Study On Corrosion And Anticorrosion Measures Of Surface Gathering Pipeline In Jingbian Oilfield

Jingbian oilfield is located in the north of Shaanxi Province.With the development into the middle and late stage,the composition of produced fluid is complex.In recent years,corrosion cracking and perforation of gathering and transportation pipelines often occur.The internal corrosion problem has seriously affected the normal production of the oilfield.Therefore,it is necessary to analyze the corrosion mechanism of surface pipeline in Jingbian oilfield,study the corrosion law of surface pipeline,and formulate corresponding anti-corrosion countermeasures on this basis.In this paper,the chemical capacity method and instrument analysis are used to analyze the produced fluid of the oil transportation system and water injection system of the surface gathering pipeline in Jingbian oilfield,scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD)and DSX-500 automatic three-dimensional imaging microscope were used to analyze the surface corrosion morphology and corrosion products of the corrosion coupon.The effects of sulfide,p H,dissolved oxygen,bacteria,temperature and pressure on the corrosion of ground pipelines in the station were studied by weight loss method through indoor experiments,The influencing factors of corrosion were identified.Aiming at the equal scale modeling of the elbow section of the failed water injection pump house,CFD-fluent was used to study the flow field characteristics and erosion wear of the pipe section under the condition of fluid carrying sand.According to the results of field corrosion monitoring and laboratory test,as well as the research on the corrosion mechanism of sulfur-containing produced liquid/oil phase system,the corrosion inhibitor was selected,and the optimal inhibitor and dosage were determined.The conclusions are as follows:(1)Jingbian oilfield produced fluid has high water content,p H is weak alkaline,7.41?8.46;Cl^-content is 13338?19556mg/L,pitting probability is high;sulfide content is88.47?122.17mg/L;sulfate reducing bacteria quantity content is between 10⁴?10⁶/m L,which seriously exceeds the standard;scaling cationic Ca²⁺and Mg²⁺content is small,there is a tendency of hydrogen sulfide and chloride corrosion.(2)The results show that the average corrosion rate of the oil transportation system is0.0154 mm/a,which is mainly local corrosion and supplemented by uniform corrosion.The main causes of corrosion are high sulfide content and High Cl^-content,and the corrosion products are mainly Fe S;the corrosion rate of the water injection system is 0.0498 mm/a,The main factors causing pitting corrosion are especially high water content,high dissolved oxygen content,sulfide and SRB.a large amount of Cl^-in the produced liquid will accelerate the occurrence of pitting corrosion and perforation.The corrosion products are mainly Fe₂O₃,CaSO₄,SiO₂and hydrate of iron oxide.(3)Sulfide,dissolved oxygen and p H are the main factors affecting the pipeline,bacteria may be the important factors causing the corrosion of produced liquid,while temperature and pressure are not the main factors.With the increase of sulfide and dissolved oxygen content,the average corrosion rate of 20#steel increases;with the decrease of p H,the average corrosion rate increases step by step,and the p H of field produced liquid is 7.41?8.46,which has little effect on corrosion;with the increase of bactericide dosage,the average corrosion rate slows down slightly,and bacteria may be an important factor causing the corrosion of produced liquid,and the temperature and pressure increase with the independent variable,which leads to the corrosion rate change is very small,which is not the main cause of corrosion.(4)By collecting the field data and using CFD-fluent to study the flow field characteristics and erosion wear in the bend section under the condition of fluid carrying sand,the flow field distribution and pipeline wear are obtained;the pressure difference between the inlet and outlet is large,P_in=3.8MPa,P_out=0.2MPa;the inlet velocity distribution is uniform,V=9.5m/s,the turbulence does not reach the stable state,the flow velocity stratification in the outlet section is serious,and the inner velocity is small,V=5.45m/s;Cavitation corrosion is easy to occur at the place with large curvature,so it is necessary to improve the inner surface finish of the pipe,increase the hardness of the cast pipe material or improve the fatigue strength to inhibit cavitation corrosion;erosion occurs all at the upstream side of the elbow,and the produced fluid is corrosive,which will cause electrochemical reaction at the break of the protective film on the inner wall of the elbow formed by erosion,and rapidly aggravate the pitting corrosion and perforation of the pipe.(5)10 kinds of corrosion inhibitors were screened by electrochemical method,and the results showed that 1#,2#,3#,5#,6#had better inhibition effect;The weight loss method was optimized,and the results showed that the two inhibitors were good in inhibition of average corrosion,pitting and sulfur resistance;In the process of the study of the amount of inhibitor,the corrosion rate can be reduced effectively when the dosage of the two inhibitors is80mg/L-100mg/L.The results show that the corrosion rate of 2#inhibitor decreases obviously with the increase of dosage in the range of 0 mg/L?80 mg/L,and the corrosion rate is the lowest when the dosage is 80 mg/L;the corrosion rate of 3#inhibitor changes little when the dosage is 0 mg/L?60 mg/L,and it suddenly becomes very low when the dosage is 80 mg/L,so it can be used as corrosion inhibitor on site;Therefore,it is recommended to use 2#or 3#corrosion inhibitor at the dosage of 80mg/L?100mg/L.