The method is based upon Hooke's Law. Knowing the stretch under a particular tensile load enables the unstretched length to be calculated. This equals the length of pipe between the stuck point and surface. The stretch in the pipe is found with reference to some fixed point which will not move when extra pull is applied
26.374 x Wdp x e where L = Length free pipe in metres
Wdp = Plain end pipe weight in kg/m e = Differential stretch in mm p = Differential pull in kN
735294 xWcpxe P
where: L = Length free pipe in ft
Wdp — Plain end pipe weight in lb/ft e = ins p — lbs
Reasonable estimates of the depth of the stuck point can be obtained in this way. The values obtained are less reliable asthe deviation increases due to down hole friction. Other minor inaccuracy is introduced by neglecting the changing crosssection of the string at the tooljoints.
A stuck or free-point indicator service is offered by Schlumberger and Dia-log. A sensitive electronic strain gauge is run on the logging cable. The cable transmits measurements of torque and stretch in the pipe to the surface read-out instruments.
The indicator is lowered to various depths and at each depth tension and torque are applied to the string at the surface. The strain gauge indicates whether the pipe reacts at that depth to the applied tension and the applied torque. Pipe which appears to be free in tension does notalways react to applied torque. A back-off can only succeed if the pipe is free in both senses
Seperate slim acoustic logs are designed to indicate intervals of stuck, partially stuck or free pipe which may exist below the upper stuck point.
Drillpipe or collars can be unscrewed downhole by exploding a charge inside a selected tooljoint connection, just above the stuck point.
A succesful back-off depends upon having the following.
Select a connection which has been broken during the round trips prior to the pipe becoming stuck.
Particular care should always be taken when applying torque or releasing it from the string. Keep the forces involved fully under control and keep men out of the potentially dangerous area.
Note: - Torque should be worked down the string before the string shot is fired, this may take some time.
If the string fails to backoff after the firing of the charge, continue to work the torque down the string before trying another string shot.
sufficient left-hand, or reverse torque at the joint zero or slightly positive tension at the joint a sufficiently large explosive charge, accurately located at the joint
The ideal tensile load is zero, ie with the threads subject to neither compression nor tension. However, since a zero tensile load is difficult to achieve, pull is applied which will develop a slight tension rather than compression. Over the years there has been some debate regarding the surface pull required to achieve this condition. Since the pipe is held down then it can be assumed that buoyancy does not affect the pipe above the stuck point. However, as soon as the joint is cracked buoyancy will act on the freed pipe.
If buoyancy does not apply then the pull required to maintain the drillpipe in tension will be the total weight of pipe above the stuck point plus the weight of other equipment such as blocks.
An alternative method for finding the required pull is to use the actual hook load observed by the Driller just before getting stuck:
c -ir-,.. Hookload-wt, of blocks-weight of fish in mud , ,. , , Required Pull = y + wt. of blocks
In deviated wells with excessive drag and pull it will be difficult to develop the correct tension at the joint and more than one attempt may be necessary before a successful backoff is achieved. In a highly deviated well the pipe weight may be partially supported.
If the hookload while moving the string slowly up has been observed prior to becoming stuck, the following method can be used to estimate the required pull:
Listed below are fishing tools often kept on the rig site for various hole sizes drilled.
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