BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water Based Fluids
Special Procedure for Low pH Fluids
When the pH is below 9.0 to 9.5 it is difficult to accurately titrate the Pf value. Therefore, the
method must be modified in those cases. The procedure is as follows.
1. Increase the pH of a sample of filtrate to 11.5 using CAUSTIC SODA (NaOH) solution
and a pH meter. The strength of the caustic soda solution will determine how much will
be added. It is recommended that a known Normality solution be made up and used for
this procedure. (Forty (40) grams of caustic in one (1) liter of water is a 1 Normal
solution.)
2. Titrate and determine the Pf value of this altered filtrate sample and calculate the mg/L of
total carbonates from this Pf value by multiplying it by 1200.
3. Calculate the bicarbonates and carbonates in the original filtrate from the equations:
where,
( Total Carbonates ) = P ( Altered pH ) × 1200
Y
f
HCO
−
3
Y
1+
10
( mg / L) =
pH − 9. 7
−
( mg L) Y −
=
CO
3
/ HCO
=
3
Y = total carbonates at 11.5 pH,
pH = original filtrate pH.
Example calculation using pH/Pf method when the pH is low.
Original filtrate pH = 8.8
Pf of altered filtrate (11.5 pH) = 5.0 cc
1. Total Carbonates (Y) (both CO = 3 and HCO – 3 ) = 6,000 mg/L
Y = Pf × 1200 = 5.0×
1200 = 6000 mg / L
Bicarbonates = 5329 mg/L
Y 6000 6000
2. HCO3 = =
=
= 5329mg
/ L
( pH − 9.7)
(8.8 − 9.7)
1+
10 1+
10 1+
0.125892541
Carbonates = 671 mg/L
3. CO = Y − HCO = 6000−5329
671mg
/ L
3 3
=
Treatment of Carbonates
Removing carbonates from a mud involves adding some form of soluble calcium. Lime or
gypsum is the most common products used to provide this source. Calcium will precipitate the
carbonates as calcium carbonate, CaCO 3 . If the carbonates are in the HCO − 3 form they must be
converted to the CO = 3 form before they will precipitate. This means that some form of hydroxyl
must be present. This can come from lime (Ca(OH) 2 )or caustic soda (NaOH). In general, for low
pH muds, lime is used to treat out bicarbonates, and for higher pH muds a combination of
Baker Hughes Drilling Fluids
Reference Manual
4-12 Revised 2006