Field of the Invention

The present invention describes methods to remove metal alloy well plugs and annular seals from subterranean well bores together with associated downhole tools. In particular, the methods and tools are considered suitable for use in oil/gas well bores.

Background of the Invention

During the life of a well, such as an oil and/or gas well, some assemblies placed in the well bore may begin to leak, or otherwise become compromised, and are accordingly repaired with metal alloy seals. These seals are placed above plugs as well as annular seals of leaking tools such as production packers, liner hangers, tieback seals, packer production seals, cement, gravel pack packers, or other devices. See, for example, WO 2014/096858 and US Patent No. 10,961 ,806, both of which are incorporated herein by reference.

Downhole seals leak for a multitude of reasons. Often the seals are set inside casing or tubing that has not been properly scraped, brushed, or cleaned. Accordingly, the inner surface of the casing or tubing can contain cement stringers, drilling mud, constituents left from the drilling mud or other debris left in the well from previous operations. Additionally, wear and tear on the seals of the tools due to high force axial loads, as well as the temperature and pressure cycles in the downhole environment contribute to failing seals which in turn cause leaks to occur.

Furthermore, tubing in the well and the completion equipment might need to be changed due to erosion, corrosion, or concerns over pressure integrity. In some instances, the tubing is changed to repurpose the well from a producing well to an injection well. The tubing and completion equipment might be a different size, wall thickness, and metallurgy to prevent corrosion and other issues.

Summary of the Invention

The present invention provides methods for removing plugs and annular seals formed from metal alloys (including bismuth based alloys and low melting alloys that melt at temperatures of 300°C and below) within subterranean well bores, such as oil/gas wells. The methods of the present invention are considered particularly suitable for use in removing metal alloy annular seals so that completion equipment can be retrieved and replaced. In a first aspect, the present invention provides a method of removing a metal alloy plug and/or a metal alloy annular seal from a target zone of a subterranean well bore, said method comprising introducing an acid compound into the target zone to break down the metal alloy of said plug and/or annular seal.

As will be described hereinafter in more detail, whilst heaters can be employed in the removal of previously deployed metal alloy plugs and seals, such as annular seals, using acid compounds to break down the metal alloy in a downhole target zone offers a number of benefits.

The use of acid compounds offers not only a quicker and more cost efficient method for breaking down pre-existing plugs/annular seals, but it also actually consumes the metal alloy within the target zone.

In contrast, when heaters are used, melting the metal alloy within a target zone merely serves to change its state, which facilitates the displacement of the metal alloy to another location within the subterranean well bore. Although in most cases this is acceptable, in those situations where alloy run-off is undesirable, the use of acid compounds provides a suitable alternative.

Another advantage of using acid compounds rather than heaters is that, with the deployment of suitable isolation devices in and around the downhole target zone, a more targeted effect can be achieved by an acid compound, the impact of which is limited to the materials they come into direct contact with, when compared with the universal heat increases generated by a downhole heater.

It is envisaged that a range of suitable acid compounds could be employed to dissolve, degrade, loosen, and ultimately remove the metal alloy of the previously deployed subterranean well bore plugs and annular seals.

However, with that said, in the case of bismuth based alloy plugs and annular seals, acid compounds such as sulfuric acid, nitric acid and hydrochloric acid are considered particularly suitable. Nitric acid is considered to be particularly effective as it breaks down bismuth based alloys whilst leaving steel, from which most well tubing/casing is formed, undamaged.

Preferably the method may further comprise perforating a well tubing at a location in, or proximal to, the target zone in order to allow access to an annular space in which said metal alloy annular seal is set, wherein the perforating step takes place prior to the introduction of the acid compound.

It is envisaged that the well tubing can be perforated in a number of ways, including explosive and non-explosive perforation methods.

Further preferably the well tubing may be perforated at a location that is up-hole of the metal alloy annular seal. That is, a location that is closer to the surface that the seal. Additionally, or alternatively, the well tubing may be perforated at a location that is adjacent to the metal alloy annular seal.

