浅造斜点与钻具疲劳累积失效的关系及启示

通常定向井深度在3 000~5 000m的设计造斜点为200~500m,而国内不同地区已发生多起因浅造斜点施工作业一定时间后出现频繁刺漏、断钻具等失效问题。为此,通过对几起案例的共性分析,以及进一步从材料力学和钻具疲劳的角度开展理论研究,得出了浅造斜点与钻具累积疲劳极限的关系及结论,主要包括:1发生严重的钻具频繁刺漏、钻具断裂的主要原因是钻具疲劳损伤累积;2造斜点越浅,钻杆疲劳所需的最低弯曲应力幅越小,较小的狗腿度即可达到损伤累积极限,井越深越危险;3钻杆镦粗加厚带及其附近位置为现代钻具生产的薄弱区,是最主要的应力集中点,成为最易发生刺漏、断裂的位置。最后,提出了设计、现场施工作业的预防措施:1若非特殊设计目的施工需要,钻井设计应尽可能避免设计浅造斜点的中深定向井;2针对浅造斜点井设计时,应根据钻具疲劳损伤累积法计算出最大许可狗腿度,从源头上避免钻具进入损伤累积极限区;3施工中应对钻具疲劳寿命做出有效预测,进行更严格的管理,对钻具做出及时检验和分级。

Relationship between the designed shallow kick-off points and the fatigue accumulative failure of drilling tools and its enlightment

Usually, the designed kick off points (KOPs) range from 200 to 500 m for a directional well with a depth of 3 000 – 5 000 m. However, washout or broken failures of drilling tools frequently occur at many wells with shallow KOPs when those wells are drilled after some time. Therefore, through an analysis on the common characters of several cases, and based on the theoretical research of material mechanics and the fatigue of drilling tools, the relationship between the shallow KOPs and the fatigue accumulative limit of drilling tools was concluded as follows. (1) The cumulative fatigue and damage of drilling tools is the main reason. (2) The shallower the KOP, the less the minimum bending stress required by the fatigue of drilling tools; relatively small dogleg severity (an overall angle change rate) may reach the damage accumulation limit, so the deeper the well is, the more dangerous it will be. (3) The drill pipe upset thickening belt and the places around it are weak areas in the modern production of drilling tools and they are also the stress concentration points which will easily cause leakage and breakage. Finally, measures for design and site construction operation were proposed in this study. (1) Unless required by a special design purpose, the well drilling design should avoid such medium deep directional wells with shallow KOPs. (2) When designing a shallow KOP well, the maximum allowable dogleg severity should be calculated according to the drilling tool fatigue damage accumulative method to prevent the drilling tools from entering into the damage accumulation limit area. (3) Not only should the fatigue life of drilling tools be predicted effectively, but stricter management should be conducted, and the drilling tools should be inspected and classified in time.