BAKER HUGHES - Drilling Fluids Reference Manual

Hydraulics
Cation Exchange Capacity (CEC)
Commercial bentonite, other clays, and many chemicals exhibit a capacity to absorb a methylene blue
solution (C l6 H l8 N 3 SCl•3H 2 O). A standardized methylene blue solution is outlined in API Bulletin RP
13B-1. The testing procedure is described in the Fluid Facts Engineering Handbook. If the
absorption effects of all treatment chemicals are destroyed by oxidation with hydrogen peroxide
according to the test procedure, then the test results give the cation exchange capacity of only the
commercial bentonite and other clays in the drilling fluid.
As discussed in the section, Functions of Fluids, shales contain varying types and quantities of clays
within their structure. Some shales contain clays with characteristics very similar to that of
commercial bentonite, while other shales have relatively inert characteristics.
These characteristics are defined as bentonite equivalent and are directly related to their cation
exchange capacity. The term “bentonite equivalent” does not imply that the clays are bentonite.
Therefore, the differences in cation exchange capacities of commercial bentonite and drilled solids
allow the use of the methylene blue test (MBT) to distinguish between them.
The cation exchange capacity of a fluid is reported as the methylene blue capacity as follows.
cm
Methylene blue capacity
3 of methylene blue
= ------------------------------------------------------------
cm 3 of fluid
The methylene blue capacity is frequently reported as pounds per barrel equivalent (referring to
bentonite equivalent) by,
lbs m per bbl equivalent = 5 × methylene blue capacity
This equation is based upon commercial bentonite having a cation exchange capacity of 70 milliequivalents
(meq) of methylene blue per 100 g of dry bentonite. This is typically a high value for
most commercial bentonite. Depending upon the quality of the bentonite, the cation exchange
capacity will be in the range of 50 to 65 milli-equivalents of methylene blue per 100 g of dry clay.
Therefore, for proper analysis of commercial bentonite and drilled solids, a correction must be made
due to this difference. We know that the cation exchange capacity of the fluid is dependent upon the
quantity (as well as quality) of total low-specific gravity solids in the fluid.
The following equation can be written,
CEC fluid
=
ml of methylene blue solution
---------------------------------------------------------------------------------
grams of LGS
In the MBT test, we use a volume of fluid and want to determine the total quantity of low-specific
gravity solids.
where,
grams of LGS
=
vol % LGS
---------------------------- ( m L o f f l ui d) ( ρ
100
LGS )
% LGS = low-gravity solids, (e.g., 5.5%)
ρLGS = density of the low-gravity solids, g/cm3
Substituting one equation into the other, you derive,
CEC
( 100) ( mL of solution)
fluid = -----------------------------------------------------------------------------
(% LGS) ( mL of fluid) ( ρLGS)
Reference Manual
Baker Hughes Drilling Fluids
1-26 Revised 2006