For several years, coiled tubing offshore operations in Brazil have involved the use of nitrogen generation units. Remote locations, lack of liquid nitrogen suppliers and logistical issues, among others, have driven the main oil operator to increase the use of Nitrogen Generation Units.
Using a Nitrogen Generation Unit in remote locations, where most of the platforms have very limited space, was the ideal solution for a very old problem of supplying nitrogen. However, when the use of the Nitrogen Generation Unit was combined with coiled tubing operations, another concern arose.
In-situ nitrogen generation does not generate pure (99.9%) nitrogen gas. In-situ nitrogen generation can generate nitrogen gas compositions with oxygen contents as high as 5%. Nitrogen with oxygen contents as high as 5%, under downhole conditions of pressure and temperature, can result in severe corrosion problems in the coiled tubing string. Furthermore, together with the increased oxygen content, fluid systems such as completion brines and acids were pumped, which also increased the severity of the corrosive environment. Sometimes operations were stopped because the coiled tubing Bottom Hole Assembly (BHA) was completely plugged by oxidized/corroded metal solid flakes or the coiled tubing string became so corroded that it required inspection before further use in job operations. Based on these corrosion and safety issues the use of in-situ nitrogen generation loses its benefits.
To find a solution to this corrosion problem, a laboratory research study was performed with the most commonly used coiled tubing string material and the pumped treatment fluids under surface and downhole conditions with the maximum percentage of oxygen concentration that could be expected in operations. Several corrosion inhibitors and mixtures were tested for their ability to control the corrosion of the coiled tubing string.
After laboratory testing established which inhibitor systems would control the corrosion rate of the coiled tubing, yard tests were performed to confirm that the inhibitor would work in the field. After the field trial was performed successfully with several applications, the recommended practices were adopted from this research to avoid the corrosion problem faced in the past with the use of offshore nitrogen generation units.
This paper presents the sequence of events of this project: the coiled tubing corrosion observed in the laboratory research studies, the performed yard trials; the recommended field applications, as well as the benefits that this study and final application brought to the operations.