Souring and Microbial-influenced Corrosion in Produced Water Re-injection Systems
Figure 4: Pitting Rates (Bars) and General Corrosion Rates from Weight Loss (Black Rectangles) of Coupons from Systems Receiving Nitrate from Week 1 or from Week 14 (A) and Average General Corrosion Rates as Detected by Linear Polarisation Resistance over the Course of the Experiment (B)
A 0.60 0.50
0.10 0.20 0.30 0.40
0.10 Nitrate from week 1 Nitrate from week 14 0.15
0.05
014 625 0.00 B Nitrate from week 1 1.2 1.0
0.2 0.4 0.6 0.8
0 1 CI 2 CI & BIO Before addition BIO = biocide; CI = corrosion inhibitor.
accumulation in all tests. Independently of when nitrate was added, all biofilms became more stratified and structurally heterogeneous with time, showing alternate structures of clusters, voids and channels in the upper part of the biofilm and a black sulphidic crust on the base (see Figures 3 and 5).
Nitrate Functionality
Nitrate provides a potent electron acceptor for nitrate-reducing microorganisms and is energetically more favourable than sulphate for providing growth. Substantial biofilm growth is problematic during PWRI where it can lead to biofouling and subsequent reduced flow and injectivity loss. The conditions in the biofilm largely vary from those in the bulk water, providing niches for microbial growth under otherwise rough conditions. Once nitrate becomes depleted with increasing depth in the biofilm, sulphate is reduced by SRP and converted to sulphide that itself corrodes or forms corrosion products with the metal iron. The structural heterogeneity of the metal surface may provoke corrosion by forming differential corrosion cells.
General Corrosion
General corrosion rates were determined by weight loss of the corrosion coupons over the duration of the experiment (see Figure 4). In clean systems the highest rate was measured in the presence of CI only (Test 1) while the lowest rates occurred in the presence of CI and BIO (Test 2). When nitrate was added to sulphidic systems, higher rates occurred in the presence of CI in Test 5 relative to the control (Test 6). LPR corrosion rates were highest in the biofilm systems to
100
which nitrate was added from week 14 (Tests 5 and 6) and in the nitrate control (Test 4) before CI and BIO addition (see Figure 4). The addition of CI (Test 1) and CI/BIO (Test 2), and the application of CI/BIO/pigging (Test 3) were shown to reduce the corrosion rate. The largest relative decrease in corrosion rate occurred in (in decreasing order) Tests 3, 2, 4 and 1. In contrast, the corrosion rates in Tests 5 and 6 increased.
Pitting
Pitting rates basically showed the same pattern as general corrosion rates as determined by weight loss (see Figure 4). The deepest pits occurred on coupons which were exposed to the CI only, independently of when nitrate was added (Tests 1 and 5); the least attack occurred in Test 2 (see Figure 4). No certain pattern of pitting across the metal surface was observed on coupons among the tests, but it was positively related to the amount of biomass present in all tests (see Figure 5). Pitting particularly occurred under thick accumulating biomass as revealed by 3D profiling of the corrosion coupon surface (see Figure 5). This implies the involvement of bacteria in the apparent corrosion, either by direct attack of the metal surface or indirectly by the excretion of corrosive metabolic products. In the absence of nitrate (as initially occurred in Tests 5 and 6), sulphide produced by SRP can be corrosive. Nitrate can be reduced by heterotrophic and autotrophic nitrate-reducing bacteria (NRB) to corrosive nitrite and nitric oxide.3–5
In addition, the nitrate-driven
oxidation of sulphide can lead to the production of highly corrosive polysulphides and elemental sulphur. The latter has in fact been
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2 3 CI & BIO & pigging During addition 4 Nitrate control After addition End addition 5 CI 6 No nitrate control Nitrate from week 14
Corrosion (mm/year)
Pitting rate (mm/year)
General corrosion (mm/year)
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