Failure Analysis Of Insulated Tubing And Experimental Study On CO₂ Stress Corrosion

In recent years,the failure accidents of insulated tubing have occurred frequently,which not only caused huge economic losses,but also brought environmental pollution.Therefore,the failure of insulated tubing becomes a problem to be solved in the oil and gas field.In this paper,the failure reason of insulated tubing coupling is analyzed by means of metallographic microscope,scanning electron microscope and X-ray diffractometer.The possible failure mechanism is put forward.The CO₂ stress corrosion test and the corrosion fatigue test under different media of the insulated tubing steel are carried out to verify the failure of insulated tubing coupling,and the following conclusions are obtained:（1）The microstructure of failure insulated tubing coupling is tempered sorbite,and the chemical composition and mechanical properties of the material meet the requirements of API5CT standard.The macroscopic fracture morphology,micro-crack and micro-crack fracture surface show the characteristics of stress corrosion,and the corrosion product is FeCO₃.Combined with the working environment of insulated tubing coupling,the coupling is subjected to the expansion force of high temperature and high pressure steam and its own gravity.Under the corrosion of carbon dioxide gas and steam,the cracks originate from the clamp marks and cut marks on the outer surface of the coupling,and the cracks continue to expand.Finally,the coupling fractured at the sealing ring.Therefore,the failure of insulated tubing coupling may be caused by the CO₂ stress corrosion.（2）According to the possible failure mechanism of insulated tubing coupling,CO₂ stress corrosion test is designed.The standard solution of stress corrosion test is 5%NaCl+0.5%CH₃COOH+distilled water.When the CO₂ partial pressure is 0.2 MPa and the temperature is100℃,all C-ring specimens are corroded for 360 hours and they do not fracture.When the stress is greater,the surface corrosion of C-ring is more serious.The corrosion product is FeCO₃,formed by tight grain accumulation,which is the same as the corrosion product of the failed tubing coupling.In order to further simulate the actual working environment of the insulated tubing,the corrosion medium are water vapor and 0.2 MPa CO₂ gas and the temperature is 100℃,all notched C-ring specimens are corroded for 720 hours without fracture.When the stress is greater,the surface corrosion of notched C ring is more serious.The corrosion product is Fe₂O₃ at first,then FeCO₃ is formed on the surface of the sample and this is consistent with the corrosion products of the failed tubing coupling.（3）Corrosion fatigue test is simulated to verify the failure mechanism of insulated tubing coupling.In air medium at room temperature,the fatigue life of the notched specimen is about600 hours,and there is no corrosion product on the surface of the specimen.The crack source is located at the root of the notch,showing multiple sources.The crack source region is characterized by brittle fracture,and the crack propagation zone has both brittle fracture and ductile fracture model,and the instantaneous fracture zone shows ductile fracture feature.In distilled water at room temperature,the fatigue life of the notched specimen is about 274 hours,and the surface layer of the specimen is partially corroded off.The crack source region and crack propagation zone of the fracture surface are covered by dense flake corrosion products,and the morphology of corrosion products is different from that of failed tubing coupling.The instantaneous fracture zone is a ductile fracture mode.In the medium of co-existence of CO₂gas and distilled water at room temperature,the fatigue life of the notched specimen is about474 hours,and the surface of the specimen is corroded to reddish brown.The crack source region shows brittle fracture mode,and the fatigue striations exists in the crack propagation zone.Moreover,the corrosion products are partially covered in the crack source region and the crack propagation zone.The morphology of the corrosion products is muddy and flaky,which is different from the corrosion product of the failed tubing coupling.The instantaneous fault zone is a ductile fracture model.