BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Numerous laboratory studies have been conducted concerning the flow velocity required to
completely remove drill solids from a high-angle wellbore. The general conclusions from all of
these studies indicate that there is a critical velocity (CTFV-Critical Transport Fluid Velocity) that
must be exceeded to ensure a complete sweep of the wellbore. If the fluid velocity is below this
value (SCFF – Sub-Critical Fluid Flow), cuttings will start to accumulate in the wellbore. This
accumulation continues until the flow velocity over the top of the cuttings bed exceeds the CTFV,
at which time an equilibrium condition is created.
It has been determined that through these testing programs, the most difficult wellbore angle to
clean is between 65° and 75°. However, the increase in required flow velocity only varies slightly
over the complete range of well deviations for 55° to 90°. Since the transport mechanism changes
from rolling to lift, studies indicate that the critical velocity is significantly lower at wellbore
deviations from vertical to about 50°.
To summarize, it was observed in flow loop simulations that the removal of a cuttings bed with a
viscosified fluid was in fact detrimental in high angle wellbores (assuming zero to low drill pipe
rotation), and that low viscosity fluids are more beneficial.
Based on these studies and in-house work, Baker Hughes Drilling Fluids has settled on a value of 5
ft/sec (300 ft/min) as the recommended flow velocity to clean out the wellbore for particles. In
addition, this displacement rate has been tested and proved effective in numerous field applications,
especially before screen and gravel placement.
Indirect versus Direct Displacement
There are two general types of displacement used in the oil industry today. One is an indirect
displacement and the other is a direct displacement. Each type has its advantages and
disadvantages. Utilize BHDF’s DISPLEX Simulation Model to determine the pill volumes
required for spacer contact times and flow rates.
Indirect Displacement
An indirect displacement is usually associated with a displacement of the mud system to seawater
(or drill water) in a drilling liner or production casing before displacing to the next fluid system.
For example, if oil-based drilling mud is in use, the operator may wish to displace and clean the
drill string and casing with seawater before displacing to brine. The seawater would be preceded
by a series of spacers and solvents to clean and water-wet the casing. With this method, a thorough
cleansing can occur with minimal product usage due to the circulation of inexpensive water. Later,
the displacement to the clean brine will occur without contamination.
For indirect displacements, a good cement bond log is necessary because high differential pressures
on the casing could cause a breakdown of cement or collapse of the casing.
Indirect displacements may also be recommended for the production casing. In this instance, the
drill-in fluid would be displaced to drill-water before finally being displaced to clear brine.
Caution must also be exercised in this displacement because a possible reduction in hydrostatic
pressure across the production interval could lead to casing collapse.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-84