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12
Carbonates
Testing
The concentration of soluble carbonates in a drilling fluid filtrate can be de
-
termined using the Garrett Gas Train. Total soluble carbonates include CO
2
as well as the carbonate (CO
3
-2
) and bicarbonate (HCO
3
-
) ions. Mud filtrate
is acidified in the gas train, converting all carbonates to CO
2
, which is then
evolved by bubbling nitrous oxide gas through the sample. The gas train sep
-
arates the gas from the liquid which is collected in a 1-liter gas bag to allow
the CO
2
to mix uniformly, and is subsequently drawn through a Dräger tube at
a fixed flow rate. The Dräger tube will respond to CO
2
by progressively stain
-
ing purple along its length. A reaction between CO
2
and a hydrazine chemical
causes a crystal violet indicator to turn purple. The stain length is proportional
to the total carbonate concentration in the filtrate.
