BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
The ten-minute gel strength reading shows a progressively increasing tendency.
Fluid loss and particularly the HT/HP increases unacceptably and is difficult to control.
Mathematical modeling of fluid chemistry indicates CO = 3 /HCO – 3 .
Oven tests show a fluid problem.
Calculated values using the pH/P f method can reveal a trend or even a sudden occurrence of
CO = 3 /HCO – 3 , but these values can only be used as a guideline and thorough pilot testing should
be done.
Requirements for the pH/P f method are:
1. Accurate pH – The pH of the filtrate should be determined with a pH meter. The pH of the
fluid can be used if filtrate volume is limited.
2. Pf – Alkalinity of the filtrate.
3. Water Fraction – Calculated for unweighted fluids and retort cook-off for weighted fluid.
Equations
1. Hydroxyl concentration in mg/L (OH)
pH - 14
OH¯ = 17,000 x 10
2. Carbonate concentration in mg/L (CO 3 = )
CO 3 = = 1200 (P f – (OH / 340))
HCO =
3
CO
3
9.7
10 pH
Bicarbonate concentration in mg/L (HCO 3¯)
Example calculation using pH/Pf method given the values for pH and Pf
pH = 12.6
Pf = 5.3 cc
OH¯ = 677 mg/L
(OH - = 17,000 x 10 pH – 14)
= (17,000 x (1012.6 − 14) = (17,000 x (10 − 1.4) = (17,000 x 0.039810717) = 677
mg/l
CO3= = 3971 mg/L
CO3= = 1200 (Pf – (OH / 340)) = 1200 (5.3 – (677 / 340)) = 1200 (5.3 – 1.9912) =
1200 (3.309) = 3971 mg/l
HCO3¯ = 5 mg/L
HCO3– = (CO3=pH – 9.7) = (3971/10 12.6– 9.7) = (3971/10 2.9) = (3971/794.328) =
5 mg/l
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, calculated values should be
used within the total context of what is going on with the fluid, the hole, and the drilling
operation.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 4-11