WELL CASING/TUBING DISPOSAL

Field of the Invention

The present invention relates in general to P&A (plugging and abandonment) and work-over operations for underground conduits such as oil/gas wells, and in particular to the removal or disposal of oil well elements such as well casing/tubing.

Background of the Invention

Once an oil/gas well has come to the end of its useful life, the well needs to be safely decommissioned. An important aspect of decommissioning is the plugging and abandonment (P&A) of the well. The plugging of an abandoned well is necessary to seal it off and prevent the escape of hydrocarbons and gases from within the well.

Various methods are known in the art for providing both permanent and removable plugs within abandoned wells. In some situations a plug may be deployed within a well casing that is itself deployed within a bore hole within the ground.

In other situations it is desirable to deploy a plug which extends across the whole cross-section of a bore hole. In these situations it is first necessary to remove the well casing from the region of the bore hole where the plug is to be deployed.

It is known in the art to remove a well casing from within a well bore hole by mechanical means, such as milling or drilling. Such mechanical approaches can be time consuming and, as a result, expensive. They also produce debris in the form of swarf as the casing is broken down, which can interfere with both the milling /drilling itself and any subsequent plugging operation.

In addition, any swarf produced is classed as contaminated material that needs to be disposed of in accordance with strict regulations. This disposal can be very expensive in wells that are located away from the mainland out at sea.

Other suggested approaches involve the use of heat generating mixtures, such as thermite, to melt not only the well casing but also the surrounding materials of the well (e.g. concrete, formation sand). The aim of melting the well casing (and surrounding materials) is to make use of the materials within the well to actually form a plug rather than removing or disposing of the casing.

However, due to the mixture of materials that are melted to form the plug, the effectiveness (e.g. gas tight sealing and resistance to chemical erosion) of these plugs is more difficult to predict than plugs formed from more homogenous compositions (i.e. eutectic alloys or cement).

On other occasions, rather than carrying out plugging and abandonment (P&A) operations on a well, the aim is to carry out work-over operations so as to repair an underground conduit that has become damaged or blocked for whatever reason. The above mentioned mechanical and heat based approaches are also employed during such operations to clear obstacles from within the well to facilitate subsequent repair work to be carried out.

WO 2013/135583 A2, US 4,890,675, US 2002/0170713 A1, US 2,935,020, US3,565,177 and WO 2013/066340 A1 show related known methods.

Summary of the Invention

The present invention provides methods for use in the removal of well casing and tubing from underground conduits, such as oil/gas wells, to facilitate the subsequent deployment of abandonment plugs within said conduit.

The term 'removal' is used in its broadest sense throughout, in so far as it is the object of the various aspects of the method of the present invention to clear well casing/tubing from a targeted region of well bore hole so as to either expose the surrounding rock formation within which the borehole of the oil/gas well is formed or remove tubing (e.g. production tubing) from within a well casing to clear a path for the deployment of repair tools.

In the case of clearing casing/tubing to expose the surrounding rock formation the clearance formed facilitates the plugging of the entire cross-section of the well bore hole, which allows the formation of a better seal.

In the case of clearing tubing (e.g. such as production tubing) from within a well casing the clearance formed facilitates subsequent repair work to be carried out by removing obstacles to the deployment of the repair tools.

Whether the objective is to form a seal that extends across the entire cross-section of a well bore (i.e. from rock formation to rock formation) or to remove existing tubing from within a well casing to clear a path for the deployment of repair tools, it is envisioned that the methods described hereinafter provide effective solutions.

It is considered important that the various aspects of the method work on the physical properties of the well casing/tubing in such a way that the build-up of debris (e.g. swarf) in the target region. In this way the method of the invention acts to provide a clear work space for the subsequent formation of an effective well abandonment plug, be such a eutectic alloy plug, an ordinary cement plug, or any other known plugging technology.

