Arrangement and method for separation of tool string

The present invention relates to an arrangement and a method for releasing a cable from downhole well equipment arranged in a well, according to the introductory parts of claims 1 and 8. In particular the invention relates to an arrangement and a method adapted for separation of a cable suspended downhole well tool string, wherein this takes place at absence of movement in the tool string.

Downhole well equipment used in oil and/or gas wells are often suspended in a cable and is guided into the well together with the cable. In many cases it is desirable to be able to release the equipment from the cable. This can be done by arranging a release mechanism in connection to or in the downhole well equipment, which at command from the surface releases the equipment from the cable. It can also be a matter of releasing a part of the equipment from the rest of the equipment, so that a part of the equipment remains suspended in the cable. This can be particularly desirable if an upper part of a tool string contains expensive components which one does not want to be left in the well.

Such a situation can for instance arise if a downhole well tool gets stuck in the well. If it is impossible to pull the tool out of the well with the cable or without damaging the cable, the tool must be freed from the cable before the cable can be pulled up to the surface.

This can for instance be necessary if one shall attempt to collect the tool with a so-called fishing tool, whereby the cable must be removed before the fishing tool can be guided into the well. In addition there are cases where one wants to leave a part of a tool string in the well, even if it has not got stuck.

It is known to arrange a weak rupture point in the cable for this purpose. If the downhole well tool has got stuck in the well, it is then possible to rupture the cable at such a weak point by pulling the cable upward with sufficient force. However this not desirable because it reduces the weight that can be carried by the cable.

As mentioned above, it is also known to arrange a release mechanism which can be controlled from the surface. Signal conductors for controlling such a mechanism are led down in the cable. However, because of the conditions to which the conductors are

exposed when entering a well, ruptures may arise in these. Other methods for releasing the downhole well tool from the cable are thus desirable.

Norwegian patent publication no. 318058 describes a method and an arrangement for controlled release of a cable. Here, the cable is provided with a conductor (6) which is adapted to transmit signals down to a cable tool (8) from the surface. In the case of absence of the signals, a circuit (24) in the cable tool will after a certain time, by use of a timer, activate a disconnect device (12) which is adapted to disconnect the cable (4) from the cable tool (8). Thus, if the conductor (6) is destroyed the disconnect device will be activated, even if the cable tool has not got stuck. In such a case the cable tool must be pulled up to the surface before the activation time has expired, if one wants to avoid collecting the tool in another way.

Another solution is described in international patent application WO 2004/038171 Al. This also describes a method and an apparatus for releasing a cable from a well tool. Here a cable cutter is used that is adapted to cut the cable near the cable tool at activation. As described in Norwegian patent publication 318058, a timer is used here also for delayed activation of the cable cutter. However, in order to activate the timer at least three parameters must be fulfilled within determined value ranges. Such parameters are suggested to possibly be pressure and temperature in the well, and the pulling force from the cable. If the cable tool gets stuck in the well, the two first parameters will be substantially constant over a long time. The pulling force on the tool from the cable can be controlled by the operator, and the cable cutter can thus be activated after a certain time.

The use of at least three parameters for the activation of the timer represents an unnecessary cumbersome way to control the release. The reading / measuring of more parameters than necessary will both occupy space in the equipment, be unnecessary expensive, and also use unnecessary battery capacity if the signal conductors / power supply from the surface down to the tool is broken.

The object of the present invention is to provide an arrangement and a method which on a more favourable and advantageous manner solves the object introductorily described.

This object is reached with an arrangement according to the characterizing part of claim I ₅ as well as with a method according to the characterizing part of claim 8.

According to a first aspect of the invention, there is provided an arrangement for separation of a cable suspended downhole tool string of the type used in oil and/or gas wells, wherein the point of separation is between the tool string and the cable or in the tool string itself. The arrangement comprises a mechanism for the separation, and a timer for use in controlling the mechanism, wherein the arrangement is adapted in such way that the mechanism is activated if the timer expires after a predetermined time period. The arrangement further comprises an accelerometer for monitoring the acceleration of the downhole tool string. The arrangement is adapted to restart the timer at acceleration values above a predetermined value.

