The high productivity of pre-salt exploratory wells led to a re-evaluation of Petrobras' model of offshore oil production and exploration. We present here the study that allowed the conversion of exploratory wells into producers, based on the corrosion resistance of carbon steel in different scenarios of pre-salt field production. The aim of the test program was to evaluate the influence of the produced water content (BSW), CO2 partial pressure and oil characteristics on the carbon steel corrosion resistance. General corrosion analysis showed that for brine content below 40% corrosion rates ca. 0.05 mm/year for 28-day tests. Higher CO2 partial pressure did not lead to increased corrosiveness, unlike shear stress. There was localized corrosion in most of the evaluated test conditions, confirming the importance of using laboratory techniques that allow such analysis. Localized as well as generalized corrosion kinetics decrease over time, confirming that laboratory tests, even when replicating field conditions, are more conservative as the operation time of the production columns will be extremely longer than laboratory test times. Based on results of this test program it was possible to extend the operation of exploratory wells adopting as safety criteria for well abandonment the produced water content (i.e. BS&W content) and additionally monitoring the internal pressure of production tubing.
The production tubing of PETROBRAS exploratory wells is usually completed in carbon steel or low alloy steel as they operate for a short period, unlike columns in the production wells, which are completed with CRAs and designed for approximately 30 years. Due to high productivity of the pre-salt exploratory wells the idea of converting them into producing wells was considered, however it was necessary to establish safety criteria and operating limits since carbon steel is susceptible to CO2 corrosion on referred field conditions.
It is known that the water content in the oil directly influences the corrosiveness of the medium, but it is complex to estimate, based on existing knowledge, the corrosion rate only as a function of BSW (percentage of water and sediment), since each oil may have different characteristics, which are not incorporated in corrosion rate simulation softwares1-3.
