BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
while drilling. A defoamer such as LD-8 may be required (0.5 to 1.5 gal/100 bbl of volume) to
control foaming.
As drilling progresses, it is usually desirable to disperse or deflocculate fluid solids and lower
API filtrate. Addition of 2 to 6 lbm/bbl of UNI-CAL ® , 4 to 12 lbm/bbl of LIGCON ® or
CHEMTROL ® X, 0.25 to 1 lbm/bbl of CMC, and caustic soda as required for a 1 to l.5 P f (10.0 to
10.5 pH) results in a fluid with excellent flow properties and a filtrate value in the range of 4 to 8
cc. To aid in filtrate control and cake quality, bentonite should be maintained (by Methylene
Blue Test) in the 15 to 25 lbm/bbl range. If improved inhibition from magnesium is desired, pH
should be maintained below 10. Magnesium begins to precipitate as magnesium hydroxide at
higher pH ranges. Rheological properties and filtration control will be less difficult, however, if
magnesium is precipitated.
Seawater fluids require substantially greater additions of caustic soda for alkalinity control than
freshwater systems. This is due in part to the loss of hydroxyl ion reacting with magnesium, drill
solids, and commercial clay.
In an effort to improve inhibition, lime or gypsum base seawater fluids are sometimes employed.
UNI-CAL is normally used as a deflocculant and CMC, MIL-PAC, or MILSTARCH are
generally required for filtration control. Although some improvements in stabilizing gumbo
shales have been observed with this approach, the most effective method for preventing bit and
collar balling and flow line plugging while drilling highly bentonitic gumbo shales is controlled
drilling. This reduces the packing action of drilled solids. These hazards exist when the total
clay solids approach or exceed 10%. In some areas of drilling activity it may be a lesser amount
and in others a higher concentration of solids can be tolerated. It is the instantaneous penetration
rate that is the culprit impacting the packing action. If a penetration rate of 90 feet per hour is the
calculated rate to prevent balling, that means 90 feet per hour for the total stand. Not 20 minutes
and then reaming the hole for the remainder of the hour.
In summary, seawater is often used in offshore drilling operations to avoid transportation of
freshwater. Non-dispersed seawater fluids also offer substantial inhibition against clay swelling
and are sometimes effective in stabilizing bentonitic intervals while drilling the surface hole. If a
dispersed fluid with low fluid loss is required, treating costs will be considerably greater than
with comparable freshwater fluids. For this reason, seawater is often used on the surface portion
of the hole in many operations. Chlorides are then gradually reduced with freshwater additions as
rheological properties and filtration control requirements become more stringent.
Saturated Salt Water Fluids
Saturated salt water fluids are generally limited to drilling operations encountering salt
formations. Saturated salt fluids are prepared by adding NaCl to water up to saturation and then
adding appropriate viscosifiers and fluid loss control agents. In some instances, freshwater fluids
are converted to salt fluids by inadvertently drilling into salt formations. At such a point a
decision has to be made as to the route to take while drilling the salt interval. Let the formation
saturate the fluid and start treating the system with salt tolerant additives, or stop the drilling
process and saturate the active system or dump the active system and build a saturated salt mud
from scratch.
Preparation of Saturated-Salt Fluids
Initially, sufficient salt should be added to saturate the system. At 60°F, approximately 127 lb of
salt per barrel of freshwater is required. A volume increase of approximately 13% is obtained
when 127 lbm/bbl of salt is added.
Reference Manual
Baker Hughes Drilling Fluids
3-34 Revised 2006