Study On Microbial Corrosion Mechanism In Shale Gas Gathering And Transportation System

CO₂ corrosion and microbiologically influenced corrosion(MIC)usually occur in shale gas gathering and transportation system,which cause extremely severe general corrosion and local corrosion in carbon steel pipelines,and pose a huge threat in the production process.A large number of studies have been done on CO₂ corrosion and MIC at home and abroad,and established corresponding prevention and control measures.However,in the flowing environment,the corrosion mechanism of CO₂corrosion and MIC,and the control method of CO₂ corrosion and MIC co-existed still needs to be studied.Therefore,in this paper,the study was carried out in the CO₂erosion corrosion environment.Firstly,the corrosion mechanism of erosion corrosion was studied.Then the corrosion inhibition effect of corrosion inhibitor on CO₂ erosion corrosion and MIC was discussed.The growth process of biofilm on the metal surface and the mechanism of microbial physiological metabolites on corrosion behavior were revealed under the condition of corrosion inhibitor.Finally,the influence of corrosion inhibitor on microbial corrosion and the effect of sand on biofilm adsorption under erosion corrosion were described.In order to reveal the corrosion mechanism of erosion corrosion,a corrosion experiment was conducted using a loop system.The results showed that the general corrosion rate in the loop system was about 1.5 times higher than that in the autoclave.After corrosion,the corrosion products on the metal surface were smooth and tightly combined with the metal surface,showed an erosion corrosion characteristic.Further,through Fluent software numerical simulation,it was found that the vortex in the viscous bottom layer and the turbulent pulsation in the transition layer were the main reasons for the formation of erosion corrosion.The vortex in the viscous bottom layer accelerated the dissolution rate of the surface corrosion products and the diffusion rate of the dissolved corrosion product ions into the transition layer,and the vortex in the transition layer could carry dissolved corrosion product ions into the bulk fluid.In the fluid recirculation zone,the viscous bottom layer was thicker,and the diffusion rate of dissolved corrosion product ions to the transition layer became slower.Under the influence of fluid recirculate,the ion concentration increased in the near wall region,and the thickness of the corrosion products increased.The quaternized polyacrylamide corrosion inhibitor with the concentration of 300ppm could effectively inhibit the erosion corrosion under CO₂ corrosion environment.After corrosion,the corrosion character of the metal surface changed from general corrosion to a small amount of point corrosion.When polyacrylamide and microorganisms were presented,MIC characteristic was obvious.Microorganisms formed biofilm on metal that polyacrylamide failed to peel.Pitting nucleate under the biofilm,developed into cavity-type,showing obvious local acidification.According to biofilm element components,the mechanism of microbial metabolism leading to localized acidification and finally developing into pitting corrosion was analyzed.The effect of corrosion inhibitor on microbiologically influenced corrosion was discussed.Increasing the concentration of inhibitors and using the compound system as inhibitors to study the effects on MIC.The results showed that increasing the concentration of inhibitor only reduced the adhesion density of microorganisms on the metal surface in the initial stage,and the pitting density on the metal surface was significantly reduced.After increasing the concentration of inhibitor,the biofilm distribution changed from complete cover to cluster on the metal surface.Inhibitors limited the growth of microorganisms on the metal and also reduce the depth of pitting.However,adding a high concentration of inhibitor(1200 mg/L quaternized polyacrylamide),still cannot effectively inhibit pitting corrosion caused by MIC,and also in the compound system(1200 mg/L quaternized polyacrylamide and 200 mg/L imidazoline).The effects of fluid flow pattern on biofilm adsorption and corrosion pitting were further studied.It was found that the fluid flow pattern had no effect on the adsorption of biofilm on the metal surface,and also had no effect on the format ion and expansion of pitting corrosion.Finally,the effect of sand scouring on the adsorption of biofilm was studied.The sand with a mass fraction of 1%could peel off part of biofilms from the metal surface during the process of erosion.The content of biofilm on the metal surface was significantly reduced after corrosion.However,there were also still biofilms adsorbed on the metal surface in clusters,and the pitting depth was still higher on the metal surface.Due to the adsorption of sand on the ions in the solution,the impact of the sand on the uphill increases the mass transfer rate and accelerates the corrosion of the metal.