Assessing the Magnitude and Consequences of Reservoir Souring Antimicrobial Treatments
Periodic batch treatments with organic biocide in injection water are common practice, partly to control microbial corrosion and biofouling problems in surface injection facilities but also as an attempt to combat reservoir souring. However, numerous reservoirs have soured despite regular weekly batch biocide dosing over many years. Even where biocide treatments are rigorously applied, conventional batch chemical treatments struggle to exert a long-term anti-souring effect on susceptible reservoirs. This is due to the combined effects of mixing, dilution and reaction of chemicals in the reservoir, leading to inadequately low doses of biocide as the site of SRB activity moved further from the injection well bore owing to cooling (shown schematically in Figure 5).
In the case of mixed produced water re-injection (PWRI) and seawater injection into susceptible reservoirs it can clearly be shown from laboratory simulation studies that conventional biocide treatments do not stand a chance of alleviating biogenic souring and this has been proven to be the case in practice.
In recent years, continuous calcium and sodium nitrate treatments have gained popularity as an alternative to biocides. These treatments stimulate a widespread group of bacteria called the nitrate-reducing bacteria (NRB), which essentially outcompete SRB for the available carbon and energy sources. Nitrate must be injected continuously to be effective but that is possible because it is a much less expensive chemical than an organic biocide; the cost of batch biocide injection is often similar to that of continuous nitrate injection. Nitrate has the added advantages of being safe to handle, non-toxic to personnel and the environment and potentially effective in removing sulphide from water as well as preventing its formation. Unfortunately, nitrate is not a panacea for reservoir souring control; it has proven very effective in some field applications and disappointing in others.5 Furthermore, nitrate injection has been implicated in enhanced pitting corrosion in certain PWRI applications, though never in seawater injection.6
Sulphate removal from injection water (more accurately described as sulphate reduction) is well established for control of barium sulphate scale and has long been mooted as a means to combat reservoir souring by ‘starving’ the SRB of their sulphate supply. This has beneficial potential, but only if the process is very efficient, because most biogenic souring is not sulphate-limited and so only a
small fraction of the available sulphate is actually turned into H2S. In such circumstances, reducing the sulphate content of the injection water by even 90 % may have almost no impact on souring.
In summary, current strategies are desulphation and/or nitrate treatments where appropriate, coupled with rigorous application of batch biocide treatments and understanding and modelling the risks and potential limits of souring on a reservoir-specific basis – hoping for the best and planning for the worst.
Materials Consequences
H2S alters the corrosion behaviour of metals and introduces the risk of cracking mechanisms that do not occur in ‘sweet’ fluids. One of these cracking mechanisms is catastrophic, i.e. it results in sudden bursting of pressurised components such as pipelines and vessels.
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
Figure 5: Mixing, Dilution and Reaction of Biocide Batch Doses in the Reservoir
Plug-flow in pipeline Effective slug-dose
Many resovoirs have soured due despite regular weekly batch biocide dosing over many years
Carbon and low-alloy steels are the most widely used construction materials in production facilities. Under sweet conditions, carbon steel in production systems normally suffers general carbon dioxide corrosion, which involves the loss of metal more or less uniformly across the exposed surface and can be controlled by
chemical inhibition. The role of H2S is to cause the formation of a protective iron sulphide film, reducing the overall general corrosion rate. While this effect may appear to be beneficial, unfortunately the local pitting corrosion rate increases in the presence of sour fluids. Where the iron sulphide film does not fully form, or is otherwise lost in service, localised pitting
corrosion proceeds at an unmitigated rate. It seems that H2S diffusion into a pit is limited and this prevents replacement/repair of the sulphide film.
A range of corrosion-resistant alloys (CRAs) are used in conditions that are too corrosive towards carbon steel. CRAs, including stainless steels, nickel alloys, titanium alloys, etc., all rely for their corrosion resistance on the formation of a thin (a few µm) oxide film on their surfaces. This oxide film is susceptible to localised breakdown by the
Numerous reservoirs have soured despite regular weekly batch biocide dosing over many years.
combined presence of chloride ions and dissolved H2S, creating a risk of pitting corrosion in CRAs. That risk is a function of factors
such as pH and temperature, as well as chloride ion and H2S partial pressure. Defining complete domains of resistance to pitting
corrosion for even the most commonly used CRAs would be a massive task, therefore it has not been done. This means that operators are frequently wasting money by over-specifying in order to create a safety margin.
However, it is sulphide-related cracking, rather than pitting corrosion, that presents the most serious materials issue with respect to reservoir souring. As long ago as the late 1940s there were
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Ineffective diluted dose for souring mitigation
Mixing, dilution and reactivity in resovoir
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