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BIOFOULING OF WELLS - CAUSES AND SOLUTIONS
An On-Line Version of a Column First Published in:
By: David B. Vance firstname.lastname@example.org
Well fouling can occur through physical, chemical, or biological processes. Biological processes are estimated to be associated with 80% of well plugging events. The focus of this column is on mechanisms, diagnosis, and treatment of biofouling.
Symptoms of the fouling of well screens or the adjacent formation include:
Fouling through biological activity can occur due to four primary mechanisms, in order of importance they are:
Extracellular slimes are largely composed of polysaccharides and in general are the major component of the biofouling mass. Biomass associated with viable active cells is a relatively minor component. The figure illustrates how the maximum level of biological activity commonly occurs under redox conditions that are at the periphery of oxidative in the Eh range of -50 to +150 mv.
Biofouling events can be complex and caused by a variety of bacteria. However the appearance and odor of the bacterial slimes is diagnostic.
Treatment of well fouling falls into four broad areas:
Acidification is the chemical treatment most often applied to plugged wells. It is effective, cheap, and easily applied. In addition to the solubilization of extracellular polysaccharides and precipitated mineral species, it is also bactericidal in its action. The three most commonly used acids are hydrochloric (HCl), sulfamic (H3NO3S), and hydroxyacetic (C2H4O3).
Hydrochloric acid is readily available as muratic acid at a strength of 28 to 31 percent. It should be added into a well at a volume equal to the screened interval of the well bore plus another 25 to 50%. Hydrochloric acid is an extremely effective well cleaning agent, however it does have several drawbacks: it can be corrosive to metal well screens, casings, and pump components; it is also extremely toxic and care should be taken to avoid vapors from the concentrated acid as well a vapors that may be expelled from the well bore during and after acid addition.
Sulfamic acid is used in a solid form and can be introduced into a well bore in a pelletized or granular form. It is most often used to create a 30% solution. It is most effective for treating mineral encrustations that are due to calcium of magnesium. It is less effective for iron or manganese precipitates. Although, the addition of rock salt at a dosage rate of 20% of the acid can aid in the treatment of iron and manganese encrustations. While this material should be handled with the same caution as hydrochloric acid, its potential for fume generation is significantly less. It is also relatively non-corrosive. Sulfamic acid should not be confused with sulfuric acid. Sulfuric acid is corrosive and in my experience more dangerous to deal with than hydrochloric acid. Sulfuric acid will also form insoluble calcium sulfate precipitates within the well bore exacerbating the fouling problem.
Hydroxyacetic acid is also known as glycolic acid and is commonly available as a 70% concentrate. Hydroxyacetic acid can dissolve mineral scale, act as a bactericide, and acts as a chelating agent keeping solublized minerals in solution through the entire treatment process. Treatment dosage is 1 gallon of 70% hydroxyacetic acid for every 10 to 15 gallons of water in the screened interval of the well bore.
In conclusion, it is unlikely that a single anti-fouling procedure will work every time in every well (although acid treatment comes the closest). Seasonal variations will impact the type of biofouling activity in any given well at any given site, much less the variations between different wells or sites. Injection wells in particular have a limited life span that may range from a few years to months. Restoration of the well bore and the adjacent formation is a process that will have limits, a point will be reached where it will become more economic to install a new well near the old.
Copyright 2008 David B. Vance
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