CALCIUM CONTAMINATION

Contamination with Calcium Sulphate (Anhydrite) will result in flocculation of bentonitemuds resulting in an increase in yield point, gel strengths and fluid loss.

Polymer muds are relatively insensitive to this type of contamination, unless significantquantities of drilled clay solids are present.

Sources of Contamination

This type of contamination can occur whilst drilling Anhydrite.

Diagnosis

Calcium Sulphate (Anhydrite) contamination will result in:

an increase in filtrate calcium. a decrease in pH a decrease in Pt.

Treatment

Treat out calcium with soda ash at the rate of 0.116 lbs/bbl per 100 ppm calcium. Combine treatment with lignosulphonate to regain control of viscosity and fluid loss.

Note: If large amounts of soda ash are added, the soluble sulphate may cause “Ash gels”which are high and progressive.

CARBONAT/BICARBONAT CONTAMINATION

One of the most unrecognised types of contamination is that due to Carbonate andBicarbonate ions. Usually such contamination is gradual, resulting in increased gel strengthsand yield point. It is often incorrectly diagnosed as increased solids but, in this case,application of costly thinners have little, or no effect.

Sources of Carbonates

C02 from drilled formations or air ingress. Thermal degradation of organic additives Over treatment with soda ash or sodium bicarbonate Carbonate formations

Diagnosis by Analysis of Mud Alkalinity

Diagnosis of carbonate/bicarbonate contamination is routinely carried out by analysis of themud alkalinity, in particular the ratio of Pf to Mf . The following table shows how the resultsshould be interpreted and the effect of the differing types of alkalinity on the mud.

RATIO TYPE OF ALKALINITY EFFECT OF MUDPf = 0 Bicarbonate only Unstable and very difficult to control

Pf = Mf Hydroxide only Stable and in good condition

2Pf = Mf Carbonate only Unstable but can be controlled

2Pf > Mf Carbonate and Hydroxide Stable and in good condition

2Pf < Mf Carbonate and Bicarbonate Unstable and difficult to control

Note:

This method is rather unreliable and can be masked when buffers are used in certain mudsystems.

Diagnosis by the Garret Gas Train

This is the most accurate method. However it gives carbonates/bicarbonates in terms of totalcarbonates Refer to manufacturer's instructions for details of the test procedure. Resultsshould be interpreted as follows:

CARBONATECONCENTRATION

INTERPRETATION

300 - 500 ppm Background

600 - 800 ppm May cause problems

>1000 ppm Will cause severe problems

Treatment1. INCREASE pH to 9.5 - 10.0 with caustic.2. ADD lime at concentration shown in table.3. If the required quantity of lime exceeds 7 lbs/bbl, TREAT with gypsum at concentrationshown in table.4. Whilst treating with gypsum CONTINUE to add caustic to maintain pH in 9.5 - 10 range.

ADDITIVE TREATMENT LEVELPER 100 ppm CARBONATE

Lime 0.04-0.06 lb/bbl

Gypsum 0.1 lb/bbl

Notes:

1. When adding lime or gypsum, the mud will become very viscous if large treatments aremade.

2. Treatments should be made through a chemical barrel and must not exceed7 lbs/bbl over a six hour treatment period.

3. Concentration of bicarbonate as low as 250 ppm can cause problems.

4. In areas where carbonate contamination is known to be a problem, the pH should bemaintained with caustic and lime in the ratio two cans caustic to one sack lime.

Cement contamination

Cement contamination occurs in every well drilled. It is contamination from calcium hydroxide which can result in increasedpH, 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/flocculatedstate 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 polymersand loss of control over the mud system. Thickening may also be observed if the mud contains significant quantities ofdrilled 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 pHan increase in Pf and calciuma 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. Thisis 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:

When large quantities of cement are drilled it may not be economical to treat out the contamination due to the largeconcentrations of bicarbonate required. In this case, the best course of action may be to change out the contaminated mudfor new mud or drill cement with sea water if available.

When drilling cement, the rig crew shall be aware of the possibility of plugged/blinded screens.Onshore operations tend to drill cement with well water and or mud and dump the contaminated mud.

H2S CONTAMINATION

H2S will have no adverse effects on the mud. However, the gas is highly poisonous andcorrosive.

Sources of H2S

The gas is sometimes associated with formation fluids. (Add Biocide)

Diagnosis

Gas liberated at surface will be detected by gas sensors.

Soluble sulphides in the mud are detected by use of the Garret Gas Train. The Mud Engineermust be familiar with its use and make sure that the equipment is called out and maintained ingood working order when H2S presence is expected. Refer to the manufacturer’s instructionsfor detailed test procedures.

Treatment

1. pH must be raised to a level in excess of 10 using caustic.

2. Add a H2S scavenger such as zinc carbonate.

SALT CONTAMINATION

If salt enters a bentonite based mud, the clay platelets will flocculate resulting in gellation andincreasedfluid loss (>10ml).

Polymer systems are tolerant to salt contamination. However, the salt acts to reduce the yieldof most polymers (not Starch), and in practice, high salt concentrations will require a higherconcentration of polymer for the same effect.

Sources of Salt Contamination

Drilling of salt formations Contamination from salt water aquifers.

Diagnosis

Salt contamination will result in:

an increase in Chlorides a decrease in pH a decrease in Pf.

Treatment

In serious cases the only effective treatment for salt contamination is dilution.