腐蚀与脉动冲击载荷共同作用下超高强度钻具疲劳损伤行为与控制机制研究

At present, drilling string failures occur frequently when drilling ultra-deep slim hole in deeper reservoirs. The first imperative is to study the fatigue failure mechanism for drilling string and develop new high fatigue resistance drilling strings. The fatigue crack initiation process of drill string materials would be observed in situ with meso-scale. The effect of materials inclusion and microstructure as well as medium of external environment on fatigue crack initiation would be studied. Then the fatigue crack growth law due to microstructure, medium, impact frequency and single impact energy would be studied by repeated impact fatigue test method. And the effect of inclusion distribution, micro-alloy, structure and grain size on rotating bending fatigue properties of drilling strings would be also studied. Relation equation between characteristic parameters and fatigue life would be established based on statistics theory, then obtain the modle of micro-mechanism control material fatigue damage. Based on fatigue fracture theory, screw connector structure would be optimized using new design concept, so API standard of screw connection could be breaken. Drilling string fatigue damage mechanism under the circumstance combined by mechanics and chemistry would be revealed, then the drill string material evaluating system would be further completed, forming a new designing method for screw connection, and thus providing theoretical reference for the development and application of ultra-high strength fatigue resistance drilling strings.

当前深部油气藏超深井小井眼钻井过程中钻具失效事故频繁，弄清钻具疲劳失效机理、开发新型高抗疲劳钻具是增加油气井"延伸极限"的当务之急。本项目拟在介观尺度下观测高强度钻具材料疲劳裂纹的萌生特征，研究材料夹杂物、组织及介质对疲劳裂纹萌生的影响规律；采用多冲疲劳试验方法研究微观组织、介质、冲击频率与单次冲击能量对材料疲劳裂纹扩展的影响规律；研究夹杂物分布、微合金、组织及粒度对旋转弯曲疲劳性能的影响规律，采用统计学原理建立材料特性参数与疲劳寿命的关系方程，得出控制材料疲劳损伤的微观机制；研究螺纹缺口效应对钻具结构疲劳寿命的影响规律，依据疲劳断裂理论，采用新的设计理念对钻具螺纹接头进行优化设计，打破API钻具螺纹标准的束缚，设计出新型螺纹结构方案。揭示力学和化学共同作用下钻具疲劳损伤的机理，完善现有钻具材料评价指标体系，形成新型螺纹设计方法，为新一代超高强度高抗疲劳钻具的研制和使用提供理论依据。

当前，钻井作业的难度越来越大，钻具服役工况也越来越苛刻，钻井过程中钻具失效事故也愈发频繁，给油田的勘探开发造成巨大的经济损失。防止钻具过早疲劳失效的关键在于弄清材料微观组织、服役环境和载荷对高强度钻具疲劳裂纹萌生和扩展的影响规律，解决钢种选用、材料学设计和结构优化设计问题。.本项目主要从材料疲劳、环境因素、结构优化等方面的研究了超高强度钻具疲劳损伤行为与控制机制，主要内容包括：（1）在介观尺度下观察了高强度钻具材料疲劳裂纹的萌生、扩展直至断裂的过程，研究了夹杂物、组织、腐蚀介质对疲劳裂纹萌生的影响规律；（2）通过模拟钻井液腐蚀环境，采用动态疲劳试验方法研究了冲击能量和冲击频率、环境因素、材质因素对钻具疲劳裂纹扩展的影响规律；（3）利用旋转弯曲疲劳试验方法对超高强度钻具的疲劳性能进行测试，采用统计学原理建立了高强度钻具材料疲劳寿命的概率分布模型，并利用优选出的概率分布模型对材料的疲劳寿命进行了定量评估，建立了高强度钻具材料特性参数与疲劳寿命的关系方程，研究了夹杂物、氢损伤、腐蚀介质、频率、缺口和温度对疲劳寿命的影响规律和作用机制；（4）研究了螺纹缺口效应对钻具结构疲劳寿命的影响规律，依据疲劳断裂理论和有限元优化设计方法对钻具螺纹接头进行了优化设计，设计出新型螺纹结构方案，并得到工程应用。项目揭示了力学和化学共同作用下钻具疲劳损伤的机理，完善了现有钻具材料评价指标体系，形成了新型螺纹设计方法，为新一代超高强度高抗疲劳钻具的研制和使用提供了理论依据。

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