BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water Based Fluids
Note:
Barium chloride in the smallest amount is lethal and extreme caution should be observed to
avoid ingestion
When P 1 > P 2
When P 1 < P 2
OH
CO
−
HCO
( mg L) = 340×
( P 1
− P )
/
2
= ( mg L) = 1200×
( P − ( P − ) )
3
/
1 1
P2
( mg L) = 1220×
( P − )
−
3
/
2
P1
( mg / L) = 1200 Pf
=
CO ×
3
Garrett Gas Train Measurement (CO3= / HCO3¯)
The Garrett Gas Train (discussed in the Fluids Testing Procedures Manual and in API Bulletin
13B-1) is the most accurate test to determine CO 3
=
/ HCO 3 – concentrations. The test procedure
determines total carbonates and does not differentiate between CO 3
=
and HCO 3 – . The procedure
utilizes acid (5N sulfuric) to reduce the CO 3
=
/ HCO 3 – to CO 2 gas in a known volume of filtrate.
The gas is volumetrically passed through a Dräger Tube. A Dräger Tube is a sealed glass tube
containing chemicals that react with specific classes of compounds. When the specific compound
passes over the chemicals they stain or change colors. In the case of carbon dioxide dräger tubes
they stain purple in the presence of CO 2 gas. Using the measured sample volume, the Dräger
tube’s stain length and a tube factor, the total soluble carbonates can be determined as below.
The GGT is also recommended for sulfide (via H 2 S) analysis. For CO 3 = / HCO 3
–
determination in
mg/L and consequent treatment levels, measurement should be done on the fluid filtrate to show
soluble CO 3 = /HCO 3 – . Measurement is sometimes done on the whole fluid, but this is not the
preferred approach.
CO 2 (mg/L) = tube stain length x tube factor / ml of filtrate sample
If CO 3 = /HCO 3 – is considered a contaminant, limits are often set whenever measured with a
Garrett Gas Train since it is considered more accurate. A range of acceptable CO 3 = /HCO 3 – values
is often set at 1000 to 3000 mg/L and should be correlated with the other fluid properties. The
Garrett Gas Train should be kept in good working order at all times to ensure accurate
measurements.
pH / Pf Method
This technique was developed from a mathematical model of carbonate equilibrium and is a
simple test that requires a minimum of time and equipment. Calculation of CO = 3 /HCO – 3 using
the pH/P f model should be done on a routine basis in areas where a carbonate/bicarbonate
problem is known to exist. However, it is not without margin of error and the results should only
be used as a guideline within the total context of what is happening to the fluid, the hole, and the
drilling operation.
Steps should be taken to address a CO = –
3 /HCO 3 problem situation in a reasonable time frame
whenever the following occurs.
Viscosity becomes difficult to control.
Baker Hughes Drilling Fluids
Reference Manual
4-10 Revised 2006