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.

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理论与数值模拟预测磨损套管残余爆破强度

摘要 套管磨损是影响套管完整性和套管强度的严重问题，特别是在超深井、定向井和水平井中。套管磨损后，套管强度下降会导致套管变形、持续套管压力甚至套管失效等危险问题。因此，考虑磨损效应，预测剩余套管强度和设计套管柱具有重要意义。然而，磨损套管的残余强度计算模型仍然是套管设计和安全评估中的一个挑战。本文的目的是探索一种简单而准确的模型来预测新月形磨损套管的残余爆破强度。本文讨论和比较了最常用的数学模型。通过调查和推导，发现API（美国石油学会）标准模型假设套管内壁周围均匀磨损减少，而偏心磨损模型将磨损的套管简化为双极坐标下的偏心管。然后本文基于月牙形磨损套管的力平衡和力矩平衡提出了月牙形磨损模型。有限元分析证明，月牙形磨损模型对磨损套管的残余爆破强度预测更加严谨和准确。本文的工作可为定向井套管磨损评价和套管柱设计提供理论基础和技术依据。然后本文基于月牙形磨损套管的力平衡和力矩平衡提出了月牙形磨损模型。有限元分析证明，月牙形磨损模型对磨损套管的残余爆破强度预测更加严谨和准确。本文的工作可为定向井套管磨损评价和套管柱设计提供理论基础和技术依据。然后本文基于月牙形磨损套管的力平衡和力矩平衡提出了月牙形磨损模型。有限元分析证明，月牙形磨损模型对磨损套管的残余爆破强度预测更加严谨和准确。本文的工作可为定向井套管磨损评价和套管柱设计提供理论基础和技术依据。