Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Calcium-Ions Contamination. The sources of calcium ions are gypsum, anhydrite, cement,
lime, seawater, and 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 (CO
3
—
, HCO
3
−
) are from
drilling a CO
2
-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 H
2
S-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.)
Definition
Common Contaminants, Sources, and Treatments

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Scavenging of H
2
S is done by use of zinc, copper, or iron. 
Salt/Saltwater Flows. 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.
Producing formation damage has been defined as the impairment of the unseen by the
inevitable, causing an unknown reduction in the unquantifiable. In a different context,
formation damage is defined as the impairment to reservoir (reduced production) caused by
wellbore fluids used during drilling/completion and workover operations. It is a zone of reduced
permeability within the vicinity of the wellbore (skin) as a result of foreign-fluid invasion into
the reservoir rock. Fig. 10.8 illustrates formation skin damage.