Effects of Flow Velocity on Biofilm formation and corrosion behavior of L245 steel in the presence of sulfate reducing bacteria

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Shale gas is a type of unconventional natural gas that is found primarily within reservoir rock sequences dominated by organic-rich shale, and is usually exploited by hydraulic fracturing technology, which typically requires a large amount of water to be injected into the gas well, and when the fracturing process is completed, a portion of the injected water immediately flows back. The fracturing flow-back fluid contains a large number of microorganisms when it enters the surface gathering and transportation system, resulting severe internal corrosion of the pipelines, especially those built during the early exploitation process, using carbon steel like L245 steel[1]. The anoxic environment and large amount of fluid accumulation in the pipeline provide appropriate conditions for the growth and reproduction of microorganisms, which increased the risk of Microbiologically Influenced Corrosion (MIC). MIC is a prevalent form of corrosion instigated by the bioactivity of diverse microorganisms. Representing a substantial challenge in the oil and gas sector, it is estimated that MIC accounts for approximately 40% of all incidents of internal pipeline corrosion[2]. SRB are typically considered the primary culprits in causing MIC, mainly because SRB are often found at the sites of corrosion believed to be associated with MIC[3]. SRB is a general term for a group of bacteria that are widely distributed in anaerobic environments, such as soil, seawater, river water, underground pipes and oil & gas wells where are rich in organic matter and sulfate[4-6]. The presence of SRB will lead to the corrosion of metal pipelines and equipment, moreover, its corrosion products FeS and Fe(OH)2 and the bacteria themselves will cause the blockage of pipelines and formation, and affect the subsequent gas production and development. SRB can use sulfides with valence states above -2 as electron acceptors, including HSO3, S2O32- and element S, to reduce S to a stable -2 valence. SRB is a strict anaerobe, its growth and reproduction are inhibited when exposed to oxygen, but it can survive for a period of time[7]. The theories related to SRB-induced corrosion include cathode depolarization theory[8, 9], metabolite corrosion theory[10-12], concentration difference cell theory[13], Extracellular Electron Transfer (EET) theory[14, 15] and Biocatalytic Cathodic Sulfate Reduction (BCSR) theory[16]. According to the researches of many scholars, factors such as biofilm structure[17, 18], ambient temperature[19], pH level[20], Cl−[21], CO2[22], H2S[23], cathodic protection potential[24, 25] and magnetic field[26] all can affect the corrosion behavior of SRB. In oil and natural gas pipeline, the flow of medium is complicated, and the influence of flow conditions on corrosion behavior of SRB cannot be ignored. The change of flow regime and flow velocity can affect the mass transfer in the pipeline, and thus affect the biochemical reaction process[27]. Furthermore, the change of shear stress can affect the formation, breakage, detachment and spalling of the biofilm attached to the pipeline and lead to the change of the state of the pipe wall and the corrosion state and potential, shear stress can even affect the transport, transfer and reaction rates of materials under the biofilm[28]. It is generally believed that the flow of the medium is not conducive to the adhesion of microbial cells on the pipelines’ surface and the formation of biofilm. On the one hand, polarization agents such as H and H2O in the fluid can rapidly diffuse to the electrode surface and improve the reaction rate of cathode. On the other hand, higher flow velocity can make the anodized ions leave the metal surface quickly, improve the anodic dissolution rate, and also affect the formation of the corrosion product film or destroy the product film that has been generated[29]. Some scholars have also found that under low Reynolds number, the biofilm formed on the inner wall of pipeline has a high content of active bacteria, while under high Reynolds number, the biofilm has a high content of extracellular polymeric substance (EPS)[30]. Scholars have made a lot of contributions to the study of SRB-induced corrosion and MIC, but there are few researches focus on the influence of hydraulic conditions on SRB corrosion. In particular, the variation of metal surface and SRB corrosion rate at different flow velocities, the influence of fluid flow on corrosion characteristics and the underlying corrosion mechanism have not been reported.