In the production of fluids such as hydrocarbons from a subterranean well, it may be desirable to selectively seal or plug the well at various locations. For example, in hydrocarbon (oil and/or gas) production wells, it may be necessary or desirable to seal off a lower hydrocarbon-producing formation during the extraction of hydrocarbons from an upper hydrocarbon-producing formation. In other applications, it may be necessary or desirable to isolate the bottom of the well from the wellhead. Downhole bridge plugs are extensively used in such applications to establish a removable seal in the well.

A conventional downhole bridge plug may include a central mandrel on which is provided at least one expandable sealing element. An annular cone and ridged slip assembly may be provided on the mandrel on each side of the sealing element or elements. The bridge plug may be set in place between adjacent hydrocarbon-producing fractions in the well casing by initially running the bridge plug to the desired location in the casing on a tubing string or using an alternative method and then sliding the slip assemblies onto the respective cones using a hydraulic or other setting tool, causing the slip assemblies to expand against the interior of the casing as they travel on the cones. Simultaneously, the cones move inwardly toward each other and against the sealing rings, causing the rings to expand outwardly against the well casing. Therefore, the slip assemblies and the sealing rings together form a fluid-tight seal to prevent movement of fluids from one fraction to another within the well. When it is desired to re-establish fluid communication between the fractions in the well, the downhole bridge plug may be removed from the well casing. A backup ring may be provided on the mandrel between each cone and the sealing element or elements to reinforce the sealing element or elements after expansion against the casing.

One type of downhole bridge plug, commonly known as a drillable bridge plug, can be removed from the well casing by drilling or milling the bridge plug rather than by retrieving the plug from the casing. In this process, a milling cutter or drill bit is extended through the casing and rotated to grind the plug into fragments until the plug no longer seals the well casing. Drillable bridge plugs may be constructed of a drillable metal, engineeringgrade plastic or composite material that can be drilled or ground into fragments by the milling cutter or drill bit.

One drawback of conventional downhole bridge plugs is that the backup rings on the mandrel may inadequately reinforce the sealing element or elements in the casing after the plug expansion process. This may allow the sealing element or elements to slip on the mandrel during application of pressure to the plug. A common drawback of conventional drillable bridge plugs is that during milling or drilling and grinding of the plug, the mandrel has a tendency to rotate or spin with the cutter or drill bit while the sealing rings and other outer sealing components of the plug remain stationary against the interior surface of the well casing. This effect may reduce drilling efficiency and prolong the time which is necessary to remove the plug from the well bore.

It is the purpose of the present invention to provide a method of sealing the well using a mandrel in combination with a low temperature alloy.

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

According to the present invention there is provided a method of in a single trip into the well, to mechanically clean the casing, melt bismuth alloy, deposit the alloy into a small annular gap between the casing and the bridge plug mandrel, to perform a pull test to confirm the bridge plug is set and to release from the mandrel and retrieve the upper housing or running tool to surface.

According to a further aspect of the invention to set the bridge plug at the casing coupling, the molten alloy sets in the coupling recess and anchors the bridge plug securely to the casing or tubing.

According to a further aspect of the invention, there is provided a heating element to heat the convert the alloy from solid to liquid. According to a further aspect of the invention the combination of bridge plug mandrel and bismuth enables a long metal to metal seal to the casing with minimum use of bismuth alloy.

According to a further aspect of the invention, the mandrel is hollow and a retrieval tool can be deployed to melt the bismuth to recover the bridge plug.

According to a further aspect of the invention, a pressure sensor is situated below the plug and can enable the operator to know the pressure below the plug, so that the pressure can be equalised prior to melting the bismuth seal.

According to a further aspect of the invention, the melted bismuth is collected in a sump container and recovered to surface.

According to a further aspect of the invention, the bismuth both seals the well and self-anchors itself to the casing.

According to a further aspect of the invention a wire brush on the assembly mechanically cleans the casing where the bismuth is to be set.

According to a further aspect of the invention an elastomer seal provides a surface for the liquid alloy to rest on.

According to a further aspect of the invention two different grades of bismuth can be used, to ensure the small annular gap is filled with low temperature alloy before it solidifies.

According to a further aspect of the invention bismuth alloy pins hold the upper housing or running tool to the bridge plug mandrel, and when at the required temperature melt and release the molten bismuth from the running tool. According to a further aspect of the invention bismuth can lock to members together, until melted and then release the running tool from the bridge plug mandrel.

According to a further aspect of the invention feed back to surface of the setting operation is a loss of weight and then a regain of weight.

According to a further aspect of the invention a pull test can be performed to confirm the bismuth is set.

According to a further aspect of the invention a j slot on top of the mandrel enables the running tool to be released.

According to a further aspect of the invention a fragmentable cover can prevent debris falling into the hollow bore of the retrievable bridge plug.

According to a further aspect of the invention an annular sump is provided for debris to collect in.

According to a further aspect of the invention the bismuth can be cast around the heating element.

According to a further aspect of the invention bismuth beads (shot) can be poured into the cavity around the heating element.

According to a further aspect of the invention, the operation is performed rigless.

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 well with a first embodiment of the invention, in its position where the bismuth bridge plug will be set

Figure 2a is a similar view to figure 1 with the bismuth discharged from the running tool and deposited in the annular space around the bridge plug mandrel.

Figure 2b shows part of the J slot of the bridge plug mandrel.

Figure 3 is a similar view to figure 2. With the running tool disengaged from the bridge plug mandrel.

Figure 4a is a section side view of a well with a second embodiment of the invention, in its position where the bismuth bridge plug will be set

Figure 4b shows a perspective view of a possible suitable brush.

