Over the past 20 years or so a large number of offshore structures have been constructed which are now or will soon be exhausted and will need to be abandoned. These offshore structures may comprise production platforms which are either steel or concrete structures resting on the sea bed or floating platforms. Numerous conduits are connected to these offshore structures to carry the various fluids being gas, oil or water etc., which are necessary for the production of oil and/or gas from the well.

In abandoning a well, consideration has to be given to the potential environmental threat from the abandoned well for many years in the future.

In the case of offshore structure there is usually no rig derrick in place which can be used to perform the required well abandonment procedure. Therefore it is typically necessary to install a new derrick or alternatively a mobile derrick can be positioned above the well. This requirement adds considerable expense to the task of abandoning the offshore well, compared to a land based well.

A typical production well will comprise a number of tubular conduits arranged concentrically with respect to each. The method of abandoning the well which is presently known in the art involves the separate sealing of each of the concentric conduits which requires a large number of sequential steps.

In the abandonment method known in the art the first step is to seal the first central conduit usually by means of cement or other suitable sealant. The first annular channel between the first and second conduits is then sealed and the first central conduit is then cut above the seal and the cut section is removed from the well.

The second annular channel between the second and third conduits is then sealed and the second conduit cut above the seal and the cut section is removed from the well. This process is repeated until all the conduits are removed. The number of separate steps required is typically very large indeed and the number of separate operations is five times the number of conduits to be removed. This adds considerably to the cost of the well abandonment due to the time taken and the resources required at the well head.

It is the purpose of the present invention to provide a method of abandoning a well which avoids the disadvantageous and numerous operations which are required by the existing known methods. This will greatly reduce the costs of safely abandoning a well.

It is a further objective of the invention to provide a method of abandoning a well without the requirement of a rig which involves significant expense particularly in subsea based wells.

It is a further advantage of the invention to sever the tubing inside the well to get access to outside the tubing.

According to the present invention there is provided a safe method of igniting a downhole ignitor without risk of it being activated at surface

According to another aspect of the present invention the safe firing head is locked in its safe position by a bismuth plug

According to another aspect of the present invention the safe firing head can only become active when it reaches the melting point of the bismuth

According to another aspect of the present invention the lubricator into which the tool are deployed into the well can be safely pressure tested without setting off the safe firing head

According to another aspect of the present invention different alloy blends of bismuth provide different activation temperatures According to another aspect of the invention, the safe firing head can be hydraulically locked in its safe position

According to another aspect of the invention, the hydraulic fluid can only be drained when the tool reaches a pre-determined depth in the well to release a hydrostatic locked piston

According to another aspect of the invention, the tool is activated by surface pressure applied to the tubing.

The following is a more detailed description of an embodiment according to invention by reference to the following drawings in which:

Figure 1. is a section side view of a first embodiment of the tool with hydraulic fluid locking a moving piston preventing it from moving to its active position

Figure 2 is a similar view to figure 1 , when the tool reaching a pre-determined depth in the well, a piston shears pins and moves to occupy the space of an atmospheric chamber, allowing the hydraulic fluid to drain and unlock the activation piston.

Figure 3 is a a similar view to figure 2, the tool now at the setting depth, tubing pressure is applied and the activation piston shears pins and completes the electrical circuit to activate the ignitor

Figure 4 is a section side view of a second embodiment of the invention, with a molten bismuth set into a recess which locks the activation piston from moving

Figure 5 is a similar view to figure 4, with the tool reaching a depth in the well where the bismuth melts and drains out of the recess, arming the tool Figure 6 is a similar view to figure 5, the tool now at the setting depth, tubing pressure is applied and the activation piston shears pins and completes the electrical circuit to activate the ignitor

Referring to figures 1 to 3

There is shown a tool housing 1, with two male electrical connector 2,3, open circuit and connected to an ignitor cartridge 4. Above the male connectors 2,3 and matching female connectors 5,6 connected to a battery pack 7. These are mounted inside a piston 8 which has shear pins 9 holding to the housing 10. In addition, there is hydraulic fluid 11 located partly outside and partly in a channel through the piston 8 providing a primary safety feature, preventing the piston 8 moving in the downward direction. A second piston 12 engages with the piston 8 and traps the hydraulic fluid 11 and is held in position by shear pins 13. The piston 12 has wellbore pressure on its low side 14, via port 15, and atmospheric pressure 16 on its upper surface 17

At a predetermined depth, for example if we want to activate the tool at the mud line, the temperature inside the tubing will be low and would not be compatible with the version of the tool described in figures 4 to 6. However in this version, the shear pins 13 would be sized to fail 18, allowing the piston 12 to travel in the upward direction and come to rest near the top 19 of the chamber 16, a void space 20 would be created at the low end of the piston 12 allowing the trapped hydraulic fluid 11 to drain and allowing the piston 8 now to move when required.

When the tool is at the required operating depth, tubing pressure is applied outside the tool, shear pins 21 fail 22 and allow the piston 8 assembly to move in the downward direction and engage the electrical connectors 23,24 closing the electrical circuit of the batteries 7 to the ignitor 4

Referring to figures 4 to 6

There is shown a similar assembly to figure 1, the difference being molten bismuth 30 is poured into the recess 31 where it sets and locks the piston 32 into that position, this results in an extremely good primary safety feature. Different bismuth alloy grades can be selected to determine at what depth in the well the bismuth will melt. The following table shows various bismuth alloy compositions and there different melting points.

When the tool reaches the depth in the well where the well temperature is equal to the melting point of the bismuth alloy selected, the bismuth melts and voids recess 33, via ports 34,35

When the tool is at the required operating depth, tubing pressure is applied outside the tool, shear pins 36 fail 37 and allow the piston assembly 32 to move in the downward direction and engage the electrical connectors 38,39 closing the electrical circuit 40 of the batteries 7 to the ignitor 4