Page 394 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 370

[ ] Practical Wellsite Operations
3.3.5

The overshot is dressed with the correctly sized basket or spiral
grapple. The spiral grapple is used when the OD of the fish is at the
largest sizes of the overshot catch range. With a basket grapple a mill
control can be run, which gives some beveled milling action to the top
of the fish. Unless there is a good reason for not doing so, run a mill
control with a basket grapple.
At the bottom part of the overshot is the guide shoe. This guides
the overshot over the top of the fish. It may open out to almost full hole
size and have a spiral cut bottom end to aid in rotating over the top of
a fish leaning onto the low side of the hole.
It is a good idea to paint the inside of the grapple and guide shoe.
If the fish is not caught, you will be able to see if the grapple went over
the fish but had perhaps slightly too great an ID to latch on.
Where the overshot has to catch on an OD some distance below
the top of the fish, extensions can be run above the grapple to allow
this. Be careful not to add too long an extension because if the grap-
ple slips over the fishing neck and onto a section of smaller OD (such
as at the top of a zip type drill collar), it may catch the fish but be
impossible to release from the fish. The grapples are not designed to
open up over the catch diameter going upwards because the back face
is not beveled at all. If this is likely, then run a safety joint to allow the
string to be pulled out if the overshot cannot be released and the fish
stays stuck.
If the fish is likely to be plugged and circulation through it impos-
sible, it is advisable to run a circulating sub at some convenient point
above the overshot and/or leave out the packoff seal.
When above the fish, break circulation at a slow rate. Note the cir-
culation pressures, drags, rotating weight, and torque. Run in slowly
and tag the top of the fish, marking the string and noting the exact
depth. Pull back a couple of meters, start circulating, and rotating slow-
ly. Come down over the fish watching pressure, weight, and torque
carefully for indications of going over the fish. The action of rotating
to the right opens up the grapple as it contacts the fish. (If the top of
the fish is clean, it will usually be possible to latch on simply by low-
ering onto the fish and applying weight without rotating.) If the fish
cannot be circulated through, the pump should be stopped when pres-
sure starts to build; bleed off the pressure if necessary to prevent pis-
toning off the fish.

370