Effect of Sour Acidizing Treatments on the Fatigue Crack Growth and Fracture Toughness Behavior of C-Mn Line Pipe Steels

Abstract

Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion. In the presence of hydrogen sulfide (H2S), these acidizing treatments could cause environmentally assisted fatigue and fracture (i.e. increased fatigue crack growth rates and reduced fracture toughness). A test program is underway to evaluate and quantify the effect of sour acidizing treatments on the fatigue and fracture behavior of welded C-Mn line pipe steels. This paper describes the preliminary findings from fatigue crack growth rate (FCGR) and fracture toughness (FT) tests on as-welded (i.e. unstrained) pipe. All tests were conducted at room temperature (RT) using compact tension (CT) specimens notched in the parent pipe (PP). Frequency scan FCGR tests were performed in the following sour acid conditions: simulated production environment (PE), spent acid without inhibitor and spent acid with residual corrosion inhibitor. The PE consisted of a simulated brine with pH = 4.5 and partial pressure of H2S (pH2S) = 0.21psia. FCGRs in the sour PE were of the order of 20 times faster than in air. The pH2S was the same for the tests in spent acid environments, but the pH was lower (approximately 3.5). As would be expected, the FCGRs were much higher in the low pH environment. The highest FCGRs were observed in the inhibited sour spent acid environment and were up to 100 times faster than in air. Sour FT tests were also conducted in the PE and in spent acid with and without inhibitor. In all cases, the measured FT values were significantly lower than in air. The test in PE exhibited higher FT than in the sour acidizing environment. The lowest FT values were observed in spent acid with inhibitor. Future work will investigate the effect of reeling on the fatigue and FT performance of pipe girth welds in sour acidizing environments.

Environmentally Assisted Cracking of Subsea Pipelines in Oil & Gas Production Environments—Effect of Static Loading

“…The sample exhibited a stable crack growth rate of 1.16 × 10 −7 mm/s, as seen in Figure 6. In a similar environment the measured K th from rising displacement associated with J th (point of first crack extension) and J 0.2 mm were 72 MPa p m and 90 MPa p m to 100 MPa p m. 18 This suggests that the results from the rising displacement tests in this environment may not be a conservative estimate of performance. The data in Figure 6, corresponds to the constant K segment at 44 MPa p m. There was a power interruption in the test, after which the test was restarted under constant K conditions.…”