Preferably, the arrangement further comprises a temperature gauge for measuring the temperature, and is adapted to take a deactivated mode at temperatures below a predetermined temperature threshold and an activated mode above said temperature threshold, wherein the timer only is started in said activated mode. Such an embodiment of the invention will contribute in saving battery time, if the arrangement is run on battery power. It also will reduce the possibility of an accidental activation of the release mechanism in case the arrangement is prepared and left outside the well.

In the activated mode, the arrangement is advantageously adapted to calculate temperature differences between the temperature at the previous start of the timer and the temperature at a real time, hi such a case, the arrangement is arranged to restart the timer if the calculated temperature difference is larger than a predetermined temperature deviation threshold. Thus, if a temperature gauge is arranged in the arrangement, it can be used to control the timer (i.e. release mechanism), in addition to the accelerometer.

Preferably, it can comprise both a positive and a negative predetermined temperature deviation threshold. It can thus detect upward and downward movement of the tool string in the well, since the temperature rises downwardly in the well.

In order to save power, especially if powered by battery, the arrangement is adapted to measure the temperature and compare it with the predetermined temperature threshold maximally once per minute.

Preferably, the arrangement is arranged to restart the timer when receiving a voltage signal sent from the surface. Thus, the operator can avoid an actuation of the release mechanism from the surface, even if the tool string is located in the same place (not moving and constant temperature) over a long time.

Advantageously, the arrangement also comprises a pressure gauge. In that case, the arrangement is such adapted that it takes a deactivated mode at pressures below a set pressure threshold and an activated mode above said pressure threshold, and the arrangement is adapted to start the timer only in the activated mode. This embodiment, in a similar manner as with the temperature gauge, will contribute in saving power.

According to a second aspect of the present invention there is provided a method for separation of a cable suspended downhole tool string of the type that is used in oil or gas wells, wherein the point of separation is between the tool string and the cable or in the tool string itself. The method comprises arranging the tool string in an oil or gas well. Furthermore, the method comprises a) measuring the acceleration of the tool string by means of an accelerometer arranged in the tool string; b) restarting a timer in the tool string when measuring accelerations above a predetermined acceleration value; and c) initiating separation with a separation mechanism arranged in the tool string if the timer runs for a predetermined time period without being restarted.

Preferably, the method also comprises measuring temperature with a temperature gauge arranged in the tool string and perform steps a) to c) only if the measured temperature is above a predetermined temperature value. In this advantageous embodiment, temperature differences between a temperature measured at a previous start of the timer and a temperatures measured later are calculated, and the timer is restarted if a calculated temperature difference is larger than a predetermined temperature deviation value.

According to the invention, an accelerometer is sufficient for detection of whether the tool string is moving or not. This gives a considerable advantage in relation to known solutions, since the accelerometer can be adapted not to use energy as long as it does not detect any movement/acceleration. In this way battery time is saved when the arrangement is battery powered.

Example

In the following, an example embodiment of the present invention will be described with reference to the accompanying drawings, in which Fig. 1 illustrates a principle sketch of a tool string;

Fig. 2 illustrates a principle sketch of the tool string in Fig. 1, arranged in a well; and Fig. 3 illustrates a flow diagram for controlling the mechanism for separation of the tool string.

In Fig. 1 an example embodiment of an arrangement 1 according to the present invention is shown. The arrangement 1 comprises a control part 10 comprising battery and control electronics, as well as a separation mechanism 12. The arrangement 1 can constitute a part of a tool string that is suspended on the end of a cable, for performing well operations.

The control part 10 comprises a PC-interface which makes it able to communicate with a computer. Through the interface, the operator can adjust and change desired parameters. Furthermore, the control part 10 comprises a battery for power supply to a logic control (see Fig. 3). The battery can also supply power to the separation mechanism 12, for activation of this.

The separation mechanism 12 is arranged below the control part 10 in the arrangement 1.

Fig. 2 shows the arrangement 1 arranged in a well 16, supported on a cable 18 in its upper end. Additional well equipment (not shown) can be connected to the lower part of the arrangement 1.

