Casing Failure Mechanism Analysis of Well L3 in L Gas Storage in Daqing Oilfield

For the early drilling completion time of L gas storage wells in Daqing and the material selection of casing without considering the long-term anticorrosion demand, the corrosion failure of casing after many years of service caused surface gas leakage. In order to clarify the corrosion failure mechanism of casing, the fracture of casing thread was analyzed by macroscopic and microscopic morphology observation, physical and chemical properties, energy spectrum and XRD detection after sampling. The results show that the crack of casing thread is stepped along the circumference. The mechanical properties and chemical composition near the fracture were in line with the relevant standards, and the metallographic structure was not abnormal. The corrosion products near the fracture contain ferrous sulfide, and the fracture morphology shows that the fracture source region has typical brittle fracture characteristics of intergranular leavage. Combined with the analysis of casing service condition, it is considered that the cracking of casing thread is the stress corrosion cracking caused by the contact between hydrogen sulfide gas inside the casing and the water outside the casing at the gap between the top and bottom of the thread teeth after the tightness of the thread decreases, which is caused by corrosion environment and axial tensile load. At the same time, the paper puts forward some suggestions on corrosion protection measures for L gas storage well, which can guarantee long-term production safety of L gas storage well.