STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 669
many formations represent a significant advance over dispersed non-inhibited
types of fluids. Inhibition is sought through three mechanisms: addition of
calcium (lime, gypsum), addition of salt, and addition of polymer.
Lime Muds
Lime muds are muds treated with caustic soda, an organic thinner, hydrated
lime, and, for low filtrate loss, an organic colloid. This treatment results in muds
having a pH of 11.8 or higher, with 3 to 20 ppm of calcium ions in the filtrate.
Lime-treated muds exhibit low viscosity, low gels, good suspension of weighting
material, ease of control at mud weights up to 20 lb/gal, tolerance to relatively
large concentrations of flocculating salts, and easily maintained low filtration
rates. One of the most important economic advantages of lime-treated mud is
its ability to carry large concentrations of clay solids at lower viscosities than
other types of mud. Except for a tendency to solidify under conditions of high
bottomhole temperatures, lime-treated muds are well suited for deep drilling and
for maintaining high weight muds. Pilot tests can be made on the mud to
determine if the tendency to solidify exists; if so, solidification can be inhibited
by chemical treatment for periods of time sufficient to allow normal drilling
and testing activities. A lime-treated mud that exhibits a tendency to solidify
should not be left in the casing-tubing annulus when the well is completed.
Lime-treated muds are prepared from freshwater drilling muds. The conversion
should be made inside the basing. The initial step in conversion of freshwater mud
to a lime mud involves dilution of the mud with water to reduce the clay solids
content to avoid excessive mud viscosity (breakover). The recommended sequence
of material addition is
a. Dilution water: 10-25% by volume
b. Thinner: 2 lb/bbl
c. Caustic soda: 2-3 lb/bbl
d. Lime: 4-8 lb/bbl
e. Thinner: 1 Ib/bbl
f. Filtration control agent: 1-3 lb/bbl
The maintenance of lime-treated muds consists of monitoring the calcium content,
Le., the proper lime solubility. Since the lime solubility is controlled by the amount
of caustic soda present in the mud, the proper alkalinity determination is of great
importance. The recommended value of Pf is 5 to 8, and it is maintained with caustic
soda; the recommended value of Pm is 25 to 40, and it is maintained with excess
lime. The amount of excess lime should be from 5 to 8 lb/bbl.
The limitation of lime-treated mud is solidification at bottomhole temperatures
higher than 250°F. Low lime mud was designed to minimize this tendency toward
solidification and can be used at bottomhole temperatures as high as 350°F. In
low lime mud, the total concentration of caustic soda and of lime is reduced.
The recommended P, is from 1 to 3, and the recommended Pm is from 10 to
15; the excess lime should be from 2 to 4 lb/bbl.
Gypsum-Treated Muds
Gypsum-treated muds have proved useful for drilling anhydride and gypsum,
especially where these formations are interbedded with salt and shale. The
treatment consists of conditioning the base mud with plaster (commercial
calcium sulfate) before the anhydride or gypsum formation is penetrated. By