BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
With good mixing conditions (good shear) 20 to 25 lbm/bbl of SALT WATER GEL generally
yields a funnel viscosity in the range of 35 sec/qt.
MILSTARCH should be mixed through the hopper slowly to prevent high-viscosity peaks. Four
to six lbm/bbl of MILSTARCH usually produces a 5 to 10 cc API fluid loss, depending upon the
amount and type of solids present.
Caustic soda should be added to obtain a pH of 10.0 to 11.0 (l.0 or greater P f ). Higher
alkalinities reduce the tendency to foam and minimize corrosion. Calcium and magnesium levels
are also reduced.
Organic deflocculants such as UNI-CAL and DESCO ® are beneficial in some instances for
thinning (UNI-CAL offers maximum effectiveness if dissolved in freshwater prior to addition),
but viscosity control is achieved primarily through dilution and with solids control equipment. In
some instances, small additions of MIL-PAC or CMC have been noted to have a thinning effect
on saturated salt fluids.
Since MILSTARCH will begin to degrade at temperatures in excess of 250°F, it may be
necessary to utilize supplementary additions of MIL-PAC. MIL-PAC is stable at temperatures to
about 300°F. Salt-saturated systems display a foaming tendency, but this problem can be
minimized or eliminated with defoamers such as LD-8 or W.O. DEFOAM.
Starch fermentation is generally not a problem if the system is saturated with salt or if pH is 11.5
or above. However, to insure against starch fermentation, add a suitable biocide.
Where high salinity, salt saturated produced brines are used to prepare starch based fluids, it may
be necessary to run the resulting system without pH control. Certain produced brines can exhibit
an acidic pH and be high in hardness (Ca ++ and Mg ++ ) thereby negatively affecting the addition of
Caustic Soda. In this case is usually more economical to run the system without pH control,
therefore eliminating this measure as a means of preventing fermentation. The high salinity of
the brine alone may not be a deterrent to fermentation.
Anaerobic bacteria can exist in certain produced brines. Starch based systems are a food source
for such bacteria and once exposed to these circumstances, degradation and fermentation of the
starch can proceed at a rapid rate. It will be necessary to treat the system with a biocide to
prevent fermentation prior to the addition of the starch. Once fermentation starts, it may be
impossible to get enough biocide into the system to offset the problem. In this case, treatment
with the biocide prior to adding any starch will be necessary.
Case History No. 1
Extremely high-density (20.0 to 21.0 lbm/gal) saturated-salt fluids are commonly employed in
southern Iran. These fluids are prepared with barite, saturated salt water and 12 to 18 lbm/bbl of
starch. The starch furnishes the desired filtration control. API filtrate is usually below 1 cc with
this concentration of starch.
The local barite often contains a high percentage of low-gravity solids and some of the formations
penetrated are “fluid making.” For this reason, it is essential to utilize fine screen shakers and
centrifuges. The pH is maintained in the 7.0 to 7.5 range with Caustic Soda.
Case History No. 2
The Nisku formation on the Western flank of the Williston Basin in Montana has been analyzed
to contain certain strains of bacteria. Drilling this formation releases the bacteria into the mud
system. If a starch based mud is used to drill the Nisku, it will be necessary to pre-treat the
system with a biocide to kill the bacteria before the formation is penetrated. Otherwise, the
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-35