BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
of available lime in the fluid. A lime fluid should be conditioned to handle excessive
temperatures by dilution and centrifugation of the solids to a minimum and by reducing
the alkalinity and excess lime content to approximately that of a low-lime fluid.
Reduction of alkalinity and lime content should be done while diluting and adding UNI-
CAL ® over a period of several circulations. ALL-TEMP ® or MIL-TEMP ® are beneficial
(0.5 to 4 lbm/bbl) to prevent high temperature solidification.
Gypsum (Gyp) Muds
1. Gypsum fluids are maintained with higher filtrate calcium and lower alkalinity than lime
fluids to increase their inhibiting affect on clays.
2. Gypsum fluids are used when large sections of gypsum or anhydrite are to be drilled.
Because of the limited solubility of CaSO 4 in water, additional gypsum or anhydrite will
not dissolve into the fluid system but will be carried as a solid.
3. Gypsum fluids are more resistant to salt and salt water than lime fluids, as long as solids
are kept in line.
4. Gypsum fluids are less susceptible to high temperature solidification than other calcium
fluids because the alkalinity is maintained at a lower value. A temperature of 350°F is
usually considered to be the upper limit for well-treated gypsum fluids.
Factors Which Affect Gypsum Fluid Breakovers
1. Since the degree of viscosity change during and after conversion to a gypsum fluid
depends upon the total solids concentration of the fluid system, dilution with water is
necessary prior to conversion. Good solids control equipment is strongly recommended.
A centrifuge is recommended for weighted fluids.
2. The type of clay solids present in the fluid influences the viscosity during conversion.
Because MILGEL has a much higher Base Exchange capacity than most drilled clays, the
viscosity increase is much more pronounced need when converting fluids that contain high
concentrations of MILGEL.
Note:
Methylene Blue Test for Base Exchange gives a general idea of the severity of conversion.
3. The type and amount of deflocculating agents added to the fluid prior to conversion will
have a significant influence on viscosity during the conversion. When necessary,
treatments should be made with small increments of UNI-CAL and caustic soda. If a
phosphate has been used, the conversion will normally exhibit drastic viscosity humps.
These are the result of the reaction between phosphate, lime, and solids all caught up in a
mass ion impact while displacement and sequestering is all taking place simultaneously.
4. Maximum agitation should be maintained in the surface active system.
5. Pilot tests should be run prior to conversion to help determine necessary amounts of
dilution and concentration of conversion chemicals.
6. The pit volumes should be reduced to allow room for dilution.
7. Necessary amount of gypsum and deflocculant should be stacked near the hopper, and
caustic soda should be carried to the top of the suction (mixing) pit. Gypsum and
deflocculant are added through the hopper. Caustic soda is added directly to the suction
(mixing) pit near the point of maximum agitation.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-43