Page 327 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 303

2.9.2
Drilling Problems--Avoidance Planning [ ]

Stiff assembly. Where a stiff BHA is run after a build or drop assem-
bly, or after a directional motor, lay down enough drillpipe to ream the
complete section drilled with the more flexible assembly. When ream-
ing a kickoff in softer formations, take precautions to ensure that a
sidetrack is not drilled. Use low WOB/higher RPM and if necessary
1
make a special trip with a bit-sized hole opener and a /2 in undergauge
bullnose to ream out the hole before drilling ahead.
Ledges. Can occur when there are large changes in formation hard-
ness. Note this in the program. Run slowly when tripping in at these
points. Ream any resistance seen on trips.
Mobile formations. Sticking in salt may occur fast enough for the bit
to get stuck as it drills. If this condition occurs, it is hard to avoid alto-
gether but may be reduced by using a water-based mud that leaches the
salt slightly as drilling progresses. Higher mud weights may (or may
not) help to keep the salt under control. Bits are available that drill
slightly off center and so cut an overgauge hole (bi-center PDC bits);
these are helpful when drilling in problem flowing salts.
If the salt moves more slowly and causes problems, mainly when
tripping back through it, try programming wiper trips through newly
drilled hole after a short time. Sometimes a wiper trip is effective after
18 hours drilling—up to the previous wiper trip depth. Field experi-
ence will show the best way to handle flowing salts.
Stuck pipe: solids related problems. The deviation profile has an
effect on hole drags/torques and on the potential for sticking.
Computerized models may predict likely downhole drags and allow
the wellpath to be designed for the lowest figures. Select drillpipe to
give at least 100,000 lbs overpull over up drag at section TD (after
applying a safety factor; 85% is common).
Unstable formations are better drilled at low inclinations, if pos-
sible, to reduce the likelihood of formation compressive failure due
to overburden.
Cuttings beds. Ensure that mud rheology and AVs are sufficient to
clean the hole. This will be more difficult as inclination increases.
Circulate clean before pulling out, displacing a pill around if necessary.
If cavings are causing drags on trips, a slight increase in mud weight
may help to stop the caving (if this will not cause losses). Cavings may
cause a dirty annulus during the trip, even if the hole was clean at the
start of the trip. If there are indications during the trip of a dirty annu-
lus, then stop and circulate clean before continuing out.

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