fracturing pipe string design, which leads to limitations in fracturing displacement and inadequate reservoir stimulation. Under complex working conditions such as pump startup, the pipe string is subjected to water hammer-induced bulging stress, coupled lateral and axial vibration stress, and collision stress with the inner wall of the casing, ultimately resulting in overloaded stress levels in the string. Current design methods for pipe strings lack accurate prediction of these multisource loads, leading to either insufficient safety margin reserves or excessive redundancy and overdesign. There is an urgent need to develop a more refined safety verification method for pipe strings based on quantitative load characterization to meet the growing demands of ultradeep reservoir development. This study focuses on the theoretical calculation methods for loads under complex fracturing conditions in ultradeep wells. Numerical models for solving three types of additional stresses in the string during ultradeep well fracturing are established, and a method for determining the design margin of the string safety factor based on the maximum cumulative stress is proposed. Taking an example well from the southern margin of the Junggar Basin in Xinjiang as a case study, the fluctuations of the three additional loads over time, their maximum values, and occurrence positions under different pump working cycles and pipe segment length ratios are calculated. On this basis, the required additional safety factor design margins are derived. The results show that when the fracturing pump operation cycle is extended from 0.625 s to 1.875 s, the total additional load on the string decreases by 53% and the additional safety factor margin drops from 0.262 to a negative value of -0.244. Reducing the length of large-diameter pipe segments while increasing the length of small-diameter pipe segments can mitigate the additional load caused by the water hammer effect but increases the additional loads induced by the vibration and collision. Under different pipe segment length ratios, the safety factor margin ranged from 0.262 to 0.392. The findings and methodologies presented in this study provide theoretical guidance for the refined design and optimization of pipe strings in ultradeep oil and gas wells.