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Acidizing of oil and gas wells is one of the most established and widely used stimulation methods to improve productivity of wells. The process involves pumping acid into a wellbore or geological formation, to dissolve restrictive material or formation rock and enhance the flow-paths or channels for oil or gas to the well-bore. Hydrofluoric acid (HF) in combination with hydrochloric acid (HCl), also known as mud acid, is typically used for dissolution of sandstone formations. Even in the presence of corrosion inhibitors, these highly corrosive acids have a strong influence on the corrosion fatigue performance of pipelines.
For the first time at TWI, a programme of fatigue endurance and fatigue crack growth rate (FCGR) testing was undertaken in order to investigate the effect of an HF-containing proprietary solution on the fatigue performance of C-Mn steel girth welds.
The following objectives were proposed for tests in the acidizing solution containing corrosion inhibitor in order to determine:
Due to the corrosivity and toxicity of the HF-containing test solution, additional precautions were made. All relevant staff and personnel undertook a special training course on safe handling of HF. Special PPE was worn by technical staff (Figure 1), emergency protocols established and a dedicated shower was installed prior to the start of the project.
In corrosive media, the loading frequency is known to influence FCGRs. A series of frequency scanning tests were conducted to determine the saturated loading/plateau frequency where a decrease loading frequency does not further increase the FCGR. FCGR “Paris-law” tests were carried out to determine the enhancement of the FCGR compared to that in air. These tests were conducted on single edge notched bend (SENB) specimens and the direct current potential drop (DCPD) method used to monitor crack length.
Fatigue endurance tests were conducted in both air and the HF-containing environment to produce S-N curves. A test machine equipped with a polyvinylidene fluoride (PVDF) vessel for the environmental tests is shown in Figure 2. Comparison of the fatigue lives in both environments resulted in an EKDF, which was vital for the fatigue design of pipelines subjected to these acidizing environments.
A strong frequency dependence of FCGR was observed in this proprietary HF-containing solution. FCGRs were significantly higher in this medium compared to the data obtained in air. The Paris law constants derived from the testing were useful in engineering critical assessment to determine weld flaw acceptance criteria. Fatigue lives were considerably lower in the acidizing solution and the EKDF obtained was useful for design purposes. The client was very satisfied with the outcomes of the project and provided excellent feedback.
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