It is envisaged that by perforating the well tubing at a location that is adjacent to the metal alloy annular seal, the perforation step pre-weakens the annular seal prior to the introduction of the acid compound, which improves the plug/annular seal removal process.

Preferably the method may further comprise the step of at least partially isolating the target zone within the subterranean well bore to retain said acid compound within the target zone long enough to break down the metal alloy.

It is envisaged that the step of at least partially isolating the target zone serves to focus the action of the acid on the metal alloy. This not only renders the removal process more efficient (by reducing the amount acid compound lost from the target zone) but it also reduces the impact of the acid compound on untargeted downhole tubing and equipment.

Isolating the target zone also allows stronger acid compounds to be used without necessarily increasing the risk of collateral damage to untargeted downhole tubing and equipment.

Preferably the acid compound may be introduced into the target zone using a dump bailer delivered down the well bore. It is envisaged that the dump bailer may form part of a larger downhole tool that, in use, is deployed downhole using traditional approaches, such as cable, wire or work strings.

Alternatively, the step of introducing the acid compound comprises pumping the acid compound into the target zone.

In a further alternative, the step of introducing the acid compound comprises deploying the acid compound down the well bore to the target zone in a delivery container that is configured to break upon impact within the target zone and disperse the acid compound in the target zone.

Preferably, although not essentially, the method may further comprise heating the target zone and/or the acid compound. Various types of heaters may be used to achieve the heating step, with chemical heaters that employ thermate or thermite based heat sources being one preferred example. Further preferably the acid compound may be heated before it is introduced into the target zone. It will be appreciated that by heating the acid compound and/or the target zone in general it is possible to further enhance the effectiveness of the acid compound at breaking down the metal alloy in the downhole environment.

Preferably the method may further comprise subjecting the well tubing to additional loads via tension, compression, internal pressure and/or external pressure either before or afterthe acid compound is introduced to the target zone.

It will be appreciated that using mechanical actions in combination with the chemical action of the acid compound can enhance the effectiveness of the metal alloy removal.

In a second aspect of the present invention there is provided an alternative method for use in removing a metal alloy annular seal from a target zone of a subterranean well bore that employs heat rather than acid compounds to break down a pre-set annular seal formed from a metal alloy, such as a bismuth-based alloy.

In this regard the present disclosure provides a method of removing a metal alloy annular seal from an annulus within a target zone of a subterranean well bore, said method comprising: perforating a well tubing at a location adjacent to said annular seal; and delivering heating means downhole to the target zone via the well tubing and operating such to melt the metal alloy annular seal.

Perforating the well tubing prior to heating serves to create one or more points of egress that the molten alloy can use when the heating step begins. It is envisaged that without the initial perforation step, upon heating of the target zone the molten alloy would have no means of escape and would essentially be retained in the annulus by the well equipment that the metal alloy was originally used to repair (e.g., production packer, cement).

It is envisaged that the well tubing can be perforated in a number of ways, including explosive and non-explosive perforation methods. Suitable perforation methods will be readily appreciated by the skilled person.

Preferably, the method further comprises the step of providing a junk backet a location down hole of the heating means prior to the operation of the heating means such that any molten alloy flowing through the perforations can be collected by the junk basket.

Although the method of removing metal alloy plugs and annular seals according to the first aspect of the present invention and the method of removing metal alloy annular seals according to the second aspect of the present invention can be used to simply remove the metal alloy leaving other well bore equipment in place, it is envisaged that both methods could be employed in order to extract, or pull, well tubing and other completion equipment (e.g., production packers, liner hangers, nipples) from a subterranean well bore.

In view of this, a third aspect of the present invention provides a method of pulling completion tubing and/or completion equipment from a subterranean well bore, said method comprising: a) using the method of either the first aspect or the second aspect of the present invention to remove a metal alloy annular seal; and b) engaging said completion tubing and/or completion equipment and retrieving such from the well bore.

