Study On The CO₂ Corrosion Of The Gathering Pipeline For Gas In Ya Kela-Da Gas Field

Based on the failure analysis of gathering pipeline rupture elbows of Ya kela-Da gas field, corrosion tests were carried out to simulate oil and gas field environment in order to compare corrosion rates of different pipe materials. The polarization curves and impedance spectroscopy were also tested to analyze corrosion mechanisms by means of eletrochemical methods. In the end, reference data is provided for materials selection and corrosion prediction of gathering pipeline in oil and gas fields through comprehensive analysis.Based on analysis of the running condition of gathering pipeline of Ya kela-Da gas field, failure gathering pipeline elbows of oil and gas fields were analyzed through tests of macroscopic feature, metallographic structure, chemical compositions and X-ray diffraction. Corrosion failure analysis tests indicated that the material of gathering pipeline rupture elbows was 16Mn/Q345 steel, whose chemical compositions accord with the national standard. Through analyzing the running condition of gathering pipeline, it was found that there was condensate water at the gathering pipeline elbows, which dissolved CO2 in the oil and gas and generated carbonic acid as a result of the corrosion of the gathering pipeline. Cl-, HCO3-, Ca2+ and Mg2+ contained in the oil field water resulted in local corrosion of pipeline, and the scouring action with the reason of fluid flow in the gathering pipeline aggravated the corrosion of the pipeline. Comprehensive analysis indicated that though there were many factors leading to the corrosion of gathering pipeline of Ya kela-Da gas field, at present the most severe damage was not uniform corrosion caused by CO2 and other media, but CO2 local corrosion aggravated and pitting corrosion at the elbows of the pipeline caused by the change of media flow pattern and erosion corrosion.By use of high temperature and pressure reactor, the running condition of oil and gas field's gathering pipeline was simulated, and the corrosion rates of common gathering pipeline, such as 16Mn/Q345, 13Cr, 20 steel, and X60 pipeline steel were tested.Eletrochemical methods were used to test polarization curves and impedance spectroscopy of pipes, discuss the corrosion mechanism. The results indicated that: (1) The corrosion tendency of 16Mn/Q345 steel was higher than that of other materials, because 16Mn/Q345 steel corrosion electrical was the highest; (2) 13Cr steel was not basically corroded, 16Mn/Q345 and 20 steel were corroded moderately, and X60 was corroded severely. The corrosion rates of 16Mn/Q345 and 20 steel were much lower than that of X60, and 13Cr had the excellent CO2 corrosion resistance. (3)16Mn/Q345 steel could not generate protective corrosion film in the range of 20 to 60℃, and its corrosion rate was determined by both the carbonic acid generation rate through hydrolyzation of CO2 and the diffusion rate of CO2 to the metal surface, which made its corrosion mainly uniform corrosion.