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14.3.2.15 specific surfactant classesAlkylaminomethylenephosphonic acid amphiphilic compounds having the general formula R′R2NCH2P(O)(OH)2, such as octylaminomethylenephosphonic acid, have been claimed as biocides. It is possible that hydrophobically modified phosphonate scale inhibitors with several phosphonate groups might also show biocidal properties but this was not claimed.104 Sulfamic acid surfactants such as dodecyl sulfamic acid have also been claimed by the same group as biocides.105

14.4 Biostats (control “Biocides” or metaBolic inhiBitors)

Biostats do not necessarily kill bacteria but interfere with their metabolic processes, controlling their growth. As discussed in the section on organic biocides, isothiazo-lones are one class of biostats, preventing the formation of iron sulfide scale (via hydrogen sulfide) by maintaining a low level of SRB and inhibiting their metabolic activity. Metabolic inhibitors deprive SRB of the ability to produce ATP, and as a result, cells are unable to grow and/or divide. This inability to grow or divide may eventually cause the death of some of the SRB; however, the cell death is not a direct result of exposure to the metabolic inhibitors as it would be for biocides. Alkylbenzyldimethylammonium salts are also a commercially significant class of biostats. A combination of biocide and biostat has been shown in laboratory stud-ies to inhibit biogenic sulfide production at significantly lower concentrations than would be required if the biocide or biostat was used alone.106

Examples of biostats that are not biocides are discussed in this section. They include:

anthraquinone•azide ions•nitrite and nitrate ions•molybdate or tungstate ions•selenate ions•

14.4.1 AnthRAquinOnE AS COntROl BiOCidE

Anthraquinone has been used as a biostat in a number of projects since the late nineties (Figure 14.20). Anthraquinone is not water-soluble, but 9,10-anthracenediol

ON

O

C2H5

N

O

CH3H3C

N

O

H2CN

O

Figure 14.19 7-Ethyl bicyclooxazolidine (left), 4,4-dimethyloxazolidine (middle), and methylene bis-oxazolidine (right).

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352 Production Chemicals for the Oil and Gas Industry

disodium salt is water-soluble and works as if it were anthraquinone.107 Anthraqui-nones have been shown to inhibit sulfate respiration in SRBs effectively shutting down the sulfide-producing mechanism but having little effect on other classes of bacteria.108–109 One water injection program managed to eliminate continuous injec-tion of a quaternary biocide with slug doses of the easier-to-handle anthraquinone product. Slug doses of synergistic acrolein biocide were used in addition.110–111 Batch treatment of anthraquinone together with THPS biocide has also been shown to be effective in stopping biogenic sulfide production in produced water tanks in slop-handling systems.112–113

14.4.2 nitRAtE And nitRitE tREAtmEnt

Nitrite (NO3−) and nitrate ions (NO2

−) as found in calcium or sodium nitrate/nitrite salts are cheap, easy to handle, environment-friendly inorganic chemicals, and becoming more and more used in the oil industry to inhibit SRB. The corrosion con-sequences of using nitrate or nitrite in oilfield brines have been reviewed.114

Nitrite directly inhibits sulfate reduction by SRB because it is reduced more slowly than sulfite by the final enzyme in the sulfate reduction pathway, dissimilatory sulfite reductase. Thus, addition of nitrite ions to produced water or seawater injection sys-tems can control biogenic sulfide formation if present in high-enough concentra-tion (depending on pH, nitrite, but not nitrate, ions also react directly with any H2S already present to form sulfur and reduced nitrogen compounds). In one field study, pulses of nitrite were more effective than the same amount of nitrite added continu-ously. Nitrite was more effective at inhibiting souring than was glutaraldehyde, and SRB recovery was delayed longer with nitrite than with glutaraldehyde.115 Nitrite injection has also been used successfully in producer wells to scavenge H2S and prevent SRB activity.116 In one case, oil production increased immediately following the treatment, probably due to the dissolution of precipitated iron sulfides in the zone surrounding the wellbore.117

There exist nitrate-reducing and sulfide-oxidizing bacteria (hNRB and NR-SOB) in most oilfields that can reduce nitrate ions to nitrite ions.118–119 Thus, the upstream petroleum industry has introduced a nitrate-based microbial treatment technol-ogy, useful for both the prevention and removal of biogenic sulfide from reservoirs, produced water, surface facilities, pipelines, and gas-storage reservoirs, as well as

O

O

Figure 14.20 Anthraquinone.

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Biocides 353

increasing oil recovery. This reservoir treatment technology works by replacing SRB with a naturally occurring suite of beneficial microorganisms enhanced by the introduction of an inorganic nitrate-based formulation.120 If the reservoir is a poor source of carbon for SRB (low in small organic acids), injection of nitrate ions will stimulate growth of “hardier” denitrifying bacteria (nutrient augmentation), thereby, dominating the system and inhibiting the growth of SRB. Production of nitrite and sometimes nitrous oxide (NO) from nitrate ions by the hNRB and NR-SOB will also directly inhibit SRB growth by acting as toxins. In addition, hNRB and NR-SOB may produce compounds that raise the oxidation-reduction potential of the environ-ment to a level that is inhibitory to the growth of SRB.121–129

Calcium or sodium nitrate are environment-friendly and complement the natu-rally occurring organic acids in the reservoir, selectively stimulating and increas-ing the targeted nitrate-reducing bacteria. Many North American gas fields have been treated successfully this way and several fields in the North Sea have already successfully used this treatment strategy for reducing, but not totally eliminating, reservoir souring, both by reactive and proactive strategies. It is a simple method for preventing biogenic sulfide formation, which can mostly eliminate the use of organic biocides.130–134

For the reasons discussed above, the use of a mixture of nitrate and nitrite ions may perform superior to a simple nitrate treatment.135 One study showed that nitrite treatment alone may be preferable in reservoirs with only SRB present.136 In addition, the use of a molybdenum compound (molybdate) may also enhance the treatment effect. Molybdenum is a known enzymatic inhibitor of the hydro-genase enzyme found in SRB.137 It should be noted that nitrate/nitrite treatment may not totally eliminate the use of biocides if they are injected very late in the system. For example, a deepwater field offshore Nigeria that has been treated with calcium nitrate still uses a biocide (THPS) to prevent biofilms in the injection facilities.138–139 A potentially large advantageous side effect of using nitrate-based water injection treatments is microbial-enhanced oil recovery (MEOR). Formation of NR-SOB biofilms in the reservoir may help to release more oil from the rock surface, which can then migrate to the producer wells. The use of nitrate, together with other nutrients such as vitamins and phosphate, has been suggested as an MEOR improvement.140

Instead of relying on indigenous bacteria, injection of non-SRB bacteria with nutrients such as nitrate ions has also been carried out, eliminating, in one case, the use of hazardous acrolein biocide.141 A novel class of bacteria that oxidize sulfide as well as oil organics with nitrate has been reported.142 There is still a lot to learn about nitrite/nitrate treatments, such as the relative effectiveness of nitrate versus nitrite ions and determining which of the several mechanisms of reducing biogenic sulfide production is dominant.

14.4.3 OthER BiOStAtS

Azide salts such as sodium azide (NaN3) have long been known as biostats and have been suggested for use in preventing biofouling of wells.143 Sodium azide acts as a

© 2009 by Taylor and Francis Group, LLC