In a fourth aspect of the present invention there is provided a downhole tool that can be used in the various methods of the present invention. In this regard there is provided a downhole tool for use in the removal of metal alloy plugs and annular seals from a target zone of a subterranean well bore, said tool comprising a dump bailer with an internal cavity that is resistant to an acid compound used to break down the metal alloy of said plugs and annular seals; wherein dump bailer further comprises release means that, in use, can be actuated to facilitate the egress of the acid compound from the internal cavity into the target zone.

Preferably the internal cavity may be formed from stainless steel and/or it is provided with a protective coating or lining that is resistant to said acid compound.

The release means can be actively actuated using suitable mechanisms, such as an electric line, a pressure signal, radio communication, and time delay circuits.

Alternatively the release means can be passive. One example of a passive release means comprises a breakable element, such as a glass seal, which retains the acid compound in the cavity until is broken. It is envisaged that the glass seal, for example could be smashed upon impact with a platform within the target zone. The platform might take the form of an existing downhole plug.

Preferably the tool may further comprise a heater that is configured to heat an acid compound housed within the internal cavity.

Preferably the tool may further comprise at least one isolation device configured to engage with a well tubing within the well bore and form a barrier to retain said acid compound within the target zone.

It is envisaged that in some preferred embodiments said isolation device may take the form of a bridge plug. In such arrangements, in addition to retaining the acid compound in the target zone, the bridge plug can act as a platform against which the breakable element of the dump bailer’s passive release means.

Preferably the tool may further comprise a piston configured, in use, to eject an acid compound from within the internal cavity of the dump bailer and introduce it into the target zone. It is envisaged that by ejecting the acid compound from the dump bailer under pressure, the acid compound can be delivered through perforations in the well tubing and into the annular space within which the annular seal may be located.

In a further aspect of the present invention there is provide a method of temporarily plugging an oil/gas well bore, said method comprising: deploying a temporary bismuth-based alloy plug or annular seal within a target zone of an oil/gas well bore; and using the method of the first aspect of the present invention to remove said temporary bismuth-based alloy plug or annular seal.

Brief Description of the Drawings

The present invention will now be described with reference to the drawings, wherein:

Figure 1 shows a subterranean well bore with a metal alloy annular seal and a preferred embodiment of the downhole tool of the present invention deployed therein; and

Figure 2 is a diagrammatic view of the key stages of a preferred embodiment of the method of pulling completion tubing that uses an acid compound in accordance with a first aspect of the present invention to remove the annular seal; and

Figure 3 is a diagrammatic view of the key stages of a preferred embodiment of the method of pulling completion tubing that uses heat in accordance with a second aspect of the present invention to remove the annular seal.

Detailed Description of the Preferred Embodiments

As disclosed above, bismuth based alloy plugs and annular seals can be dissolved, degraded, loosened, weakened and/or removed, in order to pull completion tubing and completion equipment from a well by using the following methods. Also, with suitable preparation, bismuth based alloy annular seals can also be removed from an annulus using heating means, such as downhole chemical heaters of the type described herein. These various aspects of the present invention will now be described with reference to Figures 1 to 3.

Figure 1 depicts an annular seal (18) placed above a leaking production packer (21) which was repaired during some point in time of the well’s life. Other components of the well shown in figure 1 include includes production tubing (3), casing (6), perforation(s) (15), and service tools including an upper isolation device (12), heater/dump bailer (9), and a lower isolation device (24). Lower isolation (24) and upper isolation (12) devices might include bridge plugs, cup seals, or removable packers.

The first method includes perforating above the alloy seal to allow access to the annular seal placed above a leaking production packer, liner hanger, cement column or the like. After perforating, an isolation string is run in the well with some type of cable, wire, or work string. Normal methods of running a service string in the well include coiled tubing, jointed tubing, electric line, or slick line.

A special dump bailer (9) can be used to introduce an acid compound into the area with the annular, alloy seal (18) as shown in Figure 1. Acid based compounds that can break down with bismuth based alloy seals include sulfuric acid, nitric acid, hydrochloric acid and the like. Concentrations, well temperature, heating methods, and soak times are very sensitive variables in determining the rate of reaction. Either the above acid compounds or other suitable acid compounds can be injected into the target area to break down the material included in the alloy seal.

