BAKER HUGHES - Drilling Fluids Reference Manual

Hydraulics
% LGS % Bentonite
+ ( CEC DS )( ρ DS ) ⎛---------------------------------------------------------
– ⎞
⎝ 100 ⎠
Solving for % Bentonite gives,
% Bentonite
% L GS[ ( CEC avg )( ρ LGS )–
( CEC DS )( ρ DS )]
= ----------------------------------------------------------------------------------------------------------------------
( CEC bent )( ρ bent )( CEC DS )( ρ DS )
If we again assume that all low-gravity solids have a density of 2.6 g/cm 3 then,
% Bentonite
% LGS( CEC – CEC avg DS )
= -----------------------------------------------------------------------
CEC – CEC bent DS
To convert the volume % Bentonite to pounds per barrel,
lb /bbl Bent onite = % Bentonite
m ( )( 9.1)
After the volume % drilled solids is found, conversion is made to pounds per barrel,
Summary
lb m
/bbl DS
= (% DS) ( 9.1)
The solids analysis equations in this and other chapters are based upon numerous assumptions and test
results from a fluid check. The potential errors are obvious; however, systematic use of these
assumptions and test results will provide information during the drilling operation in which trends
should be analyzed rather than a single value. In some cases, wrong assumptions or poor testing can
lead to erroneous calculated values.
CEC values for the bentonite and drilled solids should be measured whenever possible. However,
when measurements are not possible, assume CEC bent to be 60 milli-equivalents (meq) per 100 g.
Table 1-3 can be used to find a typical value for CEC DS in the region.
Drilling Fluids pH and Alkalinity
The pH of a drilling fluid may be defined as the negative logarithm of the hydrogen ion (H + )
concentration. At any particular hydrogen ion (H + ) concentration, there is a corresponding hydroxyl
ion (OH – ) concentration which will result in equilibrium. The hydrogen ion represents the acidic
portion and the hydroxyl ion the alkaline or basic portion of the solution. Freshwater normally has an
equal concentration of hydroxyl and hydrogen ions and a pH near 7, which indicates a neutral
condition. Addition of a basic material such as caustic or lime would increase (OH – )concentration
and pH, whereas an acid would increase (H + ) concentration and reduce the pH. The maximum
concentration of hydroxyl ions would result in a pH of 14, whereas the maximum concentration of
hydrogen ions would result in a pH of 0.
The pH of a drilling fluid is determined either by the colorimetric method or the electrometric
method. The colorimetric method utilizes chemically-treated pHydrion paper which is placed on the
fluid's surface until a color change is noted. The color observed is matched with a color chart on the
side of the dispenser. If the salt concentration is greater than 16,000 mg/l Cl¯, pH paper is not
recommended. The electrometric procedure employs a pH meter with a glass electrode. Although
more accurate than pHydrion paper, it is quite sensitive to shock and difficult to maintain under field
conditions.
Reference Manual
Baker Hughes Drilling Fluids
1-30 Revised 2006