Cement contamination occurs in every well drilled. It is contamination from calcium hydroxide which can result in increased pH, severe thickening and increased fluid loss of bentonite based mud systems (flocculation of the bentonite clay).

The severity of the problem will depend on the level of pre-treatment and thinner concentration in the mud.

In a highly treated Lignosulphonate system, the viscosity may actually drop as the clays go to an aggregated/flocculated state rather than a dispersed in flocculated state on the introduction of cement.

In polymer systems, cement contamination acts to raise both the Calcium and pH which can lead to precipitation of polymers and loss of control over the mud system. Thickening may also be observed if the mud contains significant quantities of drilled clay solids.

Sources of Contamination

The main source of this contamination is the drilling of green cement or adoption of poor placement procedures.

Diagnosis

Cement contamination will result in:

• increased pH
• an increase in Pf and calcium
• a large increase in Pm.

Treatment

• Prior to drilling cement, pre-treat the mud with 0.5-1.0 lb/bbl of sodium bicarbonate.
• If this is insufficient, treat cement contamination at the rate of  0.15 lb/bbl sodium bicarbonate per 100 ppm calcium (determine calcium using procedure below)

If sodium bicarbonate treatment levels are based on filtrate calcium only, an incorrect treatment level will be obtained.  This is because the majority of the cement will stay in suspension rather than go into solution due to the high pH.

To obtain the total calcium level, the following procedure is recommended:

1. Add 90 cm³ distilled water to 10 cm³ mud.

2. Titrate with 0.1 NH2S0₄ to a pH of 7.5 - 8.0.

3. Continue mixing for two minutes to ensure no pH rise.

4. Filter the slurry on the standard filter press.

5. Titrate 10 cm³ of filtrate with standard Versenate (001 molar).

Calcium = mls versenate x 4000

Note:

1. When large quantities of cement are drilled it may not be economical to treat out the contamination due to the large concentrations of bicarbonate required. In this case, the best course of action may be to change out the contaminated mud for new mud or drill cement with sea water if available.
2. When drilling cement, the rig crew shall be aware of the possibility of plugged/blinded screens.
3. Onshore operations tend to drill cement with well water and or mud and dump the contaminated mud.