Longitudinal crack failure analysis of box of S135 tool joint in ultra-deep well

Abstract

Tool joint failures of deep wells and ultra-deep wells not only affect drilling speed, but also spend lots of manpower and financial resources on dealing with these accidents. Therefore, studying causes and mechanisms of tool joint failures are of great significance. Based on longitudinal crack accident of box of the S135 tool joint in an ultra-deep well of Western China, failure analysis has been done from five aspects, which includes material, crack characteristics, thread tolerances, environmental conditions, and service situation by using the failure analysis technology. The results showed that the chemical composition, mechanical properties, metallographic structures of the joint material were in accordance with API SPEC 7 NEQ standards, and the detection results of thread tolerance were in accordance with API SPEC 7 NEQ standards too. However, SEM analysis of fracture showed that the fracture mode of fresh fracture was transgranular fracture, but the fracture characteristic of old fracture was an intergranular fracture and the crack propagation was an intergranular fracture. EDS analysis showed that corrosion products adhering to the fracture surface were the decomposition products of sulfonated mud in the form of sulfide and oxide. The X-ray analysis of corrosion product film showed that the material was corroded by sulfonated mud, then the α-FeO(OH) corrosion was generated. Furthermore, the difference of axial impact energy and lateral impact energy was obvious. The match of strength and toughness was poor and material anisotropy was obvious, which caused stress concentration. The mutual friction among threads led microstructure rheological phase-out, and the corrosion from sulfonated mud induced to the micro-crack initiation. Then that caused longitudinal crack failure of the joint. The alternating forging of joint original material in horizontal and longitudinal is suggested for reducing the degree of anisotropy, and it is necessary to consider the environmental fracture factors when making the decision of material selections. The research results play an active role in the reduction and prevention of drill string failure, and have a certain significance and reference value for using, ordering, and management of drill string in oilfields.