Utilisation of a free-point indicating tool
A stuck- or free-point indicator service is offered by the wireline logging companies. A sensitive electronic strain gauge is run on the logging cable within the stuck string and anchored to the inner surface of the pipe. Tension and torque are then applied to the string at the surface and the strain gauge readings are transmitted to surface, indicating whether the pipe reacts at that depth to the applied tension and the applied torque. By repeating this procedure the deepest point to which tension can be transmitted can be identified, and similarly the deepest point to which torque can be transmitted. These are the points below which the pipe cannot be moved up or rotated respectively. The effective stuck point is the lower of these.
Note that pipe which appears to be free in tension does not always react to applied torque, and vice versa. A back-off can only succeed if the pipe is free in both senses.
Separate slim acoustic logs are available that are designed to indicate intervals of stuck, partially stuck or free pipe which may exist below the upper stuck point.
Bang!
BACKING OFF
Drillpipe or collars can be unscrewed downhole by exploding a charge known as a string-shot (prima-cord folded up inside a piece of tubular plastic) inside a selected tool-joint connection, just above the stuck point. A connection should be selected which has been broken during the round trip prior to the pipe becoming stuck.
A successful back-off depends upon having the following :
• zero or slightly positive tension at the joint
• sufficient left-hand, or reverse torque at the joint - 50% to 75% of make-up torque is suggested
• a sufficiently large explosive charge, accurately located at the joint
For a safe operation carry out the following checks :
• ensure that tong and slips dies are clean, sharp and the proper size for the string above the rotary
• check that tong, snub and jerk lines are in excellent condition
• ensure that slip handles are tied together with strong line, to prevent the slips being kicked out of the table and thrown clear when the pipe breaks out
• ensure that elevators are latched around the pipe and slackened off under a tool joint with the hook locked when torque is being applied to the string
• ensure that no torque remains in the string when it is picked out of the slips, unless the pipe is properly held with a back-up tong
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.
The following two pages give information about the tension and torque to be applied.
Note: Torque should be worked down the string before the string shot is fired, this may take some time. If the string fails to back off after firing the charge, continue to work the torque down the string before trying another string shot.
PROCEDURE
BACKING OFF
MAINTAINING THE APPROPRIATE TENSION
The ideal tensile load is zero, i.e. 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 :
Required Pull = Hook load – weight of blocks – weight of fish in mud + weight of blocks Buoyancy Factor
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 back-off is achieved. In a highly deviated well the pipe weight may be partially
supported.
If the hook load while moving the string slowly up has been observed prior to becoming stuck, the following method can be used to estimate the required pull:
• Calculate the theoretical weight of the whole string in air (using approximate weight for drillpipe)
• Subtract from this the observed weight of the string (hook load – blocks)
• This gives the weight loss due to buoyancy, friction and wall support which can be expressed as a percentage.
• Calculate the theoretical weight of pipe in air down to the stuck point (using
approximate weights) then subtract the percentage weight loss due to buoyancy and wall support etc.
• Add the weight of the blocks etc. and this will be the tension prior to back-off.
TORQUE VERSUS NUMBER OF TURNS (PIPE BODY)
* Premium Class = Minimum remaining wall thickness = 80% (Uniform wear) K-factor
inch mm lbs/ft kg/m field units S.I. units 31/2
The approximate weight of the DP = 31.08 kg/m The weight of free pipe in air is
3,630 x 31.08 x 9.81 = 110,600 daN Allowable torque is 3,200 daN-m (page C-35) Turns per 100 m = 3,200 x 10 = 0.519
61,700
Number of turns is 0.519 x 36.30 = 18.8
Field units :
5" 19.5 lbs/ft, new, grade E, DP Stuck at 11,900 ft.
The approximate weight of the DP = 20.89 lbs/ft The weight of free pipe in air is
11,900 x 20.89 = 248,600 lbs.
Allowable torque is 23,000 lbs-ft (page C-34) Turns per 1000 ft = 23,000 = 1.575
14,600 Number of turns is 1.575 x 11.9 = 18.7 Torque in N-m(lbs-ft) = K x turns/100m (turns/1000 ft)
where K is given in the following table:
Note :
in S.I. units : K = 0.00051 (D4 - d4) [D and d in mm]
in field units : K = 50.16 (D4 - d4) [D and d in inches]
These factors are based on a shear modulus of 8.274 x1010 N/m2 (11.71x 106 psi)
Note:
Remember that if the tool joint make-up torque is less than the allowable pipe body torque then when applying left hand torque the pipe may back off before the allowable pipe body torque has been reached. If this is not desired the upper torque limit is determined by the lowest actually used tool joint make up torque, reduced by a safety factor.
FISHING TOOLS
Type of fishing tool Connecting tools
Taper tap (poor class of tool: overshot always preferable if available)
Spear (provides very good connection, Bent drillpipe single
Jar, hydraulic or mechanical Bumper sub Flash cutter (Schlumberger, etc.) Jet cutter (Halliburton, etc.) Chemical cutter (Baroid, etc.) Electrical cable back-off
Reverse circulating globe-type basket Magnet