Bactericide: Definition, Biocide Types, and SRB Control

Key Takeaways

• Sulphate-reducing bacteria and reservoir souring: Sulphate-reducing bacteria (SRB), including the genera Desulfovibrio, Desulfotomaculum, and Archaeoglobus in thermophilic applications, are strict anaerobes that use sulphate ions (SO4^2-) as the terminal electron acceptor in their metabolism, reducing sulphate to hydrogen sulphide (H2S): SO4^2- + 8H+ + 8e- produces H2S + 2H2O + OH-. When sulphate-bearing injection water (common in surface freshwater or seawater used for waterflooding or hydraulic fracturing) is injected into a reservoir that initially had low or zero H2S, SRB colonise the wellbore or near-wellbore injection zone and generate H2S from the injected sulphate, souring the reservoir and ultimately souring the produced gas and oil stream. H2S souring has economic consequences ranging from the need for additional sulphur processing at the gas plant (increased operating cost) to the need for material upgrades of all wetted equipment to NACE MR0175 sour-service specifications (increased capital cost) to regulatory reporting obligations and potential personnel safety incidents. In Montney and Duvernay wells where the formation water may contain naturally occurring SRB in dormant state, the injection of sulphate-bearing hydraulic fracture water can activate these bacteria and initiate souring even without SRB arriving in the injected fluid. Bactericide treatment of frac water to less than 10 SRB cells/mL before injection is the standard preventive measure in the WCSB for wells drilled into formations where H2S is absent or below regulatory threshold.
• Glutaraldehyde: mechanism, dosing, and limitations: Glutaraldehyde (glutaraldehyde, C5H8O2, a dialdehyde) is one of the most widely used non-oxidising biocides in oilfield applications. It is bactericidal by cross-linking amine groups in bacterial cell-wall proteins and enzymes, denaturing the proteins and disrupting cell membrane integrity, which leads to rapid cell death. Glutaraldehyde is effective against SRB, APB, GHB, and most fungi at concentrations of 50 to 500 mg/L in aqueous systems. Typical batch-treatment doses for frac water are 100 to 300 mg/L with a 30-minute contact time before injection, or 50 to 100 mg/L for continuous injection into a produced-water injection system. Limitations of glutaraldehyde include its reaction with H2S to form non-biocidal thio-aldehyde products, which means it is ineffective in H2S-bearing systems without prior H2S scavenging; its activity reduction at pH below 4 or above 8.5 (which limits its use in some high-pH completion fluid systems); and its relatively high mammalian toxicity (glutaraldehyde is regulated under Canadian Environmental Protection Act (CEPA) as a toxic substance at workplace concentrations above 0.05 ppm airborne), requiring personnel protective equipment during handling and restricted aqueous discharge concentrations in produced-water management.
• THPS: performance and environmental profile: Tetrakis hydroxymethyl phosphonium sulphate (THPS) is a quaternary phosphonium biocide that kills bacteria by disrupting cell membrane proteins through a different mechanism than glutaraldehyde. THPS has several operational advantages: it remains active in H2S-bearing environments (it does not react irreversibly with H2S the way glutaraldehyde does), it is effective at both acid and alkaline pH (active from pH 3.5 to 9.5), it degrades rapidly in the environment to non-toxic phosphate and formaldehyde (biodegradable in 7 to 14 days in aerobic conditions), and it has low fish and marine toxicity, giving it regulatory approval for overboard discharge in offshore applications under North Sea OSPAR regulations. For WCSB Montney frac water treatment, THPS is often the preferred biocide for its H2S compatibility and environmental profile: Montney produced water may contain trace H2S (200 to 1,000 ppm dissolved) that would deactivate glutaraldehyde, and WCSB environmental regulators are increasingly scrutinising the discharge and disposal of biocide-treated flowback water, making THPS's faster biodegradation a regulatory advantage. Effective bactericidal concentrations for THPS in fresh-to-brackish Montney source water are 100 to 250 mg/L with a 30-minute contact time, similar to glutaraldehyde, and THPS is commonly supplied as a 75 percent active solution diluted to 1 to 2 percent on location before metering into the frac-water storage or blending tank.
• Bacterial testing and monitoring methods: Selecting the appropriate bactericide dose requires baseline testing of the source water and monitoring of treated water to confirm adequate kill efficacy. The serial dilution culture (SDC) method, also called the most probable number (MPN) test, is the traditional method: water samples are diluted in a series of bacterial growth media tubes (Postgate medium B for SRB, which contains sulphate as the electron acceptor and is incubated anaerobically for 28 days), and the dilution at which growth ceases (indicated by blackening from FeS precipitation in SRB media) gives a count of colony-forming units per mL (CFU/mL) or cells/mL as the most probable number. While inexpensive, the SDC/MPN method requires 28 days of incubation for definitive SRB results, making it unsuitable for real-time process control. Rapid alternative methods include impedance microbiology (which detects bacterial metabolism as changes in electrical impedance of the growth medium within 6 to 24 hours), adenosine triphosphate (ATP) bioluminescence (which measures total microbial biomass as a proxy for bacterial count in minutes), and 16S rRNA gene sequencing by quantitative PCR (qPCR), which can enumerate and identify specific bacterial species in a water sample within 3 to 6 hours at a sensitivity of 1 to 10 cells/mL. The WCSB industry is transitioning from SDC/MPN toward qPCR for frac-water quality assurance because the 28-day incubation of SDC is incompatible with the 2 to 5 day frac-water treatment and injection timeline on a Montney well pad.
• Biofilm control in injection and production systems: Bacterial populations in oilfield systems rarely exist as free-floating planktonic cells; they predominantly attach to metal surfaces and form biofilms: structured communities of cells embedded in an extracellular polymer matrix (polysaccharide slime) that provides mechanical protection, nutrient concentration from the flowing water, and a reduced-oxygen microenvironment that supports SRB even in nominally oxygenated surface systems. Biofilm communities are 10 to 1,000 times more resistant to bactericide than planktonic cells of the same species because the polymer matrix restricts biocide diffusion to the cell surface, the outer layer of the biofilm acts as a sacrificial sink for the biocide before it reaches the inner layers, and the metabolically dormant cells in the biofilm interior are less susceptible to mechanisms that require active metabolism. Effective biofilm control requires biocides that penetrate the polymer matrix, which typically favours smaller-molecule non-oxidising biocides (DBNPA, THPS) over larger quaternary ammonium compounds; alternating between biocide classes to prevent resistance development; combining a biocide with a surfactant that disrupts the biofilm polymer matrix and exposes the cells to the biocide; and periodic mechanical pigging of pipelines and vessels to physically remove established biofilm deposits before they become thick enough to shield the inner cell layer from any chemical treatment.