Brittle fracture of titanium alloy tubing in ultra-deep sour gas well: Root cause, mechanism and countermeasures

Owing to its outstanding properties including high specific strength, excellent corrosion resistance, and remarkable mechanical properties, titanium alloy has emerged as a promising alternative for carbon steel tubing in ultra-deep and highly corrosive environments. However, an unprecedented brittle fracture occurred during the first large-scale field deployment of 567 LH02-grade titanium alloy tubings in a 5515 m high-sulphur gas well in Southwest China. To elucidate the root causes, we conducted a comprehensive investigation that integrated macroscopic and dimensional inspection, non-destructive testing, physicochemical analysis, SEM/EDS micro-characterization, full-scale tensile tests, and finite element analysis (FEA) under both factory hydrostatic conditions and field environments. The key findings indicate that improper heat treatment resulted in a coarse-grained Widmanstätten microstructure and, more notably, the formation of a 0.5mm-thick oxygen-rich α-layer on both the inner and outer surfaces. This layer exhibited significantly increased hardness and brittleness. Micro-cracks initiated simultaneously at multiple locations within this α-layer and propagated rapidly through the wall due to reduced fracture toughness, ultimately leading to catastrophic brittle failure without any observable macroscopic necking. The hydrogen content in the failed tube was nearly four times greater than that of the intact tube, thereby further compromising ductility. In response to these findings, targeted process improvements have been implemented—these include stringent hydrogen content control measures, limitations on furnace loading (≤5 tubes per batch), in-situ temperature monitoring protocols, mandatory inspections of the α-layer, and an elevated Charpy impact requirement—to ensure long-term reliability in ultra-demanding sour-gas environments. • The first large-scale application of 567LH02-grade titanium alloy tubing in a 5515m ultra-deep high-sulfur gas well in China encountered unprecedented brittle fracture. • Improper heat treatment is identified as the root cause, characterized by a coarse-grained microstructure, a 0.5mm-thick oxygen-rich brittle α-layer on both inner and outer surfaces, and a hydrogen content approximately four times that of intact tubings. • Microcracks initiate in the α-layer, with axial tension dominating the rapid penetration of cracks through the wall thickness, leading to brittle fracture with nearly no macroscopic necking.