BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
Hole Cleaning Sweeps
Hemphill and Rojas conducted an in depth study of hole cleaning sweeps for coiled tubing
operations, i.e. static drill string.. They concluded that high-density sweeps were more effective
than high-density high-viscosity sweeps based on field measurements. The higher viscosity of the
latter caused flow diversion to the high side of the wellbore. Not surprisingly, high-viscosity
sweeps also performed poorly.
Sewell and Billingsley concluded that the best hole cleaning sweeps were high-density lowviscosity
sweeps, with a density 3 – 4 ppg higher than that of the drilling fluid. They made the
following recommendations:
• Calculate sweep height in the vertical part of the annulus, and added hydrostatic head in the
annulus.
• Rotate at 80+ rpm while pumping the sweep, 180+ rpm is preferred. It is not recommended
to pump a weighted sweep while sliding.
• Keep pumping until the sweep is out of the hole. If pumping stops while the sweep is in the
deviated hole, it falls out of the “jet stream” and re-deposits captured fines.
• Avoid increasing the viscosity of the weighted sweep.
• Plan to run 3 – 4 sweeps to achieve desired results when hole cleaning problems are
indicated. One sweep will probably not be sufficient to remedy an existing situation.
• Avoid checking for flow or closing the annulus with the weighted sweep in the drill pipe.
• If desired for preventative maintenance: 1) schedule sweeps every 6 – 8 hours or 500 –
1000 feet (whichever comes first, unless the hole dictates greater frequency); barite/silt
build-ups are dependent on time and footage: and 2) pump a sweep while circulating
bottoms up before tripping, and another once back on bottom.
Power et. al. concluded that high-density sweeps were more effective than high-viscosity sweeps.
They strongly advocated the use of sweep reports, which should be standard practice in any drilling
fluids operation.
Short Tripping
Another technique that has proven helpful in controlling problems created by the cuttings bed in
the high-angle portion of the well is to make periodic short trips. Tripping the bit through the bed
not only assures that the bottom hole assembly and bit can be pulled through it, but the action
should agitate the bed to the point that a portion will slide down the hole and subsequently be
picked up when circulation is commenced. The time intervals for these short trips will be
determined by the torque and drag conditions that exist in any particular well. Lacking specific
well history, a general rule of thumb is to short trip at least once for every 500 feet of new hole.
Drill String Rotation
The industry is in total agreement that drill string rotation improves hole cleaning, especially in
hole sizes less than 17 ½”. The general opinion in the industry is that the optimum drill string
rotation rate is approximately 120 rpm. It is believed that the drill string rotation improves hole
cleaning by mechanically disturbing the cuttings bed on the low side of the hole. There could also
be effects due to skewing the velocity profile in the wellbore and the creation of Taylor Vortices.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-14