The seal can be loosened, weakened, and/or removed by additionally applying loads via tension, compression, internal pressure or external pressure on the tubing (3) before or during the acid soak.

Heat can also be applied to the target zone in which the alloy seal is located via a down hole heater. Common types include electric or exothermic chemical heaters. A downhole heater may be included in the special dump bailer (9). The signal to the dump bailer could be the same signal to start the heater. Alternatively, a second signal or a delay timer circuit could be included in the heater starter considered to be part of the heater assembly and included as part of the dump bailer (9) of Figure 1 for illustrative purposes.

Another type of heater might be useful to apply heat in the target zone. A thermate, thermite or modified thermite type chemical compound could be carried into the well with the service string in the first run or subsequent service tool string runs. The heating, chemical compound may have one or more of the following forms including liquid, paste, gel. The reaction of this exothermic chemical heater could take place inside a heater tube or be uncontained allowing direct contact with the tubing and alloy seal. The chemical reaction can be started before leaving a special dump bailer or after being released and injected to the target area via tubing or a dump bailer with a piston.

The dump bailer could include a piston which urges the working fluid into the annular cavity through the perforations (15) after being activated. The cavity that contains the acid compound can be lined or coated with protective, inert, materials enabling strong acidic compounds to be carried into the well. The contents of the bailer might be heated during the trip as the downhole temperature is normally higher than the surface ambient temperature. Additionally, the heater part of the dump bailer assembly could be started before the assembly reaches the target area of the well bore in order to preheat the acid compound before contact with the alloy. Several different heat energy schedules can be designed and included in the heater to be used in the well bore. Long duration moderate output or short duration high heat output can be required depending on the volume of the alloy seal, well fluids, heat transfer, and tubing/casing sizes of the well configuration. Multiple heaters of different heat output configurations, sizes, and configurations, could be designed into the dump bailer (9) or made up to the dump bailer (9). If one application of the steps described above is not sufficient to retrieve the downhole components, then multiple applications of the steps may be needed. These steps could involve multiple runs into and out of the well bore to achieve the removal of the alloy seal (18), the completion tubing (3), and the associated well components.

Other techniques might be included with the service string (1 ,9) run in the well including assemblies that induce vibration which could be helpful to loosened, weaken and/or remove the alloy seal and the tubing.

Removing or weaking the production tubing (3) and/or packer components (21 ) located adjacent to the alloy seal might be necessary. Extra perforations, tubing cuts via down hole radial torch, milling, section milling, abrasive jet cutting, laser cutting, or hydraulic knife cutting might be useful to place or circulate acidic compounds to dissolve, degrade, loosen, weaken and remove the alloy seal.

If the alloy seal (18) and the production packer (21) are not removed with the completion tubing (3), then multiple runs into and out of the well with common downhole fishing tools can be used such as jars, overshots, grapples, spears, etc. These could be combined with the steps above or used to pull the alloy seal, packer, or packer components in combination with the steps described above. Also note that alloy fishing tools can be used to hold different down hole components as described in the international patent application number WO2016/024121 , incorporated by reference herein.

Note that that the tubing (3) of Figure 1 might be small or large diameter oil field tubing or casing if the leaking device is a liner hanger instead of a production packer (21). If an alloy seal above a liner hanger is being removed it might be possible to remove the tieback casing and the tieback seals before the acid soak and other steps described above. In the event the tieback cannot be removed the methods described herein could be used to remove an alloy seal.

Signals can be sent to the dump bailer and/or start the heater using normal downhole techniques including electric line, pressure signature, radio communication, time delay circuits, etc. Gas operated dump bailer designs exists and are in use in downhole methods today. If a conduit is used (coiled tubing or jointed tubing), the service tools can be simplified by replacing the heater/dump bailer (9) combination with a heater and a ported sub. The acid compound could be pumped in, or spotted by volume and pushed through the perforation(s) where it can soak the metal alloy.

