Offshore projects inject more than 1.5 million barrels of seawater a day, and without a disciplined biocide program that water can rapidly sour a reservoir and corrode infrastructure. The fix: a high-dose primary kill, a rotating secondary maintenance dose, and continuous monitoring—backed by hard thresholds and field data.
Enhanced oil recovery (EOR) via large-scale waterflooding has a microbial problem. Operators routinely inject >1.5 million barrels/day of seawater offshore (alvimcleantech.com). That water brings bacteria and nutrients into a system that, once dissolved oxygen is stripped out—often by oxygen scavengers to <0.05 ppm—to prevent aerobic corrosion (slideshare.net)—becomes an anoxic (oxygen-free) haven for sulfate-reducing bacteria (SRB). SRB turn sulfate into hydrogen sulfide (H₂S), a highly corrosive and poisonous gas that degrades oil quality and raises O&M costs. One industry estimate pegs souring at ~2% added operating cost (alvimcleantech.com).
Left untreated, injected water risks plugging and souring—threatening well integrity and revenue. The industry answer is a two-tier biocide program tied to measurable targets and aggressive monitoring, supported by pre-treatment steps like filtration and deoxygenation. In practice, that means carefully metered chemical feeds—often via dosing pumps—and verification of residuals and microbial load.
Microbial load and souring risk
Operators aim to remove dissolved oxygen to <0.05 ppm to limit aerobic corrosion (slideshare.net), a step that inadvertently favors SRB under anoxic conditions. SRB-driven souring elevates H₂S—corrosive, poisonous, and a quality detractor—adding ~2% to operating costs in one estimate (alvimcleantech.com). To manage oxygen and sulfide chemistry upstream of the reservoir, some facilities pair deoxygenation with oxygen/H₂S scavenger systems.
Primary biocide shock dosing
The first line of defense is a high-dose “bulk” or shock treatment of incoming water to kill planktonic microbes (free-floating organisms). Fields often start with oxidation and filtration—continuous chlorine or hypochlorite on seawater headers dosed to ~0.5 ppm residual chlorine knocks down microbial load in minutes (alvimcleantech.com). After chlorine disinfection, dechlorination (e.g., sodium bisulfite) protects downstream equipment, especially reverse osmosis (RO) membranes used for sulfate removal (alvimcleantech.com), which can include RO trains such as membrane systems. At that point, an anoxic protocol is established and a non‑oxidizing biocide is added.
Typical primary biocides include aldehydes (glutaraldehyde or formaldehyde releasers), DBNPA (2,2-dibromo-3-nitropropionamide), and THPS (tetrakis-hydroxymethyl phosphonium sulfate), among similar broad-spectrum agents (alvimcleantech.com; oilfieldtechnology.com). Shock doses are engineered for 3–5 log reductions in total bacteria. In field water treatments, glutaraldehyde is “widely applied” at 25–500 ppm (with occasional short-term spikes to ~2,500 ppm in severe cases) (pmc.ncbi.nlm.nih.gov), while ≥100 ppm DBNPA or THPS is common for well shut-ins or plant sterilization.
Laboratory data back the need for robust primary dosing. Shi et al. (2021) found that a one-time treatment with 750 ppm of an aldehyde-releasing biocide (ARB) achieved lasting control, whereas 100 ppm glutaraldehyde failed to suppress souring beyond ~7 days (pmc.ncbi.nlm.nih.gov). In that study, the 750 ppm dose drove total microbial and SRB abundances down by ~10³× in the first week (pmc.ncbi.nlm.nih.gov). Low (100 ppm) doses had only modest, short-lived effects. Because once biofilms form they can require ~10–1000× more biocide to remove (alvimcleantech.com), the primary kill has to be decisive.
In practice, a primary program may include chlorination (~0.5 ppm Cl₂) and filtration—often with self-cleaning hardware such as an automatic screen—followed by batch injection of glutaraldehyde or a similar biocide at several hundred ppm for hours to days, cycling as needed. Targets are typically heterotrophic plate count (HPC) ≤10³ CFU/mL (colony-forming units per milliliter) or “no detectable” SRB immediately after treatment. Operators engineer pulses based on flow: for example, a 500‑ppm slug for 4–8 hours daily, or periodic pigging (mechanical pipe cleaning) and soak. Where seawater isn’t available, analogous high-dose treatments are applied to aquifer or produced water streams.
Secondary maintenance and rotation
After the initial sanitation, a follow-up program prevents rebound and resistance. This typically uses much lower biocide levels—single‑digit ppm maintained continuously or periodically. Operators often inject low‑dose glutaraldehyde or THPS continuously into pipelines or headers at roughly 5–30 ppm, suppressing survivors between shocks. If a shock slug has just been applied, the continuous feed can be paused until background levels recover. Some systems add a periodic “squeeze” treatment downhole (injecting biocide into the formation) to complement topside control. Maintenance dosing is often delivered as part of a broader biocide program.
Crucially, the same product should not be used alone for long stretches; surviving populations can adapt. In one study, glutaraldehyde treatment enriched spore-forming SRB such as Desulfotomaculum, which can endure lower-level glutaraldehyde and repopulate the reservoir (pmc.ncbi.nlm.nih.gov). A classic response is to rotate chemistries or use blends: apply a quick‑kill slug (e.g., glutaraldehyde, DBNPA), then switch to a different maintenance biocide such as an isothiazolinone mix at low ppm. A Rohm & Haas patent (1991) recommended an isothiazolone mixture (5‑chloro‑2‑methyl‑3‑isothiazolone plus 2‑methyl‑3‑isothiazolone) continuously at ~0.25–2.5 ppm to maintain SRB control between high‑dose treatments (patents.justia.com). Because aldehydes and isothiazolones act differently, tolerance to both is less likely.
