Summary for casing and tubing failure modes

Casing and tubing is an important material and equipment in the petroleum industry and it has a great amount of use. Casing pipe and pipe string failure will lead to the entire pipe string can not work normally and you will see a lot of casing and tubing corrosion in the construction site. Based on this, the applicability evaluation of casing and tubing with suspected defects is proposed, not only you can make a judgment on the quality of the tubing, but also you can regulate the tubing which doesn’t meet the standards.

The working condition of casing and tubing is relatively poor, and the stress state is complex, so the failure mode is varied, and the failure analysis is summed up in the following 3 types:

Deformation failure

In practical applications, due to the radius of casing and tubing is much smaller than that of the license, instantaneous plastic bending deformation is usually occurred in casing and tubing. It will cause permanent bending of casing and tubing (commonly known as dead bending) in a long time.

In addition, flexible deformation beyond the tensile limit, exceeding the limit of deflated or compressed phenomenon are all part of the deformation failure.

Fracture failure

Fracture failure in the casing and tubing is a large proportion of failure fracture, especially the fatigue fracture that damage is more serious. The main fracture forms are:

• Overload fracture   The speed of pipeline into the wells is too fast, resulting in its snap off when encountered downhole plug or during the job the underground fallen objects lead to pull off and so on.
• Low stress brittle fracture   Brittle fracture of the casing and tubing weld.
• Stress corrosion fracture   Sulfide stress corrosion cracking can be caused when working in oil and gas wells with high sulfur content. The characteristics of the stress corrosion fracture usually have 3 areas, fault source area, crack propagation region, fast break or tearing zone.
• Hydrogen embrittlement   When the tubing material contains too much hydrogen, it’s easy to produce hydrogen embrittlement in tensile force. The characteristics of hydrogen embrittlement fracture are in the fracture edge can be observed in white or white light ring.
• Fatigue fracture and corrosion fatigue fracture   Since the coiled tubing have to constantly import or winding roller, it will be subjected to cyclic stress, and the well often has a corrosive medium erosion. Therefore, the working condition of the coiled tubing subjected to fatigue or corrosion fatigue is the most.

Surface damage failure

Surface damage mainly includes 3 forms:

• Corrosion   Including uniform corrosion (such as in a long time storage process leading to corrosion), pitting corrosion (such as the internal and external appearance pitting in the process of using or storing) and crevice corrosion (such as the corrosion between oil pipe welds and materials).
• Wear   The wear between the downhole tubing and casing or the wear between oil pipe and grip system for injection head and so on.
• Mechanical damage   Casing and tubing in transit surface bumps; The scratch of the wax scraper which falls into the well in the paraffin removal operation; In the injection process because of holding tight, left on the surface indentation.

Measures

Most of the tube failure accident occurred in the transition zone of the thickened transition, which accounted for 70%. The reason of failure is that the structure is not reasonable, resulting in stress concentration and corrosion fatigue. Central tube failure accident accounted for 30%. The cause of failure is the interaction of wall thickness thinning caused by abrasive wear and corrosion pit.

Proposed to strengthen the inspection of the pipe body before welding and deal with the serious internal corrosion to ensure the quality of the tube;
Ultrasonic inspection is carried out around the thickening transition zone of the oil pipe to check whether there is any defect in the transition zone. Length of transition zone and arc radius of oil pipe should be checked before welding, where thickened inner transition zone <100mm, R <300mm are not to be welded;
Strengthen the management of drilling fluid, the pH value of drilling fluid should be maintained at 10 or more than 10 under the condition permission.

Conclusion

With the increasing demand of the drilling industry, the oil pipe is easy to damage under the long-term high temperature and high pressure and unexpected accidents are common occurrence. So it is urgent and important to analyze the failure of tubing. Timely measures can improve the utilization of resources, avoid economic losses, and bring greater social and economic benefits.