Empirical Random Kick Model and Casing Reliability

Abstract

Summary The development of exploratory wells is based on the interpretation of seismic readings by geotechnical field experts. Correlation wells, when they exist, are located very far, leading to large uncertainties and to different interpretations by different experts. Exploratory wells are expensive and risky endeavors, in particular deep wells with high-temperature and high-pressure conditions. Reaching reservoirs at record depths often requires challenging usual safety margins, which should be based on reliability analyses. In this paper, we present an empirical random kick model to combine the opinions of different specialists and the reliability analysis of casing subject to kick loads. The empirical kick model has a random kick volume and intensity, gas gravity, and influx depth. Three tubular strength models are considered: Barlow formula, von Mises stress, and Klever-Stewart model (Klever and Stewart 1998; Stewart and Klever 1998), with random yield stress and tube thickness. A typical 19,000 ft well is considered as a case study example, and the First-Order Reliability Method (FORM) is used as a probabilistic solver. Results show the differences between safety factors and reliability indices obtained for the three strength models. Sensitivity factors show that yield stress and model error are the most relevant random variable at shallow depths. Kick volume and influx depth are found to be the most relevant random variables of the kick model, especially at greater depths. The proposed empirical kick model is shown to have a significant impact on casing reliability, in comparison with a kick in which uncertain kick variables are assumed with uniform distribution.