BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
• lift out debris and junk
• water wet pipe surface
• remove pipe dope
• effectively displace mud
The best fluid for thinning the mud is the continuous phase of the mud. 50-75 bbl of the base oil can
be pumped in the case of oil based muds. For water based mud pumping 50 bbl of water as the first
displacement pill will effectively thin the mud. Before any displacement the compatibly of the spacers
with the mud and the ability to water wet steel surfaces should be checked at room temperature and
85° C (185° F) to confirm compatibility (in deep water lower temperature tests may be necessary). All
tests should be done on field mud samples to ensure mud is representative. In high mud weight the risk
of inducing barite sag needs to be considered, because displacement pills can thin the mud to the point
it can no longer support barite.
Water pumped without surfactants to displace oil muds (e.g. for an inflow test) prior to clean up, can
form gunk, which are not broken down by subsequent clean up pills. Aggressive solvents are required
to remove gunk and pipe dope.
XAN-PLEX TM D and Xanthan Gum are the preferred materials for mixing viscous pills. When
mixed in calcium brines, these require special procedures by lowering pH before adding the
polymer, and then raise pH to get the fluid viscous. HEC-viscosified fluids do not have good solids
support. Polymers should be checked for suitability at the anticipated bottom hole temperature.
Lead Spacer (Push Pill) Systems
The function of a lead spacer is to move the drilling fluid from the wellbore without contacting
other incompatible fluids. Moving a system from a wellbore is best accomplished by utilizing a
high pump rate, pipe rotation and a viscous (weighted) spacer. The high viscosity helps maintain
the integrity of the spacer by enabling it to stay in “plug” or laminar flow at high pump rates. The
spacer must be large enough to allow for 5 to 10 minutes contact time based on the pump rate.
Pipe rotation helps break up the gelled pockets of mud that may accumulate in some sections of the
annulus, especially in highly deviated wellbores. If required, the density of the lead spacer should
be adjusted for well control reasons and should be at least or slightly more dense than the fluid
being displaced. Although the lead spacer may be in plug flow, the middle spacer systems are
moving in turbulent flow and will remove any residual debris. It is recommended that each spacer
cover at least 1000 feet of the annulus at its largest diameter.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-93