Corrosion Mechanism And Prediction In Gas Well Containing High CO₂

In the process of oil and gas exploration, carbon steel has been widely used because of its good economy and mechanical properties. Nowadays carbon steels, especially N80and P110, are accepted in downhole for low cost. While, on the contrary, CO2corrosion gets more severe with the increasing number of oil and gas fields. So giving more attention to oil casing corrosion mechanism is very significant.This paper focused on the corrosion behavior of comman N80and P110oil casing steels. The high temperature and high pressure autoclave and in-situ electrochemical autoclave were utilized for corrosion simulation in CO2environment. The influence of temperature, CO2partial pressure and flow velocity was analyzed by weight loss and modern surface analysis methods (SEM and XRD). The localized mechanism and corrosion prediction model of oil casing steels in CO2environment were put forward. Also galvanic corrosion was studied. The main conclusions are shown as follows:(1)The corrosion rate increases with CO2partial pressure and flow velocity, while decreases with temperature increasing.(2)The corrosion products of oil casing steels change with temperature. the main corrosion product(s) was/were Fe3C in room temperature, FeCO3in medium temperature, FeCO3å'ŒFe3O4in medium and high temperature, FeCO3and calcium magnesium double salt in high temperature.(3)Pitting corrosion can gradually developed to mesa attack. The initiation of mesa attack was a series of pits caused by the localized broken down of corrosion product. In static state, metal matrix at the bottom of pits was protect by the corrosion products, while the edge of pits was still naked in bulk solution, which made the high corrosion rate in crosswise direction. In dynamic state, scouring and corrosion have a synergistic effect, which may cause severe mesa attack.(4)By analyzing the main controlling factors of high CO2content oil and gas fileds and experimental data, the corrosion prediction model and sensitive well depth prediction model were put forward.(5)The galvanic couple area ratio and corrosion film situation can accelerate or inhibit corrosion rates.