Fig. 3 shows a flow diagram for the logic control of the separation mechanism 12. The logic control can be supplied with power from either a battery 20 or through cable supply

22. Under normal operation the arrangement will receive power from the surface through a wire in the cable 18. If the power supply from the surface should fall out, for instance due to wire cut-off in the cable 18, the arrangement will receive power from the battery 20. When the operator has set the desired parameters in the logic part (Fig. 3), the battery 20 is connected to the arrangement. The battery is connected to a bridging plug (not shown) which will arm the arrangement, and which will activate the separation mechanism if all activation parameter conditions are fulfilled.

In a particularly preferred embodiment there is arranged a temperature gauge 24. This part can preferably also comprise a pressure gauge. The arrangement is adapted to take a deactivated mode ("low power mode") at temperatures below a predetermined value, and an activated mode above the predetermined value. Such a predetermined temperature value can for instance be 80 °C. The temperature can advantageously be registered about once a minute.

As the arrangement or tool string, respectively, is guided down through a well, the temperature will rise, and when it exceeds the set temperature value (80 °C), the arrangement will accordingly change from the deactivated mode to the activated mode. Not until being in the active mode will the logic part start to monitor acceleration measurements by means of the acceleration sensor 26. The operator has preset a parameter value for acceleration of the arrangement 1 (as described above). If the monitored movement or acceleration is below the set parameter value, a timer 28 in the logic part will be started.

As long as no acceleration above the set parameter value is monitored, the timer 28 will run. The logic part is adapted in such way that if the timer 28 runs a predetermined time period without being disrupted or restarted, the logic part will actuate the separation mechanism 12 (Fig. 1). The operator can for instance set such a time period equal to 48 hours. The timer 28 is connected to a separation control 30 which controls the separation mechanism 12.

However, if the logic part detects an acceleration of the arrangement 1 that is above the predetermined value, the timer 28 will be restarted. That means that it starts from the

beginning, and a possible timer-initiated actuation of the separation mechanism 12 can first take place after the time period set by the operator.

In this embodiment, the timer 28 can only be started in the activated mode. Also, the logic part performs acceleration monitoring only in the activated mode. In this way, battery time is saved when the tool string is out of its region of operation, such as high up in the well or outside the well 16.

In the embodiment with a temperature gauge 24, temperature deviation can also advantageously be implemented as a parameter that can disrupt the timer. In the activated mode, temperature differences between temperature measurements performed after the timer 28 was last started can be registered. The logic part is adapted to calculate temperature differences between the temperature when the timer 28 was last started and temperatures measured later. If the difference between two such temperatures is larger than a value predetermined by the operator, the logic part will restart the timer 28. The operator can advantageously set both positive and negative values for temperature deviation. Such a value can for instance be 50 °C. That means that if the temperature has changed more than 50 °C since the timer 28 was last started, the timer 28 will again be restarted. This can happen if the tool string is moved upwards or downwards in the well 16, which has different temperatures at different depths.

In stead of, or in addition to the temperature gauge 24, there may also be provided a pressure gauge. In the same way as the arrangement 1 is adapted to take a deactivated mode or an activated mode depending on the temperature, the arrangement 1 can also be adapted to take a mode depending on the surrounding pressure. In this way the arrangement will for instance only take an activated state when it is arranged so deep in the well 16 that a certain pressure surrounds it.

If the timer 28 expires, the separation control 30 will provide current from the battery 20 to the separation mechanism 12 for ten seconds. The separation mechanism 12 will then split in two. The separation control 30 will wait for one minute and then repeat the process. This will repeat until the battery is dead.

As mentioned above, the separation control 30 controls the activation of the separation mechanism 12. In the description above, the separation control 30 receives information from the timer 28. However, the logic part can also advantageously have a decoder 32 for surface commands, which is adapted to give input to the separation control 30. The decoder 32 is adapted to receive commands from the operator through signal conductors from the surface. In this way the operator can control the separation mechanism 12.

The operator can for instance give command for actuation of the separation mechanism 12 by feeding the decoder 32 with voltage peaks with a certain length and a certain value. Such an actuation command can for instance be to feed the decoder 32 with a voltage above a predetermined threshold value in 15 seconds, then reducing the voltage within three seconds, and then repeat this five more times. Such a command can advantageously activate the separation mechanism 12 independently of the arrangement 1 being in the deactivated or activated mode.