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A mud is said to be contaminated when a foreign material enters the mud system and causes undesirable changes in mud properties, such as density, viscosity, and filtration. Generally, water-based mud systems are the most susceptible to contamination. Mud contamination can result from overtreatment of the mud system with additives or from material entering the mud during drilling.
Common contaminants, sources, and treatments
The most common contaminants to water-based mud systems are:
- Solids (added, drilled, active, inert)
- Gypsum/anhydrite (Ca++
- Cement/lime (Ca++
- Makeup water (Ca++
, Mg++ )
- Soluble bicarbonates and carbonates (HCO3−, CO3—)
- Soluble sulfides (HS−, S—)
- Salt/salt water flow (Na+
Solids are materials that are added to make up a mud system (bentonite, barite) and materials that are drilled (active and inert). Excess solids of any type are the most undesirable contaminant to drilling fluids. They affect all mud properties. It has been shown that fine solids, micron and submicron sized, are the most detrimental to the overall drilling efficiency and must be removed if they are not a necessary part of the mud makeup. The removal of drilled solids is achieved through the use of mechanical separating equipment (shakers, desanders, desilters, and centrifuges). Shakers remove solids in the size of cuttings (approximately 140μ or larger). Desanders remove solids in the size of sand (down to 50μ). Desilters remove solids in the size of silt (down to 20μ). When solids become smaller than the cutoff point of desilters, centrifuges may have to be used. Chemical flocculants are sometimes used to flocculate fine solids into a bigger size so that they can be removed by solids-removal equipment. Total flocculants do not discriminate between various types of solids, while selective flocculants will flocculate drilled solids but not the added barite solids. As a last resort, dilution is sometimes used to lower solids concentration.
The sources of calcium ions are:
- Hard/brackish makeup water
The calcium ion is a major contaminant to freshwater-based sodium-clay treated mud systems. The calcium ion tends to replace the sodium ions on the clay surface through a base exchange, thus causing undesirable changes in mud properties such as rheology and filtration. It also causes added thinners to the mud system to become ineffective. The treatment depends on the source of the calcium ion. For example, sodium carbonate (soda ash) is used if the source is gypsum or anhydrite. Sodium bicarbonate is the preferred treatment if the calcium ion is from lime or cement. If treatment becomes economically unacceptable, break over to a mud system, such as gypsum mud or lime mud, that can tolerate the contaminant.
Biocarbonate and carbonate contamination
The contaminant ions (CO3—, HCO3−) are from drilling a CO2-bearing formation, thermal degradation of organics in mud, or over treatment with soda ash and bicarbonate. These contaminants cause the mud to have high yield and gel strength and a decrease in pH. Treating the mud system with gypsum or lime is recommended.
Hydrogen sulfide contamination
The contaminant ions (HS−, S—) generally are from drilling an H2S-bearing formation. Hydrogen sulfide is the most deadly ion to humans and is extremely corrosive to steel used during drilling operations. (It causes severe embrittlement to drillpipe.) Scavenging of H2S is done by use of zinc, copper, or iron.
The ions, Na+Cl- , that enter the mud system as a result of drilling salt sections or from formation saltwater flow cause a mud to have high yield strength, high fluid loss, and pH decrease. Some actions for treatment are dilution with fresh water, the use of dispersants and fluid-loss chemicals, or conversion to a mud that tolerates the problem if the cost of treatment becomes excessive.
Noteworthy papers in OnePetro
Nyland, T., Azar, J.J., Becker, T.E. et al. 1988. Additive Effectiveness and Contaminant Influence on Fluid-Loss Control in Water-Based Muds. SPE Drill Eng 3 (2): 195-203. SPE-14703-PA. http://dx.doi.org/10.2118/14703-PA.