Various studies have indicated that sessile bacteria in biofilms, not planktonic bacteria suspended in liquids, are directly responsible for pitting attack on metal surfaces in Microbiologically Influenced Corrosion (MIC). MIC has been detected not only in static fluid systems, but also in flow systems. Fluid flow directly impacts mass transfer and biofilm formation. A sufficiently high linear flow velocity can prevent biofilm establishment or even dislodge an established biofilm. It is difficult to perform experiments using a large flow loop to achieve high linear velocities. Instead, an electrochemical glass cell bioreactor with a cylindrical coupon on a rotating shaft can be used to simulate pipe flow with high linear velocities in MIC research. Mass transfer and wall shear stress similarities can be used to relate the coupon rotational speed in a glass cell and the average linear velocity in the corresponding pipe flow. In this work, ATCC 7757 strain of Desulfovibrio desulfuricans, a common strain of sulfate-reducing bacteria (SRB), was used in glass cell experiments to study biofilm behavior under flow conditions. The results confirmed that a high linear flow velocity could indeed prevent SRB biofilm formation.
MIC is a major challenge to the oil and gas industry and the problem is growing due to aging equipment and practices such as water flooding used to extract oil from depleting reservoirs. A group of bacteria known as sulfate reducing bacteria (SRB) are often found to be responsible for MIC1. Acid producing bacteria may also be directly involved in MIC. Some other bacteria may be indirectly involved, such as aerobic biofilm-forming bacteria that provide an anaerobic microenvironment for SRB to thrive.
Water injection or flooding is a common oilfield practice used to increase well pressure in aging wells. Injection water typically comes from nearby aquifer or more often the sea. Seawater contains nutrients sufficient for some bacteria to grow. Untreated seawater carries harmful bacteria such as SRB. Even treated seawater can be a source of SRB inoculums2. It was initially believed by some people that MIC normally occurred under stagnant conditions. However, experimental and field results showed that MIC also occurred under flow conditions. At linear velocities of about 0.35 m/s, MIC on AISI 1018 mild steel was reported3. Hydrodynamics and nutrient availability are two key factors influencing biofilm growth in MIC investigations. Fluid flow can enhance mass transfer but it may also produce a high shear that inhibits cell attachment and causes even detachment of an established biofilm4. Stoodly et al. 5 observed that the Pseudomonas aeruginosa biofilm pre-grown in a glass flow cell was detached at 3.5 m/s. It was reported that the structure and material properties of SRB biofilms were influenced by the fluid shear6. Laboratory research at Ohio University indicated that the fluid flow rate had a considerable impact on MIC corrosion rates of carbon steels7. This work presents additional information on the SRB biofilm growth under flow conditions.