BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water Based Fluids
Therefore, M f values in excess of five (5) may be indicative of carbonate and bicarbonate anions
or could be caused from a highly treated lignosulfonate/lignite fluid as well.
Figure 4-2
Ions Present at Various pH Levels
11.4
pH
OH¯
or
OH¯ and CO 3
=
Filtrate pH
8.3
OH¯
or
OH¯ and CO 3 =
or
CO 3 = and HCO 3¯
P f
ml
0.02N
H 2SO 4
4.3
HCO 3 ¯ and CO 2
Interfering ions
CO 2
The common alkalinity producing ions which can be present at various pH levels are shown in
Figure 4-2 in relation to the P f and M f alkalinities.
Estimation of Concentration of Carbonates / Bicarbonates
Treatment of carbonates/bicarbonates usually means lime or gypsum and caustic soda additions
and requires a reasonably accurate measurement of CO = 3 /HCO – 3 ions. Over treatment with either
lime or gypsum at a high temperature could lead to temperature flocculation or solidification.
Therefore, caution should be exercised in most cases before treating a CO = 3 /HCO – 3 problem and
treatment amounts should be supported with documented pilot testing. Before treatment can start,
an estimation of the amounts of carbonates in the mud should be made.
The API has standardized three methods for the measurement of carbonates in drilling fluids.
The recommended method is the Garrett Gas Train (GGT) which is the only method of the three
which directly measures carbonates. All other methods use alkalinity data to infer the
concentration of carbonates and bicarbonates in the fluid. The GGT procedure is discussed
briefly in this chapter and a complete description of the test is given in API Bulletin RP 13B-1.
The other two methods are the traditional P f /M f titration and the P1/P2 titration. The P1/P2
method was developed to overcome limitations in the P f /M f method which tends to greatly over
estimate the bicarbonate concentration, particularly in highly buffered systems. The P1/P2
titration uses barium chloride to precipitate barium carbonate. Because barium chloride is
poisonous and because the titrations are very sensitive, caution must be exercised when the
method is used. Figure 4-1 compares the advantages/disadvantages of the two methods.
A fourth alternate mathematical model has been developed using the pH/P f relation to estimate
probable carbonate/bicarbonate concentrations and is illustrated in this chapter.
Baker Hughes Drilling Fluids
Reference Manual
4-8 Revised 2006