| With the continuous exploitation of CO2/H2S offshore oilfield in China,the water content gradually increases,and the corrosion problem of carbon steel pipes and equipment becomes increasingly prominent,causing huge economic losses every year.Therefore,in order to better control corrosion,the influence of temperature,CO2 and H2S partial pressure ratio and flow rate on the corrosion behavior of carbon steel were studied by simulating the oilfield conditions in the South China Sea.The study found that the corrosion rate varied with H2S concentration in two completely different ways:at 40? and 60?,when less H2S was added,the corrosion rate decreased,and then gradually increased with the increase of H2S concentration.However,when the temperature exceeds 70?,the corrosion rate will gradually decrease with the increase of H2S concentration,which is closely related to the film morphology and composition of corrosion products.This finding may well explain the actual corrosion conditions in some offshore oilfields.In the judgment of corrosion control factors,it was proposed by others that the ratio of partial pressure of CO2 and H2S was rel ated.When the ratio was less than 20,corrosion was controlled by H2S.When the ratio is greater than 20 and less than 500,it is CO2/H2S mixed corrosion control.When the ratio is greater than 500,it is CO2 corrosion control.The findings are now accepted by some researchers.However,according to the actual production situation of the oilfield,the temperature and flow rate in different oil areas when the corrosion is the most serious,as well as the factors of corrosion control,are different.In particular,the determination of corrosion control factors often deviates,resulting in the wrong selection of anti-corrosion measures,which consumes a lot of manpower and material resources.This study found that this critical partial pressure ratio is closely related to temperature,the higher the temperature,the smaller the ratio,that is to say,the higher the temperature,the more likely it is to be CO2 corrosion control.When the temperature rises above 60?,the critical ratio of CO2 and H2S partial pressure has dropped to 10 when the corrosion is controlled by H2S corrosion.When the temperature rises above 100?,the critical ratio of partial pressure of CO2 and H2S has dropped to 300 when the corrosion is controlled by CO2 corrosion.So the influence of temperature should be considered when determining corrosion control factors and taking targeted measures.At present,corrosion is controlled or alleviated by adding corrosion inhibitor to pipelines and equipment in offshore oilfields.According to statistics,the application amount of organic corrosion inhibitors is imidazoline corrosion inhibitor and its derivatives in oilfields.The thiourea imidazoline TAI was widely applied on offshore oilfields,and have excellent corrosion inhibition efficiency in CO2 environment.But its corrosion inhibition efficiency is reduced in CO2/H2S coexistence system.Therefore,it is of great significance to develop corrosion inhibitors in CO2/H2S coexistence system.In this paper,a new imidazoline derivative MTAI,which is resistant to CO2/H2S corrosion,was obtained by introducing unsaturated triple bond on TAI molecule.To explain these two kinds of corrosion inhibitors in CO2/H2S corrosion performance differences,TAI and MTAI were studied by dynamic weightlessness experiment,electrochemical experiment,contact Angle test,quantum chemical calculation etc.According to the study,TAI has one active center,while MTAI has two active centers,and the energy gap of MTAI is significantly lower than that of TAI,indicating that the adsorption of MTAI is stronger,the adsorption film is more stable,and the corrosion inhibition rate is higher.The interaction energy between MTAI and the iron surface is greater than that between TAI and the iron surface.The adsorption of MTAI on iron surface is mainly chemical adsorption.MTAI also has a significantly larger radius of rotation than TAI and is more resistant to corrosion ions.So MTAI has better corrosion inhibition.In this paper,the HS-concentration distribution around the two corrosion inhibitor molecules was got by the molecular dynamics simulation.It found the HS-concentration around MTAI molecules significantly higher than that around the TAI molecular.MTAI molecules and HS-there is a strong force,so this paper presents this method can be used to predict corrosion inhibitor efficiency in CO2/H2S systems,and rapidly screening and determine the molecular structure of the best corrosion efficiency.In practical applications,single corrosion inhibitors often fail to meet the protection requirements in complex environments,and it is necessary to utilize the synergistic effect between corrosion inhibitors to develop composite corrosion inhibitors.However,the study of synergistic effect often relies on a large number of weightlessness experiments,lacking microscopic analysis and theoretical guidance.To solve the above problems,a large number of experiments were carried out under simulated working conditions,and it was found that there was a good synergistic effect between MTAI and hexadecyl pyridine quaternary ammonium salt(16BD)at an appropriate concentration ratio.The synergistic corrosion inhibition mechanism of the two is as follows:firstly,the MTAI molecule is adsorbed on the surface of carbon steel with HS-,forming a layer of corrosion inhibitor film.When 16BD exists at the same time,it can form 16BD cation in aqueous solution,and then interact with HS-adsorbed on the surface of carbon steel through electrostatic interaction,filling in the defect of MTAI film and forming a more compact composite corrosion inhibitor protective film.When the concentration of 16BD is too high,competitive adsorption occurs between the two corrosion inhibitors,resulting in antagonistic effect.Further research in this paper shows that the synergistic effect of corrosion inhibitors can be predicted by using the change value D of free volume fraction.When D is greater than 0,the two corrosion inhibitors have a synergistic effect.The higher the value,the better the synergistic effect.When D value is less than 0,the two corrosion inhibitors show antagonistic effect.The greater the absolute value is,the more obvious the antagonistic effect is.It can be used as a new method to judge the synergistic effect of corrosion inhibitor.The compound corrosion inhibitor developed in this study was tested on a platform of offshore oilfields,and through the analysis of corrosion probe and coupon detection,the results showed that the product has excellent effciency,can effectively control the corrosion on site and meet the oilfield requirements.In order to control CO2/H2S corrosion,in addition to the use of corrosion inhibitor,the desulfurizer was used to reduce the content of hydrogen sulfide,especially in high H2S area.The application of desulfurizer and corrosion inhibitor combined protection can achieve twice the result with half the effort.In this paper,the desulfurizer used in offshore oil fields is improved,and the best synthetic raw material and proportion are obtained. |
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