复合磨损模式下的套管失效风险预测方法--以Ø193.68 mm×12.7 mm套管为例

Prediction method of casing failure risk under compound wear mode:A case study on Ø193.68 mm×12.7 mm casing

深井、超深井、大位移井等复杂油气井钻井过程中套管磨损严重,大大增加了套管失效风险。现场测量数据表明,某些井段的套管上存在多个磨损区域,磨损形状呈现出比经典“单月牙”形更加复杂的复合模式,常规方法难以准确预测套管失效风险。为此,在分析复合磨损模式与单月牙模式区别的基础上,基于室内实验与数值模拟分析,揭示了复合磨损套管在内压与外挤作用下的失效机理,得到套管剩余强度的主控因素及其作用规律,构建了磨损套管剩余强度智能预测模型,形成了磨损套管失效风险预测方法。研究结果表明:①套管磨损形状不仅是单月牙形,还会出现多月牙重叠、交叉和分离的复合磨损形状;②磨损套管的内压失效是一种强度失效,发生在磨损区套管壁中部的等效应力达到材料的屈服强度时,而外挤失效是一种稳定性失效,发生在套管截面形状的椭圆度达到临界值时;③磨损深度、径厚比和屈服强度是套管抗内压强度的主要影响因素,抗外挤强度除上述因素外还包括磨损宽度和分离角,设计如忽略套管磨损的影响,可能引发井筒完整性失效风险。结论认为,该方法在中国西北部某超深井进行了现场应用,预测精度在90%以上,吻合度较高,为油气勘探开发向深层、超深层发展提供了技术和安全保障。

Casing wear is serious in the drilling process of complex oil and gas wells such as deep wells,ultradeep wells and extended reach wells,which greatly increases the risk of casing failure.Field measurement data show that there are multiple wear areas on the casings in some hole sections and the wear shape presents more complex compound modes than the classical single crescent,which makes it difficult to accurately predict the risk of casing failure by conventional methods.After analyzing the differences between the compound wear mode and the single crescent wear mode,this paper reveals the failure mechanisms of compoundly worn casing under the action of internal and external pressure based on laboratory experiments and numerical simulation.In addition,the main controlling factors and action laws of residual casing strength are figured out.Then,the intelligent prediction model of residual strength of worn casing is established,and the prediction method of failure risk of worn casing is developed.And the following research results are obtained.First,casing wear shape is not only single crescent but also multi-crescent overlapping,crossing and separating.Second,the internal pressure failure of worn casing is a kind of strength failure,which occurs when the equivalent stress in the middle of the casing wall in the wear area reaches the yield strength of the material;while the external pressure failure is a kind of stability failure,which occurs when the ovality of a casing section reaches the critical value.Third,wear depth,diameter-to-thickness ratio and yield strength are the main factors affecting the internal pressure strength of casings,and the main factor affecting the external pressure strength include wear width and separation angle besides the above mentioned factors.If the influence of casing wear is neglected,the risk of wellbore integrity failure may be aroused.In conclusion,this method is applied on site in a certain ultradeep well in the northwestern China,and its prediction accuracy is over 90%with a higher coincidence rate.It provides a technology and safety guarantee for the shift of oil and gas exploration and development to deep and ultradeep strata.