Investigation On Stress Corrosion Cracking Of L80-13Cr Stainless Steel In Brine With High CO₂ Concentration

The oil recovery is increasing with the development of CO₂ flooding technology,but the risk of stress corrosion cracking(SCC)is also increasing,which caused by the high pressure or even supercritical CO₂.Because L80-13Cr stainless steel has the excellent CO₂corrosion resistance,it is widely used in downhole pipe string equipment in high CO₂ oil and gas fields.However,the SCC mechanism and detection methods of L80-13Cr in the high CO₂ downhole environment are still not clear.In this paper,the SCC germination and development process of L80-13Cr steel in the downhole environment were studied by slow strain rate tensile(SSRT),electrochemical noise(ECN),acoustic emission(AE)and environmental scanning electron microscopy(ESEM).The main conclusions are as follows:(1)Under the normal pressure environment,by studying the SCC behavior of L80-13Cr stainless steel in NaCl solution,it was found that:with the decrease of p H value and the increase of temperature and NaCl concentratin,the SCC sensitivity of the L80-13Cr is increasing.The fracture mode is mainly changed from ductile fracture to brittle fracture,with obvious brittle cleavage surface.The SCC mechanism is mainly anodic dissolution type.(2)In the high-pressure CO₂ environment,the temperature range from 30??150?and the CO₂ pressure range from 0 to 7.5 MPa,NaCl concentration range from 3.5%to 20.0%.With the increase of temperature and CO₂ partial pressure,the SCC sensitivity of L80-13Cr first increases and then decreases,and the river-like cleavage surface appears in the fracture.With the increase of salinity,the SCC sensitivity of L80-13Cr steel first decreases and then increases.When the salinity is higher than 10%,significant brittle fracture occurs in the fracture,and the crack propagation mode is a mixture of transgranular and intergranular.The mechanical properties of L80-13Cr are significantly deteriorated.The SCC mechanism is a mixture of anodic dissolution and hydrogen induced cracking.(3)ECN and AE were used to monitor the in situ SCC of L80-13Cr steel,combined with polarization curve and electrochemical impedance spectroscopy.It is found that different types of potential-current noise peaks are related to different corrosion events.The noise peak with short life and low amplitude corresponds to the formation of metastable pitting,while the noise peak with long life and high amplitude indicates steady pitting or crack initiation.In addition,there are significant differences in the frequency and count of acoustic emission in different stages of SCC.In the elastic strain region,many signals with short duration and low count value appear,which are related to the metastable pits.In the fracture period,the high-duration and high-count signals were detected,which are related to the growth process of SCC cracks.The non-destructive monitoring technology based on ECN or AE could be used to monitor the SCC process of L80-13Cr steel in real time,which is expected to provide early warning for SCC risk of the downhole string and reduce the incidence of downhole accidents.