TECHNICAL FIELD [00011 The present invention relates to a method for releasing cables from an at- tached well tool, where a cable cutter is arranged between the main part of a cable and a well tool, said cable cutter being controlled by a timer. The invention relates also to an apparatus for exercising the method.

PRIOR ART TECHNIQUE [0002] A well bore apparatus being adapted for being releasably coupled to suspen- sion cables is disclosed in the US Patent 4,237, 972 in the name of C. P. Lanmon. In this prior art the inventor explains the idea of cutting the cable when the tool has fastened in the well. The basic idea is to avoid a long piece of a cable being left and destroyed be- tween the tool and the fishing tool. Therefore, it is necessary to cut the cable close to the tool, and the cutting device is in this prior art controlled by a time delaying retain- ing means for preventing the actuation of the cutter, said retaining means having a chemically-corrodible element within a chamber, where a corrosive chemical can act upon the chemically-corrodible element. It is even suggested, that the chemically- corrodible element is a metal and the corrosive chemical is either an acid or a caustic.

The corrosive element when released by the chemical degradation fires an explosive driving the cutting blade. To activate the cutting means, before the apparatus is lowered into a well bore a corrosive chemical is released in the apparatus for causing the failure of a restraining means only after the elapse of a predetermined time interval that is or- dinarily more than sufficient for carrying out the intended operation of the apparatus.

This has, however, the drawback, that the apparatus is time sensitive in relation to de- lays in the well operation, and on the other hand, that much time will be spent waiting, when the apparatus has stuck in the well bore.

[0003] In US Patent 3,073, 388 another actuating means is used for the cutter. This actuating means is driven by explosives disposed in a casing means and controlled by firing means and a timing mechanism operatively connected to the firing means. The

timing means are preset before being sent down the well and are initiated by position- ing of the casing means at a remotely inaccessible location in the well bore. This timing means is not initiated until the positioning and much time will, also in this case, be spent waiting for the firing of the explosives. A quick release through an early cut is thus excluded. On the other hand, if, for some reason, the tool is released again, there is no way to stop the timing means and the cable will thus unavoidably be cut after the lapse of the preset time.

[0004] A normal procedure for using tools in a well bore is to send down the tool suspended in a cable. The cable is capable to carry a certain load before being destruc- ted by the tensile stress. The tool has a mass far below the limiting tensile stress of the cable. If the tool one way another is stuck in the well bore, it is important to release the tool and pull up the cable, as exerting further tensile stress will pass the yield point of the cable material thereby permanently reducing the cross section of the cable and its possible ability to carry large electrical currents.

[0005] In cases, where the cable is also an electric wire, the electric current is de- pending on the tensile stress as the tensile stress reduces the cross section of the wire or the single strands and is thereby increasing the energy loss through the cable. Thus, if the cable is stretched too much, it will introduce a large specific resistance or short- circuit the different cords.

SUMMARY OF THE INVENTION [0006] Therefore, it is a primary object of the present invention to provide a method for the cutting of the cable, where the cutting and the time for cutting can be decided from the surface above the well. To this end, the inventor has suggested that at least three parameters are used for controlling the cutter, e. g. a pulling force exerted by the cable on the well tool, the pressure in the well location, and time, where the timer is working electronically. The term"cutting"is to be interpreted in a broad sense to in- clude any intended disconnection between the cable and the well tool, except rupture.

Preferably, the temperature can also be used as a controlling parameter. The electronic timer is controlling a cutter of any appropriate type being driven by explosives or any other means. With such control of the reset of the timer the cutting can only be per- formed, when the cable for some time is without any current and the conditions given

by the surroundings are according to the set values of the pressure, the force, or tem- perature, respectively. Further, by being able to reset the timing device by changing the current through the cable, the cutting can be avoided at any time before the cutting is performed.

[0007] In a preferred embodiment the timer is reset from currents in the cable through the use of an interface pick-up. Thereby, a steady detection of the currents in the cable will be possible for both large currents, i. e. measured in A or kA, or smaller currents as measured in mA.

[0008] If the cable incidentally has been destructed by increasing the tensile stress beyond the rupture limit, no current can be guided through the cable. The location of such rupture is normally closer to the surface than to the tool, as the upper part of the cable also is carrying the load of the lower part of the cable. The cutting will therefore take place after the actual preset delay. This is just a security measure, so that the cable sooner or later will be cut. In order to avoid such destruction the inventor has suggested that the pulling force exerted by the cable on the well tool is limited by a mechanical unit. It is further suggested that the activation of the limitation unit can be used for trig- gering the timer.

[0009] Sometimes, the delay will be too long, if the tool must be retrieved in a short time. To this end, the electronic timer--in an advantageous embodiment--can be con- trolled by pulses smaller than the reset pulses, thereby making it possible to the opera- tive crew to remotely control the preset timer delay and the triggering of the cutter. In an advantageous embodiment a capacitor can be arranged between the cable and ground so that small pulses or their flanks can be directed to the timer, without seri- ously influencing the circuits for the larger working currents.

[0010] To exercise the invention it is suggested to use an electronic timer comprises a receiver for receiving reset signals generated by the currents in the cable, an auxiliary inlets for signals from sensors for physical parameters, and an outlet for the control of a cutter control. The physical parameters could be the pressure, temperature, or tensile stress. When these parameters exceed a preset value or leave a predetermined range, a signal is given to the timer, which-after the preset timer period-fires the cutter con- trol and thereby disconnects the cable from the tool. The cutter control can comprise explosives or other cutting effective arrangements to secure the cutting of the cable,

when needed. Such cutting effective arrangements could be springs working directly with knives, scissors or shears.

[0011] More embodiments of the subject matter invention are disclosed in the fol- lowing detailed description of the invention, where an explanation is given using an ex- ample of the invention.

BRIEF DESCRIPTION OF THE DRAWING [0012] The invention is described more clearly in a preferred embodiment shown in the drawing showing an example of a diagram of a circuit for cutting the cable.

DETAILED DESCRIPTION OF THE INVENTION [0013] In the drawing an electronic timer device 1 is in connection with the cable 2 through a pick-up 3. Under influence from not directly shown sensors information about the pressure and the force exerted on the cable is directed to the timing device 1, which--provided the pressure and the force are according to the set values--after a period with no currents in the cable will activate the explosives control 4. This can take place at any time after the tool has stuck in the well. Even if the force exerted on the cable has exceeded the limit, so that the cable has been destructed, it might still be pos- sible to guide pulses down to the control circuit for the electronic timer. This is caused by the capacitor 5 arranged between the cable 2 and ground so that small pulses and their flanks can be directed to the electronic timer, without resetting the electronic timer circuit. For influencing the reset of the electronic timer, larger working currents are needed.

[0014] When the right conditions for firing the explosives are present, i. e. the tensile stress is above a certain limit, the pressure is likewise above the preset limit, the tem- perature might be above the preset limit, and the timer has been allowed to count down, the explosives control 4 will fire and activate the cutter 6, which thereby will cut the cable close to the attached well tool so as to leave as little cable as possible with the at- tached well tool. This measure should secure a safe fishing of the tool, as earlier such left-over of the cable could both be a waste of the cable and impede the fishing of the tool.