Two important problems can surface in the fishing of drill collars and drillpipe. One is that drill collars which are off bottom can fall to the bottom of the hole after they are washed over. The other is that the top of the collars, if free, can lean over in an enlarged hole or under a ledge and make it impossible to get over them with tools. The first potential problem of letting washed over collars or pipe fall to the bottom of the hole may be circumvented withawashover back-off tool." These tools connect to the fish prior to washing over. If the fish is not free after the first washover, a string shot back-off may be performed to remove all free fish. Also, these tools permit faster recovery because collars or pipe may be washed over and pulled in one trip.
The second problem in which the collars or pipe lean over in an enlarged hole or under a M restricted diameter hole may be alleviated by backing off in carefully chosen points. The factors and situations which promote trouble are the following: enlarged hole, ledges, and crooked hole (doglegs).
These three potential problems may be identified by coordinating the proposed back-off depths with the well site geologist and the rig drillers. The geologist will be aware of enlarged zones and the drillers will have noted the locations of doglegs while tripping the drill string.
It is possible that the collars or pipe will be stuck in more than one zone.
If wall sticking (differential sticking) has occurred circulation is usually possible. If cavings have stuck the usually not possible.
Cutting off of blades on stabilizers or other centralizing tools may allow the fish to lean to the side of the hole and make getting over the fish impossible.
A change in mud properties is almost always required to successfully fish stuck collars or drillpipe. The mud properties which may have the more significant affect are filter cake thickness and density. Higher mud densities usually better stabilize the wall of the hole. Thinner filter cakes reduce the risk of differential sticking.
Washpipe is very susceptible to wall sticking because of the small diametrical clearance between the washpipe and the hole.
Washing over drill collars which are frozen to the wall of casing is almost impossible. It is thought that the washover shoe wears or breaks up because of the vibration of cutting across the steel in casing and the cavings or cement freezing the collars.
string then circulation is
Backing off inside casing is the safest location, CEMENT but may not be the most practical.
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