BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
There are a number of drilling factors influenced by the drilling fluid, but the following are of
primary concern in drilling a horizontal well.
• Borehole instability
• Hole cleaning
• Torque and drag
• Formation damage
• Barite sag
• Hydraulics
Borehole Instability
Borehole instability is one of the more costly, but relatively common problem associated with
drilling oil and gas wells. Not only can it impede drilling, it can result in loss of the hole. The term
normally conjures thoughts of borehole collapse with accompanying stuck pipe. This is one of the
more critical reactions of borehole instability, but it can manifest itself in more subtle ways such as
hole-fill, bridges, hole enlargement, or hole size reduction.
These reactions lead to other associated problems including increased torque and drag, logging
difficulties, poor cement jobs, and increased drilling days, all of which add significantly to the
drilling cost. It should be pointed out that the discussions to follow do not cover hole enlargement
problems from weakly consolidated sands, or rubble zones that erode from the hydraulic action of
the drilling fluid.
Cause
In simple terms, the primary cause for hole instability occurs when the rock surrounding the hole
fails to support the load that was previously taken by the removed rock. This removal results in a
stress concentration around the wellbore. If the rock strength is sufficient to absorb this stress, the
borehole will remain stable. If not, it will fail. Aadnoy, et al, found that borehole collapse is caused
mainly by shear and tensile failure, while fracturing of the wellbore is predominantly caused by
tensile failure. However, even if the rock is strong enough to withstand this stress concentration,
exposure to the drilling fluid may weaken it to the point that failure will occur.
Control
The first consideration to controlling borehole instability is to increase the fluid density. By having
the wellbore pressure carry some of the load, the stress of the borehole wall is reduced and
compressive failure may be averted. However, if the fluid density is increased too much, a tensile
fracture may occur, resulting in loss of circulation.
Bradley showed through his stress cloud analyses, that the fluid weight required to prevent collapse
increases significantly with increasing hole angle, while the fluid weight that can be tolerated
before fracturing occurs (loss of circulation) decreases with increasing hole angle. These findings
are confirmed by the work of Aadnoy and Chenevert and others. This places strict tolerances on the
fluid density range that must be maintained in high-angle wells. Increases in mud weight of
between 0.5 ppg and 1.0 ppg per 30 degrees inclination through shale/mudstone sections are
usually necessary to prevent combat hole collapse. Experience in the local area will determine
where in this range to be. The additional annular pressure losses when circulating – ECD - can lead
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-8