STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

672 Drilling and Well Completions
used to drill below the salt beds, although high resistivity of the muds may result
in unsatisfactory electric logs.
Conventional saturated salt muds are composed of attapulgite or “salt” clay
and a starch, mixed with saturated brine water. The available make-up water or
freshwater mud has to be saturated with salt (sodium chloride). Freshwater
requires about 125 lb/bbl of salt to reach saturation; it then weighs approximately
10 lb/gal. Saltwater mud made up of 20 lb/bbl attapulgite clay has a
funnel viscosity of about 40 s/qt, and a plastic viscosity of about 20 cp.
Preparation of the saturated saltwater mud from freshwater mud requires the
dumping of approximately half of the original mud, then saturation of the
remaining original mud with salt and simultaneous extensive water dilution to
avoid excessive viscosity buildup. Starch is used for filtration control in saturated
saltwater muds at temperatures below 250°F. For higher temperatures (to 300”F),
organic polymers must be used. Polymers and starches are not effective in the
presence of cement or calcium concentration at high pH. If starches are used
for filtration control, the salt concentration must be kept above 260,000 ppm
or the pH above 11.5 to prevent fermentation. Alkalinity of the filtrate (P,)
should be kept at approximately 1 to control free calcium.
A modified saturated saltwater mud is prepared with bentonite clay by a
special technique. First, bentonite is hydrated in freshwater, then treated
with lignosulfonate and caustic soda. This premix is then mixed with saltwater
(one-part premix to three-part saltwater). The mixture builds up a satisfactory
viscosity and develops filtration control. Thinning of the mud is accomplished
by saltwater dilutions; additional premix is required for viscosity and water
loss control.
Nondispersed Noninhibited Systems. In nondispersed systems, no reagents are
added to specifically deflocculate the solids in the fluid, whether they are formation
clays or purposely added bentonite. The main advantage of these systems is to use
the higher viscosities and, particularly, the higher yield point to plastic viscosity ratio.
These altered flow properties provide better hole cleaning. They permit lower
annular circulating rates and help prevent bore hole washouts.
Also, the higher degree of shear thinning provides for lower bit viscosities.
This enables more effective use of hydraulic horsepower and faster penetration
rates. In addition, shear thinning promotes more efficient operation of the solids
removal equipment.
Low Solids-Clear (Fresh) Water Muds
It is a well-known fact in drilling practice that clear (fresh) water is the best
drilling fluid as far as penetration rate is concerned. Therefore, whenever
possible, drilling operators try to use minimum density and minimum solids
drilling fluids to achieve the fastest drilling rate. Originally, the low solids-clear
(fresh) water muds were used in hard formations, but now they are also applied
to other areas.
Several types of flocculents can be added to clear water to promote the
settling of drilled solids by flocculation. They are effective in low concentrations.
The manufacturer’s recommendations usually indicate lbs of flocculent per 100
ft of hole drilled. The typical application is prepared as follows:
a. Mix the polymer (flocculent) in a chemical barrel holding freshwater that
has been treated for hardness with soda ash; the proportions are approximately
5 lb of polymer per 100 gal of water.