BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Turbulent Plug
V=O
Figure 6 - 22
Plug and Turbulent Flow Regimes
Contact Time
Spacer contact time in the wellbore is determined by the volume and type of spacer, the annular
flow rate, the fluid and density being displaced and the wellbore configuration. Contact time is
critical in the cleanup process because removal of debris occurs gradually as a spacer flushes past
the wellbore surface. In most applications, the contact time may vary somewhere between 2.5 to
10 minutes. The concentration of the solvent in the spacer also plays a significant role in cleanup,
especially in the removal of oil-base and synthetic-base residue. In these and other applications,
the volume of the spacer and the displacement rate determine the contact time. Usually the
displacement rate is based on the annular flow rate needed to achieve turbulent flow, however, hole
or rig conditions may limit the pump output. Once the volume is calculated for optimum contact
time at the agreed-upon displacement rate, the appropriate solvent concentrations can be optimized.
For the removal of oil/synthetic debris, concentration requirements are calculated based on the
surface area of the wellbore (casing or open-hole). Programs are available to calculate precise
contact time requirements for specific applications.
HSE needs to be considered for all chemicals used. Mixtures of displacement pills and mud have to
be separated from the active mud and packer fluid for disposal (zero discharge issues).
Well Cleanliness Determination
The determination of how clean the well is usually based on the cleanliness of fluids returning from
the wellbore. The most common measures are normal turbidity units (NTU) and solids content,
neither actually relate to what is left in the well. Junk baskets or equivalent equipment do give
some positive indication of solids removal. Other indicators of a clean well are torque and drag
(related to the increasing friction coefficient because of the fluid coating the casing walls being
washed away) and cleanliness of the clean up string when pulled to surface. Rust in return from the
pumped seawater or brine (if oxygen scavenger is not used) is also an indicator of a clean well.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-94