Drilling Fluid Contamination: Carbonate/ Bicarbonate

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One of the most unrecognised types of contamination is that due to Carbonate and Bicarbonate ions. Usually such contamination is gradual, resulting in increased gel strengths and 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 the mud alkalinity, in particular the ratio of Pf to Mf . The following table shows how the results should be interpreted and the effect of the differing types of alkalinity on the mud.

RATIO

TYPE OF ALKALINITY

EFFECT OF MUD

Pf = 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 mud systems.

Diagnosis by the Garret Gas Train

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

CARBONATE CONCENTRATION

INTERPRETATION

300 - 500 ppm

Background

600 - 800 ppm

May cause problems

>1000 ppm

Will cause severe problems

Treatment

1. 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 concentration shown in table.

4. Whilst treating with gypsum CONTINUE to add caustic to maintain pH in 9.5 - 10 range.

ADDITIVE

TREATMENT LEVEL
PER 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 are made.

2. Treatments should be made through a chemical barrel and must not exceed
7 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 be maintained with caustic and lime in the ratio two cans caustic to one sack lime.

 

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