储层分段体积改造下套管接头强度损失预测方法

Study on Predication Method for Strength Loss of Casing Joint in Segmented Volume Fracturing of Reservoir

非常规油气开发的主体技术是水平井+储层分段体积改造,井下服役套管因承受周期性的循环内压、轴向载荷及地层压力,多井段、长时间改造作业加剧了套管接头的失效风险及安全隐患。针对储层分段体积改造过程套管接头滑脱掉井等失效问题,考虑动态载荷工况下材料的低周疲劳损伤特性,通过有限元数值模拟研究体积改造工况下套管接头的强度损失。结果表明:体积改造过程直井段套管接头的最大等效应力843.0 MPa超过了材料的屈服强度825.6 MPa,接头螺纹发生塑性变形。经6级、9级、15级、20级、30级改造后,接头强度降低了7.5%、10.0%、14.6%、18.4%及25.9%。室内全尺寸实物实验验证了接头强度损失预测方法的准确性。据此提出了现场压裂施工井下套管柱安全服役的极限内压及允许的井口压力范围,并成功应用在新疆油田玛湖作业区。

The key technology to the exploration and development of unconventional oil and gas resources includes drilling of horizontal wells and their multi-stage volume fracturing.In the process,downhole casing string undergoes the combination loads of cyclic internal pressure,axial tensile and formation external pressure,which aggravates the failure risks of casing joints.Aiming at the failure of casing joints in the form of slipping and dropping,the strength loss of casing joints during volume fracturing stimulation was studied using finite element simulation,based on fatigue damage characteristic of material under dynamic loads.The research result indicates that the maximum equivalent stress of casing joints reached to 843.0 MPa in direct well section,which exceeds the yield strength of 825.6 MPa,and then leads to the plastic deformation of joint thread.The strength of casing joints is reduced continuously by 7.5%,10.0%,14.6%,18.4%and 25.9%respectively after 6-stage,9-stage,15-stage,20-stage and 30-stage fracturing.The accuracy of the joint strength loss prediction method was verified through full-scale indoor experiments.The limit internal pressure and allowable wellhead pressure range for the safe service of downhole casing strings during on-site fracturing construction have been proposed and are successfully applied in the Mahu operation area of Xinjiang Oilfield.