Figure 5 is a similar view to figure 4a with the tool moving in the upward direction, and the wire brushes engaged with the casing cleaning the casing surface.

Figure 6. is a similar view to figure 5 with the bismuth discharged from the running tool and deposited in the annular space around the bridge plug mandrel.

Figure 7a is a similar view to figure 6. With the running tool disengaged from the bridge plug mandrel.

Figure 7b shows part of the J slot of the bridge plug mandrel.

Figure 8 is a section side view of a well with a third embodiment of the invention, in its set position in the well, and the running tool removed. Figure 9 is a similar view to figure 8, with a retrieval tool docked into the internal bore of the mandrel Figure 10 is a similar view to figure 9 with the retrieval tool heating element turned on, which in turn melts the bismuth which is collected in the sump attached to the lower end of the bridge plug mandrel. This bridge plug can now be retrieved to surface.

Referring to figure 1 to 3 there is shown a well casing 1, which has to be sealed. A tool assembly 2 is lowered into the well on wireline 3. The tool consists of an upper connector 4, an upper housing 5, which contains a heating element 6 with bismuth 7 either cast onto it or in the cavity as shot or beads. The bismuth pins 8 hold the housing 5 to the bridge plug mandrel 9 of the tool to be left in the well.

At the lower end of the bridge plug mandrel is a combination of one or more wire brushes 10 and an elastomer seal 11, the wire brush mechanically cleans the surface of the casing and the elastomer seal provides a surface for the molten bismuth to rest on before it solidifies.

The sequence of operation will be the tool is lowered in the well until at the setting depth, this will be adjacent to a tubing or casing coupling (as for example shown in figure 4). The tool will then be reciprocated up and down and the brushes 10 will remove any debris and clean the surface of the casing to ensure a good bond of the molten bismuth 12.

The tool will then be held stationary, and the heating element 6 turned on, the bismuth 7 inside the running tool will fully melt, and the heat will eventually melt the pins 8. Electrical power for the heating element could for example be supplied via the wireline on which the tool assembly is deployed on. The bridge plug mandrel 9 will drop and the molten bismuth 7 with it. Pins 13 connect the housing 5 to the bridge plug mandrel 9 via key ways 14, and the bridge plug travel will stop when the pins 13 reach the top of the key way 15. At surface, an indication that this has occurred with be a loss of weight and a regain of weight. It will be appreciated that a smaller volume of expensive bismuth is required to achieve a very long metal to metal seal, as the bridge plug mandrel will be a fraction of the cost of the bismuth.

After a set period of time a pull test can be performed with the wireline to confirm the bridge plug is set. At the top of the bridge plug mandrel is a J slot 16. At position 17 is the end of the mandrel travel when it is first released, when the running tool is lowered, the pins follow the slot and come to rest at position 18, the running tool can then be picked up and it disengages from the bridge plug mandrel and the running tool can be retrieved to surface.

Referring to figures 4 to 7 there is shown another embodiment of the invention. It shows also more clearly the benefit of setting the bismuth plug adjacent to a tubing or casing coupling 20 as there is almost always a cavity 21, which when molten bismuth 22 fills this cavity forms a perfect anchor for the bridge plug to the tubing or casing. This operates very similar to the tool described in figures 1 to 3. Slight differences are a field attachable heat sensitive sleeve 23,24 with cast bismuth 25 joining the two sleeves, one half of the sleeve connects to the running tool housing 26 and the other to the bridge plug mandrel 27. It performs the same function as the bismuth pins 8, in the case the bismuth is a full annular ring so can accommodate a larger shear load.

While running in the well the elastomer seal 28 can fold back and rest on chamfer surface 29 thus minimising its contact to the inside of the casing. The same feature is made for the wire brushes 30,31. When the tool is moved in the upward direction, the seal 28 comes to rest on a surface perpendicular to the casing 32, similarly the same for the brushed 33

It will also be noted that different melting point alloys can be used. In this case a 138C alloy 34 is positioned near the lower end of the cavity 35, and 270C bismuth 36 is placed. The reason for this is, as the molten bismuth flows down the annular gap 37 it could solidify and bridge off, but by heating the 138C bismuth to 270C it has a larger differential temperature before it transitions, so does not solidify until it has fully filled the annular space 37. The 270 C bismuth is deposited where the heater was located so the mandrel will be hot at this point 38

Also, to enable the tool to be filled in the field, a plug 40 can be removed from the upper connector 41 and bismuth shot or beads can be used to fill the cavity 35, this enables the tool to be quickly serviced in the field to perform another job without having to return the tool to a service shop. Referring to figures 8 to 10 there is shown a retrievable version of the tool. In this case, the bridge plug mandrel 50 is hollow 51, and above the seal 52 are passages 53 filled with 270C solid bismuth. On the lower end of the mandrel is a empty housing 54. A fragmentable cap 55 prevent debris from falling into the inner bore 51 of the mandrel 50. Inside the bore 51 is a fishing profile 56

The bridge plug in this example is set as previously described.

To remove from the well, a retrieval tool 60 is lowered in the well on wireline, its nose 61 breaks the cap 55 and enters the bore 51, until the fishing profile 56 is engaged. A pull test can be performed to confirm the profile is engaged.

A heating element 62 is then turned on, and this melts the bismuth 63 on the outside of the mandrel and in the passages 53, the molten bismuth 64 flows into the empty housing 54. At surface the weight indicator will rise indicating all the bismuth has been melted and filled the chamber 64, the bridge plug can be retrieved to surface.