An example implementation from field practice: after filtration and chlorine, inject 300–500 ppm DBNPA for 24 hours (“shock kill”), then switch to a continuous glutaraldehyde feed of ~5–10 ppm in the return line. A month later, apply another pulse of THPS or glutaraldehyde at 200 ppm for 4 hours, then revert to maintenance. Some operators dose two biocides simultaneously (e.g., THPS + glutaraldehyde) at lower levels for synergistic effect. Field reports often cite 0.5–5 ppm in continuous mode as effective if the primary kills were strong.
The goal is measurable: no significant regrowth between major treatments. If ATP (adenosine triphosphate) readings or MPN (most‑probable‑number) tests stay near baseline week to week, maintenance is adequate; if weekly HPC starts trending upward, dosing is adjusted. In one benchmark lab study, 250 ppm glutaraldehyde alone failed to suppress regrowth beyond a couple of weeks, whereas a more stable alternative preserved control for ~2 months (oilfieldtechnology.com).
Quantitative monitoring program
A robust monitoring program validates and adapts the biocide strategy. Without data, creeping regrowth goes unnoticed. Routine culture tests (e.g., MPN counts for SRB, APB—acid‑producing bacteria—or total bacteria) and modern ATP assays provide coverage. For injection systems, ATP‑luminometer tests can deliver “total active biomass” in minutes (environmental-expert.com; environmental-expert.com). Many operators use quick kits daily or weekly on fresh injection water; a sharp rise in luminescence or CFU indicates regrowth. Targets often aim for near‑zero SRB MPN; a healthy injection might be <10 MPN/mL for SRB and <10³ CFU/mL total.
Parallel indicators matter. Measuring dissolved H₂S in produced fluids or at downhole sample points offers a proxy—any sustained increase signals failing souring control. Online sulfide sensors or periodic lab analyses are used. Field test kits confirm continuous biocide residuals; even a 1–2 ppm residual at injection wells is a useful check, and a missing residual hints at reaction/consumption. Corrosion coupons or probes can flag microbiologically influenced corrosion (MIC); a jump in weight-loss often correlates with rising bacterial counts.
Advanced labs add microbial community analysis, sequencing DNA or using qPCR for SRB genes such as dsrB, which can spot trends invisible to culture. Shi et al. report genomic “alpha diversity” shifts before souring reoccurs (pmc.ncbi.nlm.nih.gov; pmc.ncbi.nlm.nih.gov). In practice, regular ATP/MPI tests plus occasional molecular checks provide a strong mix.
Data thresholds and field outcomes
Monitoring data drive decision thresholds. For example: if SRB MPN rises above 1 or ATPR (relative light units) doubles, increase the biocide slug. If H₂S at the first production well exceeds 1 ppm, an emergency bleach shot is triggered. Over time, plotting ATP, CFU, and H₂S lets engineers correlate treatments to outcomes and tune dosage or contact times. Inline gauges, test kits, and records turn the program into a quantifiable, auditable process; deviations can prompt escalation—higher dose, pigging, or a chemistry change—before a production alarm sounds.
Monitoring also guards against unintended consequences. Frequent glutaraldehyde use can cause non‑target effects or byproduct formation; lab tests should verify the chosen biocide remains effective in the actual brine. Many operators run “mini-batch” tests: incubate injection water with the biocide and measure 1‑, 2‑, and 4‑hour kill curves. If kill rates slip, tolerance may be rising.
Data-based outcomes are clear. Targets might be SRB MPN <1/mL and total ATP ≤500 pg/mL. In one case study, switching to continuous nitrate (a parallel strategy beyond pure biocide scope) cut SRB gene copies by >90% and H₂S by ~70% over 6 months (researchgate.net). Field practitioners often report proper biocide control keeps produced water sulfide <0.1 ppm, whereas uncontrolled wells can reach hundreds of ppm H₂S. Instituting chlorination plus glutaraldehyde in one field cut pigging frequency by 50%.
Two-tier program and sources
In summary, a robust injection‑water biocide program uses two tiers backed by data. Tier 1 (Primary): high‑dose oxidizing plus non‑oxidizing biocides to achieve a multi‑log kill—typical doses are O(10–500 ppm)—with a goal of near‑total sanitization (very low CFU, no SRB detected). Tier 2 (Secondary): low‑dose continuous or periodic treatment with one or more alternate biocides in the ppb–ppm range (parts per billion to parts per million), using different chemistry than Tier 1 to prevent adaptation. Plants should log dosages (ppm, injection rate) versus microbial metrics (CFU/ATP/H₂S), tracking “log kill” and duration of effect. Lab data suggest a 750 ppm aldehyde dose can maintain H₂S <0.1 ppm for ~1 year (pmc.ncbi.nlm.nih.gov), serving as a benchmark to compare against field durations.
This strategy is grounded in industry studies and field experience (alvimcleantech.com; oilfieldtechnology.com). Shi et al. (2021) experimentally demonstrate how dose levels and biocide types control SRB for fixed periods (pmc.ncbi.nlm.nih.gov; pmc.ncbi.nlm.nih.gov). Trade white papers and guidelines (Alvim Cleantech 2017; Oilfield Technology 2022) summarize best practices—e.g., 0.5 ppm chlorine, 100–500 ppm glutaraldehyde, continuous vs. batch dosing (alvimcleantech.com; oilfieldtechnology.com). The throughline across these sources is consistent: only a combined chemical strategy with real‑time monitoring reliably forestalls resistant strains and souring in EOR injection systems.
