Scavenger

Key Takeaways

• H2S scavenging in produced gas and crude oil uses triazine-based liquid scavengers (monoethanolamine triazine, MEA-triazine, being the most common) that react irreversibly with H2S through a condensation reaction to form non-volatile dithiazine compounds, providing batch or continuous treatment of H2S-containing streams without requiring an amine absorption column or regeneration system: the triazine reaction with H2S proceeds rapidly at ambient temperature (reaction half-lives of seconds to minutes at stoichiometric concentrations), consumes one mole of triazine per mole of H2S (plus an additional mole for the intermediate reaction), and produces dithiazine ring compounds that are non-volatile and non-toxic at the concentrations encountered in treated streams; triazine-based scavengers are used at concentrations of 100 to 10,000 ppm in liquid crude oil and condensate systems (providing sweetening of sour crude for pipeline and tanker transport), as continuous injection into gas gathering lines (providing H2S reduction to meet sales gas specifications below 4 ppm H2S in most markets), and in batch treatment of storage tanks and produced water systems; the limitations of triazine scavengers include their relatively high cost at high H2S concentrations (where the stoichiometric triazine dose becomes large), the tendency for dithiazine reaction products to form solids in gas streams under certain temperature and pressure conditions (creating potential for equipment fouling), and the incompatibility of some triazine formulations with specific crude oil chemistries (where precipitation or emulsion formation can occur).
• Oxygen scavenging in injection water and completion fluids is critical in systems where even trace amounts of dissolved oxygen (parts per billion level in some corrosion-sensitive applications) can cause severe localized corrosion of carbon steel tubing and casing, microbially-influenced corrosion (MIC) from oxygen-stimulated growth of sulfate-reducing bacteria, and accelerated chemical degradation of oxygen-sensitive scale inhibitors and biocides: the standard oxygen scavenger for oil and gas water injection systems is sodium bisulfite (NaHSO3) or ammonium bisulfite ((NH4)HSO3), which reacts with dissolved oxygen to form sulfate (sodium or ammonium sulfate) at a stoichiometric dose of approximately 7 to 8 ppm bisulfite per ppm of dissolved oxygen; the catalyzed bisulfite reaction (using cobalt or nickel catalysts at 0.1 to 1 ppm concentration) achieves near-complete oxygen removal within seconds to minutes at ambient temperature, while uncatalyzed bisulfite requires higher temperature or longer contact time for efficient reaction; the bisulfite reaction product (sulfate) is essentially harmless to well and formation chemistry at the concentrations encountered in water injection systems; alternative organic oxygen scavengers (hydrazine, ascorbic acid, and various proprietary formulations) are used in specific applications where bisulfite is incompatible (for example, hydrazine is used in high-pressure steam generation where bisulfite decomposes to sulfur dioxide at elevated temperature, but hydrazine's carcinogenicity has driven the industry toward less hazardous alternatives).
• Iron scavenging in acid stimulation treatments prevents the formation of iron sulfide and ferric hydroxide precipitates that can plug the formation face and the wellbore when acid treatment fluids containing dissolved iron (from corrosion of tubing or scale dissolution by the acid) mix with formation brines containing hydrogen sulfide or high pH water: the formation of ferrous sulfide (FeS) from ferrous iron reacting with hydrogen sulfide, and the formation of ferric hydroxide gel from ferric iron reacting with high-pH formation water, are both highly damaging to formation permeability because these compounds precipitate as fine particles that plug pore throats and can be very difficult to remove by subsequent acid treatment; iron scavengers for acid stimulation systems include acetic acid (which complexes ferrous iron and prevents its oxidation to ferric iron by keeping it in a soluble complex), citric acid (which forms strong chelate complexes with both ferrous and ferric iron that remain soluble at higher pH than uncomplexed iron), and erythorbic acid (which reduces ferric iron back to ferrous iron, preventing ferric hydroxide precipitation); the iron scavenger is typically added to the acid stage at concentrations of 0.5 to 5 percent by weight, with the required dose calculated from the expected dissolved iron concentration in the spent acid (estimated from the corrosion inhibitor's iron leaching potential and the iron mineral content of the formation being treated).
• H2S scavenging in drilling fluids prevents the toxic and corrosive effects of formation H2S entering the mud system during drilling through sour zones, protecting the drill crew from H2S inhalation hazards, preventing sulfide stress cracking (SSC) of high-strength drilling equipment, and avoiding degradation of the mud system by sulfide contamination: H2S that enters the drilling mud from a sour formation reacts with iron in the mud (from barite impurities and steel equipment corrosion) to form iron sulfide particles that increase the mud weight, degrade filter cake quality, and can cause differential sticking; zinc-based scavengers (zinc carbonate, zinc oxide, and zinc-polyamine complexes) react with H2S in the mud to form insoluble zinc sulfide, removing the H2S from solution at a reaction rate that is fast enough to prevent H2S from equilibrating to the gas phase and entering the breathing zone above the mud pits; the zinc sulfide reaction product is stable and non-reactive with the other mud components, remaining suspended in the mud as fine particles that do not significantly impair mud performance; the scavenger dose is calculated from the expected H2S concentration in the formation and the anticipated mud flow rate, with additional contingency for periods of high formation H2S flux during connection gas or well control events; monitoring of residual scavenger capacity in the mud (by testing the mud's ability to absorb a fixed H2S dose) confirms that adequate scavenger inventory is maintained throughout the H2S-exposed drilling interval.
• Spent scavenger disposal and environmental management is an important consideration in scavenger selection because the reaction products of many scavengers are subject to environmental regulations governing the disposal of produced water, drilling waste, and chemical waste in onshore and offshore environments: triazine-based H2S scavenger products and their dithiazine reaction products must be evaluated under the offshore chemical notification system (HOCNF in the North Sea, CHARM in Australia) before use in offshore drilling and production where spent scavenger may be discharged to the sea with produced water; zinc-based scavengers in drilling mud are restricted in some jurisdictions because zinc is a heavy metal with aquatic toxicity concerns, requiring that the drill cuttings from zinc-treated mud be managed as hazardous waste rather than discharged to the seabed; bisulfite oxygen scavengers are generally considered environmentally acceptable because sulfate is a natural constituent of seawater at much higher concentrations than the bisulfite treatment adds; the trend in scavenger development is toward more readily biodegradable, lower-toxicity formulations that meet both performance requirements and environmental acceptability criteria, driven by offshore discharge regulations and sustainability commitments by operators that make the environmental footprint of chemical treatments an explicit selection criterion alongside performance and cost.