Investigation On The Corrosion Behavior And Corrosion Inhibition Of Inhibitor On N80 Steel In CaCl₂ Killing Fluid

During workover operations in oil and gas wells, high density brine solution was commonly used to prevent formation fluid from coming into the well bore by controlling and maintaining the formation pressure. However, the killing fluids usually tend to exhibit strong corrosive effect under high temperature and high pressure in well bottom. It was important that the corrosion behavior and inhibition of well bore in CaCl2 killing fluid to be studied. Corrosion inhibitor was one of the effective ways to mitigate and reduce corrosion failures. In this paper, the corrosion behaviors of N80 carbon steel in CaCl2 killing fluid was studied by weight loss method and electrochemical measurement. Besides, the corrosion inhibition and mechanism of thioureido-imidazoline (TIM) and ternary inhibitor on N80 carbon steel was researched by the methods of weight loss method, electrochemical measurement, scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR)and X-ray photoelectron spectroscopy (XPS). The main conclusions are as follows:1. The corrosion behavior of N80 steel in CaCl2 killing fluid was investigated by weight loss, electrochemical method. The effects of temperature and density of CaCl2 on corrosion rate was analyzed. The obtained results showed that the corrosion rate of N80 was increased with increase in temperature and decreased with increase in density. The main corrosion products of N80 in CaCl2 killing fluid were Fe3Cl4 and FeOOH.2. This research aimed to study the corrosion inhibition and corrosion mechanism of thioureido-imidazoline on N80 steel in CaCl2 killing fluid. The gravimetric result showed that 81.3% inhibition efficiency was achieved after adding 5000mg/L TIM. Potentiodynamic polarzation studies revealed that TIM functions as a mixed type corrosion inhibitor which mainly inhibits the anodic metal dissolution reaction. AC impedance spectra revealed that a protective film was formed on the metal surface, and the corrosion efficiency increased at first, then tend to be stable with the immersion time. And some surface characterization techniques such as [X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to ascertain that a protective film formed on the N80 steel surface. A plausible mechanism of corrosion inhibition has been proposed.3. A protective film has been formed on the surface of N80 steel in solution at 90? using a ternary inhibitor which consists of NaGL, Zn(H2PO4)2 and MEA. Results of weight loss method indicated that 95% inhibition efficiency was obtained when using this ternary inhibitor. Polarization studies showed that inhibition efficiency increased with increasing inhibitor concentration, and the ternary inhibitor acts as a typical anodic corrosion inhibitor by shifting the potential more positive. AC impedance spectra revealed that a protective film was formed on the metal surface. The XPS spectra inferred that the protective film showed the presence of the elements iron, zinc, carbon, oxygen, nitrogen, phosphorus. The spectra of these elements in the surface film showed that each components could be adsorbed on the metal surface. Further, surface characterization techniques such as atomic force microscopy, scanning electron microscopy were used to analyse the nature of the protective film. Based on the results obtained, a suitable mechanism of corrosion inhibition is presented.