What Are Some Contaminants In Drilling Fluids

In general, a contaminant is any material that causes undesirable changes in drilling fluid properties. Solids are by far the most prevalent contaminant. Excessive solids, whether commercial or from the formation, lead to high rheological properties and slow the drilling rate. Most other contaminants are chemical in nature and require chemical treatment to restore fluid properties. While there are specific treatments for each contaminant, it is not always possible to remove the contaminant from the system.

Some contaminants can be predicted and a treatment started in advance. The predictable contaminants are: cement, make-up water, and sometimes salt, gypsum, and acid gases such as, hydrogen sulfide and carbon dioxide. Pretreatment can be advantageous as long as it is not excessive and does not adversely affect mud properties.

Other contaminants may be unexpected and unpredictable such as those whose concentration increases gradually. Eventually, the contaminant shows its effect by altering the fluid properties. This change in fluid properties often occurs at times when deflocculants are expended at high downhole temperatures. It is essential to keep accurate records of drilling fluid properties to ensure that any gradual buildup of a contaminant is monitored and detected.

Some of the more common contaminants and associated treating agents are shown in Tables1 and 2. It should be noted that water chemistry is more straightforward than the chemistry of drilling fluids. Organic materials not only interfere with the accuracy of titrations, but they also interfere with the treatment. For example, the addition of calcium ion to remove carbonates may result in the formation of calcium salts of organic acids (reaction between Ca++ and lignite) and calcium silicates. Both of these reactions are undesirable, but unavoidable. Treating contaminants, therefore, should be preceded by pilot testing and then, treating should be done with caution, particularly when high density fluids are involved.

Common Contaminants and Treating Agents

Contaminant

Treating Agent lb/bbl of Treating Agent to React with 100 mg/L Contaminant

Calcium

Soda Ash

Ü.Ü93 Ü.Ü74 Ü.116 Ü.118 Ü.Ü43 Ü.Ü43 Ü.14Ü Ü.1Ü6

Sodium Bicarbonate

Magnesium Carbonate

Caustic Soda Gypsum (if high pH) Lime Lime

Zinc Carbonate

Bicarbonate

Sulfide

Zinc Oxide

Table 2

Drilling Fluids Contaminants (A contaminant is anything that causes undersirable changes in mud properties.)

Contaminants

Source

Treatment

Barite

Bentonite

Dilution, dispersion, displacement or mechanical removal

Carbonates

CO2 intrusion; thermal or bacterial degradation of mud products; entrained air (from mud hoppers, shake screen or hydrocyclones)

Precipitate CO3 = ion with gypsum or lime

Anhydrite, Gypsum

Formations drilled; overtreatment of carbonate problem with gypsum

Convert to gypsum or lime mud; or precipitate Ca++ ion with bicarbonate of soda and/or soda ash

Salt, Saltwater

Formations drilled; formation water

Convert to salt saturated mud or treat system with water, caustic soda and thinners

Cement, Lime

Cementing operations or over treatment of carbonate problem with lime

Convert to lime mud or precipitate Ca++ ion with bicarbonate of soda and/or soda ash

Hydrogen Sulfide

Formations drilled; thermal degradation of products; bacterial degradation of products

Raise pH and/or precipitate sulfide with a zinc compound

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Responses

  • harriet mackenzie
    What are some contaminants in drilling fluids?
    6 years ago
  • janet brown
    How to treat carbonate contamination?
    3 years ago
  • Petteri
    What contaminants leave black specs in drilling mud?
    1 year ago

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