BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Note:
Quebracho, as a deflocculant, is rarely used and is only mentioned in case it is encountered
while drilling in a remote location.
Lignite materials (LIGCO ® , LIGCON ® , and CHEMTROL ® X) are normally used for filtration
control and are sometimes used in low-solids, freshwater fluids as thinners. Generally, lignitic
materials are less effective as thinners than tannates and lignosulfonates. When treating with
lignitic materials, first add sufficient CAUSTIC SODA (1:2 to 1:4 mixture) to obtain a pH of 9.5
to 10.5, otherwise the product is not soluble and non-functional. Treatments of 1 to 2 lbm/bbl are
normally recommended when these materials are used as thinners in freshwater fluids.
The tannin and lignin fluids of the 1940s and 1950s gave rise to newer modifications. Although
there are still applications for this technology - in shallow holes - today's deeper holes often
require higher temperature stability and broader tolerance to salt.
The tannins most commonly to be used are DESCO ® and DRILL-THIN ® , which is a
sulfomethylated quebracho, applicable across a wide range of pH, with the optimum being 9 to 11
pH. Salinity tolerance from freshwater to saturated is also possible.
DESCO ® CF is a chrome-free version of DESCO. Additional technical information on DESCO,
DESCO CF, and DRILL-THIN can be obtained from Drilling Specialties Company technical
literature.
Calcium-Base Fluids
Clay swelling is generally attributed to the penetration of water along the planar surfaces of the
mineral, increasing the cleavage spacing between adjacent sheets. These individual clay layers
are stacked one upon another in varying thicknesses and held together by cations such as sodium
and calcium, which occupy sites on the silica sheets.
The degree of volume increase depends upon the clay that is hydrated and the cation type and
concentration in the surrounding water. Sodium clays give the greatest hydration. Because of the
dissociation of the sodium cation, the attractive forces which bond the platelets together are
weakened to the extent that individual sheets literally float away from each other and disperse in
the aqueous environment. This dispersion is even greater with agitation and high pH. On the
other hand, calcium clays exhibit considerably less hydration because the calcium ion does not
readily dissociate from the silica sheet and attractive forces remain strong.
For equal concentrations, sodium clays have the capacity for greater expansion than calcium
clays. The degree of expansion decreases as the clay type changes from sodium to calcium.
Consequently, sodium clays have greater viscosity building potential in freshwater fluids than do
calcium clays.
Obviously, it is not always possible to predict the type of clays that will be encountered in drilling
operations, but we can control the type of environment to which these cuttings are exposed.
Since the rate and degree of cation dissociation is directly related to the type and concentration of
cations present in the surrounding water, we must also consider the process of cations leaving the
surrounding aqueous environment and replacing the cations attached to the basal plane surface.
The relative replacing power of one cation for another is,
(highest) H + → Ca ++ → Mg ++ → K + → Na + → Li + (lowest).
This indicates that the calcium ion will readily displace the sodium ion. This phenomenon is
referred to as Base Exchange. It should be noted, however, that this process is also subject to
Reference Manual
Baker Hughes Drilling Fluids
3-38 Revised 2006