In some removal operations it is considered suitable to adopt an even more simple approach by dropping a container of a suitable quality of the acid compound down the well bore via the production tubing. Upon impact with an isolation device (e.g., a bridge plug) within the target zone, the container will smash and its contents will be dispersed within the target zone. Glass containers are considered particularly preferable for this acid introduction approach.

The quantity of acid composition required for each removal method will be determined by the amount of metal alloy that makes up the annular seal and/or plug that is to be removed, the details of which should be a matter of record for each well repair.

Perforations can be placed above, into, and possibly below the alloy volume to enhance the surface area in contact with the acid compound.

The acid soak method described above can be used on alloy plugs set in the tubing or combinations of plugs and annular seals. This method will be especially attractive for temporary, high integrity, gas tight, temporary sealing applications in casing and tubing.

If the density of the acid compound chosen is greater than the completion fluid the steps can be simplified as follows: set a bridge plug in the tubing, perforate, run the acid compound in the well with a dump bailer that has a glass bottom, set down to release or other means to set weight down on the bridge plug to open. The acid will exit the dump bailer and start the soak period when making contact with the alloy.

The methods of the first and thirds aspects of the present invention will now be described with reference to the key stages of a preferred embodiment shown in Figure 2.

As in Figure 1 , the main components of a target zone of a subterranean well bore are shown in Figure 2. A production tubing (3) is shown in situ within a well casing (6). The annular space between the well casing (6) and the production tubing (3) is provided with a production packer (21 ). Following the partial failure of the production packer (21 ) a previous repair was made to deploy a metal alloy annular seal (18) into the annular space to prevent leaks. It is envisaged that the metal alloy used to form the annular seal could be a bismuth based alloy or a low melt alloy.

In order to remove the production tubing (3) and the production packer (21) the first stage of the preferred method of the present invention is to perforate the production tubing (3) to provide perforations (15) that allow access to the annular space from within the production tubing.

It is envisaged that perforations can be formed in a range of ways, that include explosive and non-explosive perforation devices. The skilled person will appreciate that perforation of the production tubing can be achieved in using a range of solutions that are well known in the art. It is also envisaged that in some situations the production tubing may already have perforations as a result of the previous deployment of the metal alloy annular seal (18).

An isolation device, which in the preferred embodiment is a temporary bridge plug (30), is delivered downhole to the target zone via the interior of the production tubing (3) and is then deployed at a location down hole of the annular seal (18).

Once the bridge plug (30) is in situ, the downhole tool (31) is delivered down hole to the target region using delivery means (1). As an aside, it is envisaged, that although the downhole tool (31 ) and the bridge plug (30) are delivered down hole separately in the shown embodiment, in a preferred alternative the bridge plug could be deployed down hole with the downhole tool in a single deployment.

The downhole tool (31) comprises a dump bailer with an internal cavity that houses an acid compound (32), preferred examples of suitable acid compounds include sulfuric acid, nitric acid, and hydrochloric acid. However, it is envisaged that any acid compounds capable of degrading alloys, and in particular bismuth based alloys, could be employed in the method of the present invention. In a preferred embodiment, nitric acid with a concentration of above 50%, and in particular between 50-70%, is employed to break down bismuth based alloys.

In order to accommodate the acid compound (32) during its delivery down hole, the internal cavity of the dump bailer of the downhole tool (31) is provided with a protective lining or coating (e.g. silicate-based paints). Alternatively, because stainless steels are resistant to nitric acid, the dump bailer may simply be formed from stainless steel.

In the first key stage shown in Figure 2, the downhole tool (31 ) has been delivered to the target zone within a subterranean well bore. Once in position, the downhole tool (31) can be operated to actuate release means (not shown) that eject the acid compound (32) from dump bailer into the target zone.

In the second key stage shown in Figure 2, the acid compound (32) has been ejected from the dump bailer of the downhole tool (31) and has flowed, via the perforations (15) into the annular space where it contacts the metal alloy annular seal (18). Within the production tubing (3) the placement of the bridge plug (30) serves prevent the acid compound from being lost down hole. Preferably, as with the internal cavity of the dump bailer, the bridge plug (30) may be formed from stainless steel or it could be provided with a protective coating (e.g., a silicate- based coating) that is resistant to the acid compound.

