Reducing Conservatism on Corrosion Prediction to Oil Production and Transportation Based on Laboratory Test and Data Analysis Considering P_H2S, P_CO2, pH, Shear Stress and BSW

The present work intends to build up the knowledge on oil effect on carbon steel corrosion rates by evaluating the results of Rotating Cage and Submerged Impingement Jet immersion tests carried at moderate pressures 2.5 MPa, 25 to 50 °C, shear stress from 25 to 175 Pa, in brines containing CO₂, H₂S and CH₄ with two levels of water cut: 20 and 60%. The results indicate temperature effect is stronger on BSW 60% and that shear stress had no influence on CR. On environments with 0.001 MPa of H₂S Partial pressure at 50 °C the CR were below 0.05 mm/y whereas without H₂S CR were 4x higher. No significant effect of H₂S was observed at 25 °C. Although, without H₂S, temperature and CO₂ partial pressure had stronger influence on carbon steel CR. To water cut of 20% changes on temperature, pH, P_H2S, P_CO2 and acetic acid concentration, had minimum effect on carbon steel corrosion rate. A 40x smaller CR was observed to BSW 20% compared with 100% brine. The water content must be taken in account to improve corrosion prediction and also associated effects, such H₂S contribution on CR decrease.

The importance of models for predicting carbon steel corrosion in the oil and gas production chain is undeniable. However, researchers are aware of the difficulty in extending the application range of existing data, to build a generic model. This is because the chemical and electrochemical reactions that impact the corrosion kinetics are affected by the characteristics of each process, such as: salinity, type of oil, oil/water ratio, pH, presence of organic acids. Furthermore, as if it were not enough work to run a test matrix that contemplates each of these variables, the interaction between them still has to be considered.

A very illustrative example of this association of factors is the impact of H₂S on carbon steel corrosiveness. Despite the harmful consequences of H₂S, such as H₂S embrittlement, experiments have shown that up to a given concentration its presence was beneficial, reducing corrosion rates of carbon steel and not inducing Hydrogen Induced Cracking, HIC^1-3.