Prediction of residual burst strength of worn casing by theoretical and numerical modelling

Abstract Casing wear is a serious problem affecting casing integrity and casing strength, especially in ultra-deep wells, directional wells and horizontal wells. After casing is worn, degraded casing strength will lead to such hazardous problems as casing deformation, sustained casing pressure and even casing failure. Therefore, it is crucial and meaningful to predict residual casing strength and design casing string considering wear effect. Residual strength calculation model of worn casing, however, is still a challenge in casing design and safety evaluation. The purpose of this paper is to explore a simple and accurate model to predict the residual burst strength of crescent-shaped worn casing. The most commonly used mathematical models are discussed and compared in this paper. Through investigation and derivation, it is found that API (American Petroleum Institute) standard model assumes a uniform wear reduction around casing inner wall, while eccentric wear model simplifies worn casing as an eccentric pipe in bipolar coordinate. Then this paper presents a crescent-shaped wear model based on the force balance and moment balance of crescent-shaped worn casing. Finite element analysis proves that crescent-shaped wear model is more rigorous and accurate for residual burst strength prediction of worn casing. The work presented in this paper can provide a theoretical foundation and technical basis for casing wear evaluation and casing string design in directional wells.