Over time, the acid compound dissolves the metal alloy of the annular seal (18) until such time that it has been broken down and, in effect, removed. It is envisaged the speed with which the acid compound breaks down the alloy could be increased by heating the target zone. To this end, it is envisaged that the downhole tool (31) may further comprise heating means.

It is appreciated that, whilst heating alloy within a target zone would simply change the state of the alloy from a solid to a liquid, the use of acid compounds actually consumes the alloy.

The heating means, which may preferably take the form of a chemical reaction heat source, such as a thermite or thermate heater, can be used to pre-heat the acid compound prior to its introduction into the target zone.

Once the metal alloy annular seal has been removed the downhole tool (31 ) and the bridge plug (30) can be retrieved from the well. This may be carried out in a single or multiple steps, depending on whether the bridge plug is provided as part of a larger downhole tool assembly. This third key stage, following the retrieval of the downhole tool and the bridge plug is shown in Figure 2.

In the case of the method of the first aspect of the present invention, in which only the metal alloy annular seal is to be removed, this would represent the end point. However, in the case of the method of the third aspect of the present invention a further key stage is carried out. In this final stage, the production tubing and the production packer are retrieved (i.e., pulled) from the subterranean well bore to leave just the well casing (6) in situ. The operator is then able to deploy new completion tubing and equipment in the well bore.

Under some well conditions with relatively low alloy volumes the alloy seal can be melted by placing a heater adjacent to the inner wall of the tubing. In some instance, the alloy seal can be melted by perforating and then placing the heater adjacent to the inner wall of the tubing. This is the approach provided for in method of the second aspect of the present invention.

Turning now to Figure 3, the method of using heat to remove metal alloy annular seals from a target zone of a subterranean well bore will be described. Once again, and as with Figure 1 , Figure 3 depicts an annular seal (18) placed above a leaking production packer (21) within the annulus between a well casing (6) and a well tubing (3), which was repaired during some point in time of the well’s life.

In the first stage a perforation tool (40), which it is envisaged could be an explosive or nonexplosive device, is delivered down hole via the central well tubing (3) to a location adjacent to the alloy annular seal (18) that is to be removed. The perforation tool (40) is then operated to perforate the well tubing (3) to form perforations (15) that are in-line with the annular seal (18).

It is appreciated that the perforation step not only serves to create a pathway between the inner diameter of the well tubing (3) and the annulus formed between the well tubing (3) and the well casing (6) but it may also do mechanical damage to the seal that could loosen it.

It is also envisaged that in some situations the production tubing may already have perforations as a result of the previous deployment of the metal alloy annular seal (18). However, as the positioning of such perforations would be unsuitable to facilitate the escape of the molten alloy, the provision of targeted perforations adjacent to the annular seal is considered essential even if the well tubing has pre-existing perforations.

Following the perforation step, a heater (41) is delivered into the target zone using known techniques and is aligned with the annular seal (18) and the perforations. In Figure 3 the perforation tool (40) and the heater (41 ) are shown being deployed downhole separately. However, it is envisaged that both tools could be deployed downhole at the same time as part of a combined tool assembly.

Once in position, the heat source of the heater (41 ), which is preferably a chemical reaction heat source such as thermite, thermate or a thermite blend, is activated and the target zone is heated. The increased temperatures within the target zone cause the metal alloy of the annular seal (18) to become molten.

Due to the presence of the perforations (15), any molten alloy that cannot readily flow past the production packer (21), can instead flow through back into the inner diameter of the well tubing (3). Although not shown, it is considered that the heater tool may be provided with a junk basket suspended from its down hole end so as to catch the molten alloy as it flows back into the well tubing (3).

Once the metal alloy annular seal has been removed (as is shown in the third stage of Figure 3), the production packer (21) is left exposed and can be readily destroyed or retrieved. The final stage of the process shown in Figure 3 has the well tubing (3) being removed from the well casing (6).