Souring and Microbial-influenced Corrosion in Produced Water Re-injection Systems Figure 1: Flow-through Systems Data recording
LPR corrosion monitoring
Discharge N2/CO2
Corrosion flow cell with steel coupons
Biofilm coupons were placed horizontally in corrosion flow-through cells inoculated with produced water and receiving de-aerated artificial produced water.
Artificial produced water
Figure 2: Tests Performed Test
number 1
2 3 4 5 6
Aim of test Effect of CI on nitrate-receiving biofilm
Effect of combined CI and BIO on nitrate-receiving biofilm Effect of combined CI and BIO pigging on nitrate-receiving biofilm Nitrate-receiving biofilm without any other addition (nitrate control) Effect of combined CI and nitrateaddition to biofilm without nitrate Nitrate addition to biofilm without any other addition (no nitrate control)
Nitrate addition BIO = biocide; CI = corrosion inhibitor. Biocide addition CI addition Pigging Week number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Coupon Surface Profilometry and Microscopy Acid-cleaned coupons were scanned by white-light axial chromatism using a non-contact 3D optical profilometer. Coupons were cut, embedded into epoxy, ground, polished, washed with isopropanol and dried. Cross-sections were inspected under a metal optical microscope and pitting depth was quantified.
Corrosion Product Analyses
The corrosion products were qualitatively analysed with X-ray diffraction. Sessile sulphide was quantified using the methylene blue method.
Results and Discussion Biofouling and Corrosion Two scenarios were assessed:
1. A clean system without biofilm. CI was added together with nitrate to a clean system from week 1 of the experiments (Tests 1–3). This simulated a system to which nitrate and CI are added to mitigate souring and corrosion from day 1 of production.
2. A system with an established sulphidic biofilm. During the second scenario (Test 5), CI and nitrate were added together to an established sulphidic biofilm. This simulated a sour field to which nitrate and CI are added to mitigate RS and corrosion in the topsides.
98
Biofouling Clean Systems
The biofilms gradually grew to become thick (200–1,500 µm) and were fluffy and yellow-brown in colour (see Figure 3). At the end of the experiment, the redox potential in biofilms treated with BIO (Test 1) and BIO/CI (Test 2) was consistently positive at +375 mV in the water and throughout the entire biofilm (see Figure 3). In contrast, the redox potential in the nitrate control (Test 4) dropped to -460 mV on the metal surface.
Systems with Sulphidic Biofilm
Before nitrate addition in Tests 5 and 6, a thin (40–70 µm), compact and crusty black (sulphidic) film developed on the metal coupons within one week after starting the experiment (see Figures 3 and 5). The redox potential, below -600 mV on the metal surface of the same coupons, indicated the presence of highly reduced corrosion compounds, such as iron(II), hydrogen, sulphide and iron sulphides,2,5
originating from corrosion and sulphate reduction (see
Figure 3). After nitrate addition, the redox potential increased to -250 mV at the metal surface within the sulphidic crust and more than -100 mV in the biofilm. This implies presence of a persistent sulphate reduction occurring at the metal surface even in the presence of high nitrate concentration in the surrounding water. Nitrate addition supported microbial growth leading to biomass
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
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