Page 330 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 306

[ ] Well Programming
2.9.2

Static overbalance. Use the minimum safe mud weight to minimize
the static overbalance on the formation. Condition the mud carefully
before running casing to the minimum safe density.
Wall contact. Use a well-stabilized BHA. Spiral or square drill col-
lars have less contact area than round ones. Run HWDP in compres-
sion to reduce the length of drill collars. Use a bit that requires less
WOB and therefore less BHA (PDC, diamond) if the formation is suit-
able and the rig cost justifies it.
Stationary string. Minimize programmed wireline directional sur-
veys. Top drive is an advantage, since fewer connections are required.
Make the initial flow check on trips brief, pull out above the permeable
zone, and carry out a full flow check. Keep the string moving slowly
during flow checks. In extreme cases rotate out on connections and
keep the string rotating until ready to latch or stab, ensure the crews
are properly briefed to carry this out safely. Centralize the casing well
over this interval.
Drilling jars and jar placement. Except in a horizontal hole, jars
are normally run in the BHA. Some operate mechanically and some
hydraulically. Jars may be able to jar both up and down—some only up.
It may be possible to adjust how much overpull sets off the jar and
internal mud pressure may have an effect (refer to the manufacturer’s
manual). They all work by allowing the driller to take a strain (or set
down weight) on the drillpipe and then they suddenly open or close.
This uses the stored strain energy in the string to accelerate those drill
collars above the jar and deliver a hammer blow to the collars below.
The strength of the blow is related to the weight of the drill collars
above the jar and how fast they are moving at the end of the jar stroke.
Jar placement has been the subject of much research and is quite
complex. As a general recommendation, the jar should be run with
either one or two drill collars above it for maximum effectiveness, if no
jar intensifier is used.
If the jar has adjustable settings for tripping, set it for a tension
approximately equal to twice the BHA weight, unless field experience
dictates otherwise. Be careful not to run the jars within 5000 lbs of the
neutral point when drilling, otherwise the jar will wear rapidly and
may not work when needed.
Ensure that a backup drilling jar is on site and that the jar in use is
changed out at around the interval recommended by the manufactur-
er, or less if it is suspected that the jar is no longer functional.

306