BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
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 gypsum 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 them mud
system has stabilized. Best results are obtained when the conversion is made inside the
casing prior to drilling out.
2. The amount of material needed for conversion varies depending upon the desired
properties and the condition of existing fluid. Most conversions can be made as follows.
• Drilling fluid systems low gravity solids content should be 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 below listed chemical
additions. The biggest concern is after the break over the fluids viscosity could drop
so low that the barite would fall out if a weighted system.
• 3 to 6 lbm/bbl UNI-CAL®
• 4 to 6 lbm/bbl gypsum (CaSO4 •2H20)
• 1.0 lbm/bbl caustic soda
• 0.5 to 1.5 lbm/bbl CMC, or
• 2 to 6 lbm/bbl MILSTARCH with preservative.
3. UNI-CAL 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 gypsum additions should then be mixed through the hopper along with the
UNI-CAL. The gypsum addition will cause a viscosity hump but will “break over” when
agitated and become quite fluid. If the viscosity becomes excessive, the rate of addition of
deflocculant and caustic soda should be increased along with an increase in water dilution.
4. API Filtrate is normally controlled with CMC and/or MILSTARCH. Periodic additions of
MILGEL in moderate quantities will help also help. MILGEL is usually accompanied by
treatments of UNI-CAL.
5. After all chemicals have been added to the fluid system, a check should be made of fluid
properties. Minor adjustments can then be made on subsequent circulations with
additional gypsum, UNI-CAL, caustic soda, and filtration control agents.
6. The addition of gypsum and CMC simultaneously through the hopper should be avoided
because of drastic viscosity increases.
7. Typical fluid properties after conversion and before weight-up are as described below.
8. Occasionally, it is necessary to convert weighted fluids when dilution is not practical. In
this case, the fluid can be converted in the surface pits and displaced down the hole in
stages.
9. If a high pH fluid is being converted to a gypsum fluid, more water dilution and higher
concentrations of UNI-CAL and gypsum are necessary to obtain desired properties.
10. Abnormal foaming may occur during and immediately after conversion. This foaming is
usually confined to the surface of the pits and can be reduced by adding a defoamer such
as LD-8 to the pits. Mud guns should be turned off if excessive foaming persists.
Reference Manual
Baker Hughes Drilling Fluids
3-44 Revised 2006