BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Table 3-13
Typical Fluid Properties after Conversion and Before Weight-up
Fluid Weight
10.0 lbm/gal
Funnel Viscosity
40 to 44 sec/qt
Plastic Viscosity
16 cP
Yield Point
6 lbf /100 ft2
Gels
2/10 lbf /100 ft2
API Filtrate
4 to 8 cc/30 min
Calcium
600 to 1200 mg/L
pH 9.5 to 10.5
Pf 0.2 - to 0.7
Excess Gypsum
2 to 4 lbm/bbl
Maintenance of Gypsum Fluids
1. Treatments of gypsum, caustic soda, UNI-CAL, and a filtration control agent are normally
required each tour to maintain desirable properties. The amounts added depend upon
volume, rate of penetration, amount and type of formation being drilled, and degree of
dilution.
2. The pH of a gypsum fluid is normally controlled between 9.0 and l0.5. Often the pH is
slightly higher just after a conversion but can be allowed to drift back to the desired range
as drilling progresses. The Pf should be in the 0.2 to 0.6 range.
3. Gypsum is added to maintain filtrate calcium between 600 and 1200 mg/L. The solubility
of the calcium greatly depends upon the alkalinity and salinity of the filtrate.
4. Excess gypsum should be maintained above 2 lbm/bbl. This can be determined by
titration for total calcium sulfate as shown in the Drilling Fluid Testing Procedures
Manual.
5. CMC, and/or MILSTARCH with a preservative or additional deflocculant can be added as
required for desired filtration control.
6. If starch is used for filtration control, it will be necessary to add proportional amounts of a
preservative.
7. Although gypsum fluids tolerate a higher concentration of colloidal solids than
conventional fluids, solids must still be controlled at a reasonable level to obtain optimum
rheological and filtration values. Mechanical solids control equipment such as fine screen
shakers, hydrocyclones (low-density fluids), and centrifuges (high-density fluids) should
be used to minimize dilution requirements. MILGEL should be added daily to provide
proper particle size distribution.
8. Gypsum fluids normally possess high gel strengths, but they are fragile gels and should
not be considered alarming. Gels can be decreased by reducing total solids content and
increasing concentration of UNI-CAL. Gypsum fluids are considered in good rheological
condition if initial gel is less than 5 lbf /100 ft2 and ten-minute gel is less than 15 lbf /100
ft2.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-45