System Reliability Evaluation for Complex Deep Well Casing Based on Structural Reliability

Abstract

Casing failure is very serious in ultra-temperature, ultra-deep, ultra-pressure wells, because of its severe service environments. The reliability of the casing service has become the key problem to be solved urgently. A reliability evaluation model of casing system based on structural reliability is proposed, by referring to structural reliability design method and considering the randomness and time variability of load and strength of complicated deep well casing. First, discrete casing system into several basic evaluation units, according to the structure of well body, the form of casing failure and the characteristics of stress; then establish the ultimate state equations of the different failure modes of the basic units and calculate the reliability indexes of the basic units separately, according to the structural characteristics and load modes of the basic units; Finally, complete the reliability evaluation of the whole well casing, according to the system reliability calculation method. Taking a ultra-deep well in the West as an example, the reliability index and failure probability of the whole well are calculated. The results show that the casing reliability evaluation model based on structural reliability can effectively realize the quantitative characterization of the casing reliability of the whole well and the dynamic change process of the casing reliability in the service cycle, which provides a basis for timing of well repair and monitoring of key parts. At the same time, the corresponding relationship between the reliability index and the traditional safety factor is calculated, and the evaluation results have a better guiding significance for the practical engineering.