BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
• High particle concentrations are best and at least 30 ppb of bridging mix is required for
an efficient seal.
• In the laboratory fracture sealing has been successful up to 300°F and 4000 psi
overbalance pressure in some tests.
• Drilling fluid density is not a critical factor in forming a successful bridge.
• The bridging material can be used in pills if the section can first be drilled with a drilling
fluid density below the fracture gradient, and then subsequently strengthened by
squeezing a pill across the weak zone.
On the Brent field in the UK North Sea, in the depleted reservoir zone, experience has shown that
the use of LC-LUBE can add approximately 900 - 1250 psi to the fracture propagation pressure.
Recommended LC-LUBE concentration in the active system is 6 – 20 ppb, though on Brent 20
ppb was used.
DRILLSTRING STICKING
The primary causes of stuck drillpipe include:
• differential-pressure sticking
• key seating
• junk in hole and/or collapsed casing
• wellbore instability (sloughing shale or plastic deformation of shale or salt)
• inadequate hole cleaning due to rheological properties, hydraulics, or hole
enlargement.
Many instances of stuck pipe are a result of wall sticking (differential pressure sticking) which
usually occurs in the drill collars and bottomhole assembly because of the increased surface area
exposed to the wellbore. This section will deal primarily with this problem and related preventive
and remedial measures.
Differential pressure or wall sticking is the result of the force holding the pipe against a
permeable zone while the pipe is motionless. This force is roughly equivalent to the differential
between hydrostatic and formation pressures and the surface area upon which the force is acting.
Also important, but more difficult to define, is the friction which exists between the pipe and
filter cake. The friction coefficient will vary with the fluid’s chemical composition and the type of
solids in the filter cake (coefficient is considerably higher for barite than for bentonite).
The equation suggested by Outmans to describe sticking force is,
where,
F = f × A × S
F = friction between drillstring and wall cake, or the total pulling force in pounds that would be
necessary to pull pipe free,
A = area of contact between filter cake and pipe (in. 2 ),
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-34