Aerobic

Key Takeaways

• The critical DO threshold for aerobic bacterial proliferation is approximately 5 ppb (micrograms per litre), far below the 6,000-9,000 ppb in untreated seawater and 8,000-12,000 ppb in cold freshwater at spring runoff. The water-injection industry standard target is less than 10 ppb DO at the wellhead injection manifold, with best-practice operations targeting less than 5 ppb. At DO concentrations above 50 ppb, IOB can double in population every 4-8 hours under favourable temperature conditions (20-35°C), meaning an initial colony of 100 cells/mL can grow to 10^6 cells/mL within 3-4 days if unchecked. Even at DO levels of 5-15 ppb, aerobic electrochemical corrosion reactions proceed through the cathodic half-reaction O2 + 2H2O + 4e- → 4OH-, driving anodic iron dissolution at metal pitting sites at rates of 0.1-3 mm/year of wall loss depending on temperature and water salinity. In WCSB waterflood operations drawing from Peace River or North Saskatchewan River sources, spring breakup (March-May) brings cold, fully saturated river water with DO of 11-13 mg/L, substantially increasing the oxygen scavenger demand and challenging fixed-dose injection programmes designed around summer water quality.
• Sulfur-oxidizing bacteria (SOB) are aerobic chemolithotrophs that oxidise reduced sulfur compounds to sulfuric acid via the reaction: S + 1.5 O2 + H2O → H2SO4, or for thiosulfate: Na2S2O3 + 2O2 + H2O → 2H2SO4 + Na2O. The principal genera are Acidithiobacillus thiooxidans (optimum pH 2-4, maximum SO4 production) and Thiobacillus thioparus (active at neutral pH). SOB activity at the biofilm-metal interface reduces local pH to 2-4, which is below the passive oxide film stability range for carbon steel (pH 4-12), leading to general corrosion rates of 2-8 mm/year in severely affected systems and localised pitting of 0.5-2 mm depth within weeks. In injection wells where H2S from reservoir souring reaches the surface handling system through produced water recycle loops, the sulfide substrate for SOB is continuously replenished, creating a self-sustaining corrosion cycle. Monitoring for SOB uses selective agar media (Starkey's medium, pH 3-4) with colony-forming unit (CFU) counts; an action threshold of 10^2 CFU/mL in injection water is typical for systems carrying H2S concentrations above 50 mg/L in the commingled produced stream.
• Iron-oxidizing bacteria (IOB), primarily Gallionella ferruginea and Siderocapsa species, obtain energy by oxidising ferrous iron: 4Fe2+ + O2 + 8H+ → 4Fe3+ + 4H2O. The Fe3+ hydrolyses at neutral pH to ferric hydroxide (Fe(OH)3), which forms gelatinous orange-brown precipitates with bulk density of 1.0-1.3 g/cm3 when fresh, hardening over weeks to ferrihydrite and goethite at 2.5-3.5 g/cm3. These precipitates accumulate at perforation tunnels, gravel packs, and in the near-wellbore matrix within 0.5-3 m of the injection face. Progressive plugging raises surface injection pressure at a rate of 1-3 MPa per month in heavily infected injectors. The plugging signature on an injectivity decline curve is characteristically gradual and monotonic rather than step-wise (which would indicate scale precipitation events), with the Hall plot (cumulative injection versus cumulative pressure-time product) showing progressive upward curvature. Downhole video inspection of infected perforations reveals orange-brown gelatinous deposits at perforation entries, confirming IOB as the cause. An acid wash with 7.5-15% HCl dissolves ferric hydroxide precipitates (Fe(OH)3 + 3HCl → FeCl3 + 3H2O) and restores injectivity, but re-infection occurs within 3-6 months if the oxygen source is not corrected.
• Slime-forming bacteria of the genera Pseudomonas, Flavobacterium, Serratia, and Desulfovibrio (when acting as facultative aerobes) secrete high-molecular-weight exopolysaccharides that form structured biofilms 0.1-3 mm thick on carbon steel pipe walls, injection wellbore tubulars, and packing surfaces. These biofilms are mechanically resilient: shear stress from turbulent flow at up to 5 m/s is insufficient to detach established mature biofilms, which require chemical disruption with non-oxidizing biocides. The key damage mechanism of slime-formers in aerobic injection water is the creation of anoxic micro-zones in the biofilm interior where DO drops from the bulk value (even 1,000 ppb) to less than 1 ppb within 200-500 micrometres of the biofilm base, enabling SRB to colonise the deeper biofilm layers. This means that bulk DO measurements at 5-20 ppb in injection water do not exclude SRB activity and souring within biofilms, which is why biocide treatment targeting slime-formers is an essential complement to deaeration. Glutaraldehyde (at 50-200 mg/L concentration, 4-8 hour contact time) and THPS (tetrakis hydroxymethyl phosphonium sulfate, at 100-300 mg/L) are the most widely used non-oxidizing biocides for biofilm disruption in WCSB waterflood systems, applied as monthly batch treatments alternating with a different biocide mechanism to reduce resistance development.
• Deaeration of injection water is accomplished in three sequential stages: vacuum deaeration, gas stripping, and chemical scavenging. Vacuum deaeration towers pass water over structured packing under a partial vacuum (5-20 kPa absolute), allowing dissolved oxygen to flash from solution; typical outlet concentrations are 20-50 ppb DO from raw seawater or river water at 6-9 mg/L. Nitrogen sparging applies countercurrent N2 injection to further reduce the oxygen partial pressure over the liquid, achieving 5-15 ppb outlet at wellhead manifold conditions in well-designed systems. Chemical oxygen scavengers polish residual DO below the threshold for aerobic bacterial proliferation: ammonium bisulfite (ABS, (NH4)HSO3) reacts with oxygen in the presence of a cobalt catalyst (CoCl2, 1-5 mg/L) at the reaction 2HSO3- + O2 → 2SO42- + 2H+, consuming 5-8 mg ABS per mg O2. Sodium bisulfite (NaHSO3) and sodium sulfite (Na2SO3) serve equivalent functions in freshwater and produced-water injection systems where ammonium ions must be avoided to prevent ammonia corrosion of copper alloy instrumentation. Continuous DO monitoring at the injection manifold using electrochemical membrane sensors or optical fluorescence DO meters (detection limit 1 ppb) is essential to verify that the combined deaeration and scavenging system maintains DO below the target, with automatic dosage adjustment triggered when DO exceeds 20 ppb at the injection header.