Souring and Microbial-influenced Corrosion in Produced Water Re-injection Systems
trend in sulphide was the stimulation of NRB and nitrate-reducing, sulphur-oxidising bacteria (NR-SOB) and the coinciding decrease in SRB numbers upon nitrate addition. Even though the concentration of sessile sulphide decreased towards the end of the experiment upon nitrate addition in all tests, trace amounts were detected in biofilms of Tests 4–6. This implies that with the addition of nitrate at a carbon:nitrate ratio of 1:3, souring control can be quickly achieved in the bulk water phase and is more gradually achieved in the biofilm with a persistent sulphidic layer. Souring mitigation was most likely a result of nitrate-coupled sulphide oxidation.
Corrosion Control
Comparison of the LPR rates from Tests 1–3 with Test 4 implies that the addition of a CI together with nitrate mitigated general corrosion (see Figure 4). The CI was most effective in the tests with clean systems receiving nitrate. Dosages between 100 ppm (Test 1) and 200 ppm (Tests 2 and 3) of the product were used. The surface-active compounds of the CI have presumably formed a film on the metal surface that effectively protected from general corrosion attack before the corroding biofilm developed. This is contrary to Test 5 where the CI was less effective. The sulphidic biofilm present in this system before CI addition commenced may have hindered the CI from diffusing to the metal and forming a protective film. In fact, general corrosion rates in the same test were not affected by the CI, despite a dosage increase over time from 25 to 400 ppm. The concentration of 400 ppm was eight times higher than that originally recommended by the manufacturer (50 ppm). The unexpectedly high rates of both general and pitting corrosion in Tests 1 and 5 indicate that the CI did not mitigate pitting but induced or accelerated pitting under the test conditions. Many CIs actually promote severe MIC pitting,6 particularly in high dosages, which demands careful selection and further assessment of CIs before and during field application when biofilm formation is involved.
The addition of BIO and combined pigging and BIO improved the mitigation from general corrosion in systems with nitrate present from day 1. Since BIO addition reduced the corrosion damage, this indicates that corrosion was influenced or caused by corrosive bacteria which were killed by the BIO. The proportion of MIC to the general corrosion rate is unknown from the current experiments. The removal of biofilm before shock dosing with BIO at 1,000 ppm in the presence of CI and nitrate resulted in decreased general and pitting corrosion in Test 3 compared with all other tests. Whereas thin uniform iron sulphide layers on steel surfaces may protect from corrosion, thick and layered heterogeneous biofilms, particularly when combined with scale, oil, waxes and production chemicals, often – but not always – cause destruction to the metal infrastructures in petroleum systems.1,6
For this
reason, any sort of biofilm and ‘schmoo’ is regularly removed from the pipelines and tanks by pigging and jetting. The removal of biofilm can, however, be problematic if the released planktonic cells quickly regrow to a biofilm. The elimination of planktonic cells by shock dosing with BIO may reduce biofouling and keep corrosion to a minimum. Based on LPR measurements, the treatment mitigated corrosion in the following order:
Combined BIO/CI/pigging (Test 3) > BIO/CI (Test 2) > CI alone (Test 1) •
The results show a direct interrelation of biomass and corrosion attack. They imply that the formation of differential corrosion cells on the
102 •
metal surfaces through a mixture of biogenic matter and accumulating inorganic corrosion products (iron sulphides and oxides) are causative for the pitting attack. These findings highlight the need for further investigation of the complex microbial consortia involved in MIC, the potential role of nitrate in stimulating growth of the corrosive bacteria and the microbe–metal interaction.
The redox potential at the metal surface in Tests 1 and 2 was elevated relative to the control, indicating the presence of a lower amount of highly reduced species deriving from corrosion and microbial metabolic activity. One possible explanation for this is iron sulphide mineral oxidation with nitrate as the electron acceptor by chemolithoautotrophic bacteria, such as Thiobacillus,8
using inorganic sulphur compounds for
growth. The role of the CI and BIO in the difference in redox potential inside the biofilm is unknown.
Sulphide produced by SRP reacts with iron from the anodic reaction during corrosion to form the iron sulphide products5
mackinawite, greigite
and pyrite, as well as elemental sulphur. Except for mackinawite, which has not been detected but nonetheless may have been transiently produced, greigite and pyrite have been detected in Tests 4–6. The presence of these minerals implies the presence of SRP activity. The formation of greigite from mackinawite requires the presence of an oxidant, which in the present tests is potentially an iron oxide (goethite or magnetite), oxygen or elemental sulphur. The reactions occur accordingly:
3 FeS + S0 —> Fe3S4 (greigite) Fe3S4 + 2 S0 —> 3 FeS2 (pyrite)
(1) (2)
The formation of pyrite from greigite requires strongly reducing (redox potential below -250 mV) conditions and high temperature.9
conditions occurred on the metal surface, as revealed by the microprofiles of redox potential (see Figure 3) and pH.2
These Nitrate could
have served as terminal electron acceptor during the reactions, either directly and abiotically or indirectly through microbial nitrate reduction, driving the formation of greigite and pyrite. The presence of a mixture of biogenic matter, iron sulphides, iron oxides, polysulphides and elemental sulphur might have enhanced the formation of local differential corrosion cells that either caused or accelerated pitting. This also implies the presence of interacting microorganisms of the nitrogen and sulphur cycles.
Conclusions •
The implementation of nitrate addition during PWRI requires compromise between achieving souring control by using a rather high nitrate:carbon ratio and minimising corrosion using a rather low nitrate dosage.
The observed corrosion is proposed to be largely influenced by or due to microbiological activity. The results suggest that biofouling and corrosion can be managed in PWRI systems when control of microbial activity is achieved.
•
Biofouling and MIC can be more easily controlled in clean PWRI systems to which CI and BIO are added together with nitrate from the start, and are more difficult to control in systems with existing sulphidic activity.
The recommended sequence of chemical applications to obtain the best mitigation and protection of corrosion during nitrate injection to a PWRI system is:
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
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