BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
Cuttings Bed Formation
Radial slip velocity increases with increasing angle of inclination, forcing more cuttings to the low
side of the annulus, which eventually results in the formation of a cuttings bed. Tomren, et al,
reported from their work that bed formation was dramatic in the 40° to 50° angle with the bed
sliding downward against the flow, resulting in a very high cuttings concentration.
The fact that a cutting bed forms is a serious problem, but the instability of the bed in this critical
angle of the well makes the problem even more severe. High torque and drag resulting from the
high cutting concentration at the lower part of the angle can obviously result in stuck pipe or loss of
circulation. Tomren, et al, also reported that a cutting-bed formation was almost instantaneous at
angles of inclination greater than 60°, but that the bed did not slide downward. Thus at angles
greater than 60°, the bed depth continues to build until the fluid velocity in the restricted annulus
reaches a level that causes cuttings movement, and equilibrium rate is established. Cuttings
removal is complicated by the eccentric annuli as well as pipe movement, with increasing
eccentricity decreasing cleaning ability while pipe movement provides some assistance.
Although there is a general agreement on the role of drilling fluid rheological properties related to
hole cleaning in a vertical hole, there appears to be far less agreement on its effect in high-angle
wells. Okrajni and Azar concluded from their work that in the turbulent flow regime, the cuttings
transport is generally not affected by the fluid rheological properties (yield value and yield
point/plastic viscosity ratio) in hole angles from 0° to 90°.
In the laminar flow range, however, they observed that the annular cuttings concentration was
lower for higher yield point/plastic viscosity (YP/PV) ratios throughout the entire range of hole
inclinations. Thus, maintaining a greater structural viscosity in the fluid did improve cuttings
removal in laminar flow, which became more evident at lower annular fluid velocities. Brown, et
al, reported that water gave remarkably better cleaning than a polymer drilling fluid below angles
of 50°, probably due to the turbulence of water versus the polymer.
Unfortunately, the polymer used in their study was hydroxyl ethyl cellulose (HEC), which is not a
structure-building polymer. HEC will exhibit visco-elastic behavior, however, which retards water
from breaking into turbulent flow and thus may have decreased the effectiveness of the HEC fluid
as compared to water. Thus, it is important to realize that if the drilling fluid is being circulated in
laminar flow, it should possess true thixotropic properties, or as described above, possess shear
thinning properties.
The cuttings bed in the greater than 60° angle portion of the hole is of concern, but the instability of
the bed in the 40° to 60° angles appears to cause the most problems and be the most difficult to
clean. According to Jefferson and Zamora, at about 30°, beds (although unstable) first start to form.
Beds become thicker and more stable with increasing angle, but retain the strong tendency to slump
or slide downward. This tendency diminishes as the inclination approaches 60°, after which sliding
caused by gravity stops. The maximum angle for oil and synthetic based drilling fluids usually
exceeds that for water based drilling fluids when cuttings do slide downward and cause pack-offs.
It has been reasoned that if the bed is disturbed periodically, its size can be altered to minimize the
size of the avalanche of cuttings when the hole pump is shut-off. One way of attempting to
accomplish this is through the use of sweeps.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-13