Study Of Structure And Performance Of CO₂ Corrosion Film At High Temperature Pressure In The Exploitation Of Oil And Gas

The steel corrosion of carbon dioxide has caused huge loss in petroleum and natural gas industry world widely. The structure and performance of CO₂ corrosion film are studied in simulating corrosion environment of oil and gas field. We hope this research could give some help on theory and experiment to CO₂ corrosion and its protection in the exploitation of oil and gas.In this experimental research, 1/2 cubage of the high temperature pressure kettle is full of liquid corrosion medium and the other is CO₂. The samples were installed and exposed separately to the liquid phase and gas phase.Based on the analyses of CO₂ possible phase change and interaction among of oil\gas\water in deep reservoirs and the pressure funnel in oil and gas well pipe , in the static high temperature pressure kettle, the acts those effect corrosion including oil content/pressure were tested on API common pipe steel N80\P110\J55\ 13Cr, the corrosion film was observed by SEM, the formation mechanism of corrosion film was studied such as growth process. At the conditions of pressure 7¹8Mpa\temperature 90℃\pure salt water which simulate produced water of an oil well as liquid phase \CO₂ as gas phase, the results indicated that the corrosion rate was accelerated with increasing pressure on gas phase steel samples and the peak value was at 12 MPa then declined. The corrosion rate was accelerated with increasing pressure on liquid phase steel samples and the peak value was at 9 MPa then declined. When pressure got to be 12Mpa, the corrosion rate on steel samples exposed to liquid phase went accord with that in the gas phase. The SEM image of the surface of corrosion film exhibited lots of tubercules.At the conditions of pressure 12Mpa\temperature 90℃\pure salt water add crude oil as liquid phase\ CO₂ as gas phase, the results indicated that the corrosion rate was accelerated at 30% content crude oil in liquid phase steel samples and reduced in gas phase. The SEM image of the surface of corrosion film exhibited lots of tubercules and poles together. At 70% and 90% right round oil content, the corrosion rate fell tothe lowest value and it became flat, the corrosion rate on steel samples exposed to liquid phase went accord with that in the gas phase. The growth step can be observed and the thickness of the film incrassated with time prolonging. There were some regular correspondingly crystal which is isomorphic on the corrosion film equably. It comprised of FeCC>3 mainly and a small quantities of CaCC>3, the shape of crystal conformed with calcite. The corrosion products films of the casings formed in the high temperature and high pressure conditions are consisted of many layers which indicated there is deferent strength in deferent film during the production growth.The corrosion experiments of pipe steel N80\P110\J55\X56 were conducted in a dynamic high temperature and pressure autoclave to investigate the corrosion rate\ the morphology\ structure and composition\ and factors which can effect the formation of corrosion products film by SEM\ XRD\ XPS etc. at the simulation environment of petroleum and natural gas exploitation and transportation. The mechanism of CO: corrosion was studied in this paper.At the conditions of pressure 1.5 ³5Mpa\temperature 160°C\pure salt water as liquid phase\ CO2 as gas phase, the results indicated that the corrosion rate was accelerated in liquid phase N8O\P11O\J55 steel samples with increasing of pressure firstly and then reduced, which can be described as quadratic equation relationship. At 1.5Mpa\ 6Mpa\36Mpa, the crystal grain of corrosion product film was compact and integrity, which combined fast. At 10Mpa\ 18Mpa\ 24Mpa, the crystal grain of corrosion product film was pettiness, which combined loosen. The corrosion rate was acceleratedAt the conditions of pressure 0.5 ²Mpa\temperature 30⁸0°C\pure salt water as liquid phase\ CO2 as gas phase, the results indicated that the corrosion rate was accelerated in liquid phase X56 steel samples with increasing of pressure which can be described as linear equation relationship, the corrosion rate was accelerated with increasing of temperature which can be described as exponential equation relationship. Investigating the morphology\ structure and composition of corrosion products film by SEM\ XRD\ XPS etc, the results indicated that the corrosion films mainly comprised of FeCC>3 and complex phase products (FeCa.. .)CC>3. There exists mesa erosion and cavity corrosion on the face of corrosion film. The film includes three layers .At the conditions of pressure 10Mpa\pure salt water as liquid phase\ CO2 as gas phase, the electrochemical tests are performed for steel N80 with CO2 corrosion film which was formed in deferent time and at deferent temperature. The results indicatedthat the Icorr of steel after 1 day formation film was the biggest and Icorr of steel after 2 day formation film was cline, but Icorr of steel after 3 day formation film was increasing then Icorr of steel after 4 day formation film was the smallest. With the temperature of formation, the protection for the steel become stronger and stronger, ICorr of steel with film formatting at 120°C was the lowest.At the conditions of pressure 0.5²Mpa\temperature 80°C, the electrochemical tests are performed for steel N80> the corrosion rate and structure\performance integrate the mechanism of X56 CO2 corrosion. The cathode reaction was controlled by dissect of hydrogen process and the anticathode reaction was controlled by formation speed of Fe2+. The CO2 promote the anticathode dissolution and reaction. The EIS of X56 steel indicated that it occurs half illimitably diffuseness Warburg impedance character in low frequency and content impedance in high frequency because of the presence of corrosion film. The whole reaction is controlled by diffuseness process.