STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

670 Drilling and Well Completions
adding the plaster at a controlled rate, the high viscosities and gels associated
with this type of contaminant can be held within workable limits. After the clay
in the base mud has reacted with the calcium ions in the plaster, no further
thickening will occur upon drilling gypsum or salt formations. Gypsum-treated
muds exhibit flat gels, and these flat gels depend in part upon the clay
concentration in the mud. Filtration control is obtained by adding organic
colloids; because the pH of these muds is low, preservatives are added to prevent
the fermentation of starch.
Gypsum-treated muds are more resistant to contamination and more inhibitive
(700 ppm of calcium ions) than lime-treated muds, and also have a greater
temperature stability (350°F). A freshwater mud can be converted to a gypsum
mud according to the following procedure:
a. Dilute with sufficient water to reduce API funnel viscosity to 35 s.
b. Add thinner (lignosulfonate) and caustic soda to avoid excessive viscosity
build up (breakover).
c. Add gypsum at the mud hopper.
To control the stability of gypsum treated muds, the following mud properties
should be maintained:
a. The mud pH should be 9.5 to 10.5; the alkalinity should be increased by
adding lime rather than caustic soda.
b. The calcium ion concentration in the mud filtrate should be 600 to 1,000 ppm.
c. Addition of gypsum is necessary to maintain the amount of excess calcium
sulfate (CaSO,) between 2 and 6 lb/bbl; the relevant tests on excess calcium
sulfate are subject to mud service on the rig.
Seawater Muds
Seawater muds or brackish water muds are saltwater muds. Saltwater muds
are defined as those muds having salt (NaCl) concentrations above 10,000 ppm,
or over 1%, salt; the salt concentration can vary from 10,000 to 315,000 ppm
(saturation).
Seawater muds are commonly used on offshore locations, which eliminate the
necessity of transporting large quantities of freshwater to the drilling location.
The other advantage of seawater muds is their inhibition to the hydration and
dispersion of clays, because of the salt concentration in seawater. The typical
composition of seawater is presented in Table 4-48; most of the hardness of
seawater is due to magnesium.
Calcium ions in seawater muds can be controlled and removed by forming
insoluble precipitates accomplished by adding alkalis such as caustic soda, lime,
or barium hydroxide. Soda ash or sodium bicarbonate is of no value in controlling
the total hardness of sea water.
Seawater muds are composed of bentonite, thinner (lignosulfonate or lignosulfonate
and lignite), and an organic filtration control agent. The typical formulation
of a seawater mud is 3.5 lb/bbl of alkali (2 lb/bbl caustic soda and 1.5 lb/bbl
lime), 8 to 12 lb/bbl of lignosulfonate, and 2 to 4 lb/bbl of bentonite to maintain
viscosity and filtration. Another approach is to use bentonite/thinner (lignosulfonate)/freshwater
premix, and mix it with seawater that has been treated for
hardness. This technique will be discussed in the saturated saltwater muds section.
Chemical maintenance involves control of solids concentration, pH, alkalinity,
and filtration, and pH control. Figure 4-1 12 shows the best operating range for