BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling 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 when converting fluids that contain high
concentrations of MILGEL (MBT for base exchange capacity gives a general idea of
conversion severity).
3. The type and amount of deflocculating agents added to the fluid prior to conversion have a
significant influence on viscosity during conversion. If a phosphate has been used, the
conversion will normally be drastic viscosity humps which are prolonged due to 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 amounts of dilution and
concentration of conversion chemicals needed.
6. The pits should have adequate space available to allow room for dilution.
7. Necessary amounts of lime and deflocculant should be stacked near the hopper, and
caustic soda should be carried to the top of the suction (mixing) pit. Lime and
deflocculant are added through the hopper. Caustic soda is added directly to the suction
(mixing) pit near the point of maximum agitation.
8. Accurate circulation time should be determined so that conversion materials can be added
at timed intervals over a period of one or two complete circulations.
Conversion Procedure
1. Conversion to a lime fluid should be initiated shortly after a new bit has been placed on
bottom, i.e. once the conversion starts it should not be interrupted until the system has
achieved stability. Best results are obtained when the conversion is made inside the
casing prior to drilling out.
2. The amount of material needed for conversion will vary depending upon the desired
properties and the condition of existing fluid. Most conversions can be made as follows.
3. Drilling fluid systems low gravity solids content has been diluted to a minimum solids
content of 3 to 5% by volume. If the system is weighted then the calculated dilution
volume is to be added simultaneously with the necessary chemical additions. The biggest
concern is that after the breakover the fluids viscosity could drop so low that the barite
may fall out of a weighted system.
• 2 to 4 lbm/bbl deflocculant (UNI-CAL ® )
• 2 to 3 lbm/bbl caustic soda
• 4 to 8 lbm/bbl lime hydrate [Ca(OH) 2 ]
• 1 to 2 lbm/bbl MILSTARCH ® with preservatives
• 0.5 to 1 lbm/bbl CMC
4. The deflocculant should be started through the hopper while caustic soda is added to the
pit. After five or six sacks of deflocculant and two or three sacks of caustic soda have been
added, the lime additions should begin and be mixed through the hopper along with the
deflocculant. The lime addition will cause a viscosity hump, but the system will “break
over” when agitated and become quite fluid. If the viscosity becomes excessive, the rate
Reference Manual
Baker Hughes Drilling Fluids
3-40 Revised 2006