According to the present invention there is provided a method of clearing well casing or tubing from a target region of an oil/gas well borehole, said method comprising: producing a plurality of perforations in the target region of the well casing/tubing that is to be cleared; delivering a chemical heating mixture to region of the perforated well casing/tubing; and initiating the chemical heating mixture and melting the well casing/tubing in the region to be cleared.

By perforating the well casing before delivering the chemical heating mixture it is possible to deliver the mixture to both sides of the well casing/tubing and thereby achieve a more uniform heating of the well casing/tubing.

Preferably the method may involve clearing the well casing/tubing to expose the rock formation within which the well borehole is formed so that the rock formation can be accessed from within the well casing/tubing.

Advantageously the perforations will be distributed around the entire circumference of the well casing/tubing.

Preferably an area adjacent to the outer surface of the well casing/tubing may be cleaned out by using pressure washing techniques within the well casing/tubing in the region of the plurality of perforations. In this way it is possible to create space for the heating mixture to accumulate adjacent to the outer surface of the well casing/tubing.

Preferably the delivery of the chemical heating mixture may further include squeezing the mixture into the plurality of perforations in the well casing/tubing so that the mixture is provided on both sides of the well casing/tubing as well as within the wall of the well casing/tubing itself.

By using the perforations to deliver the chemical heating mixture to either side of the well casing/tubing and also within the wall of the well casing/tubing it is possible to achieve a more uniform melt of the well casing/tubing around its entire circumference.

Preferably the method may further comprise providing an insulating material in the cleaned out area adjacent the outer surface of the well casing/tubing to maximise the impact of the temperature changes produced in the well casing/tubing.

Additionally or alternatively the method of this aspect may further comprise providing a material in the cleaned out area adjacent the outer surface of the well casing/tubing to draw the heat generated within the well casing/tubing through the walls of the well casing One group of suitable heat drawing materials are commercially available from Dow Corning Corporation under the trademark DOWTHERM™.

Suitable chemical heating mixtures include mixtures comprising thermite and thermate.

Preferably the chemical heating mixture may be provided as a gel, paste, a pseudo liquid or a solid. The gel, paste and pseudo liquid forms being particularly suitable for squeezing in to the perforations formed in the well casing/tubing.

In some applications it would be beneficial for the chemical heating mixture to be provided in the form of at least one solid block. Further preferably the solid block(s) may have a central hole to allow the escape of any steam/gas produced by the heating of down hole fluids during the reaction.

Advantageously the central hole also facilitates control of the burn and keeps it close to the well casing/tubing. It is envisaged that by providing the access/escape means in the middle of the block is better than simply reducing the diameter of the block so that there is a gap between the well casing/tubing and the block, because the block is kept closer to the well casing/tubing and thus is more effective.

It is appreciated that in some operations additional benefit may be achieved by using a combination of chemical heating mixtures in different states. For instance, a paste or gel of heating material could first be applied (e.g. squeezed) through perforations formed in a well casing/tubing to facilitate the heating of the outer surface of the well casing/tubing. Then a solid block, perhaps with the central vents, could be deployed within the well casing/tubing to provide the heat within the well casing/tubing.

It will be appreciated that the use of the various methods of the present invention will facilitate the separation of the well casing/tubing into two sections (i.e. the portion above the consumed/shattered/melted region and the portion below it.

In the case were the well casing is cleared the gap created between the two sections of the well casing exposes the surrounding rock formation and provides a region in which a plug can be formed across the entire cross-section of the well bore hole (i.e. from the rock formation on one side of the borehole to the rock formation on the other side of the borehole).

In the case were an inner tubing is cleared the removal of the tubing from within the well casing facilitates the deployment of repair tools to carry out work-over operations.

Brief Description of the Drawings

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

• Figure 1a shows a well casing in situ within a well bore hole;
• Figure 1b shows the well casing perforation stage of the method of the present invention;
• Figure 1c shows the jet washing stage of the method of the present invention;
• Figure 1d shows the step of the delivery of chemical heating mixture into and around the well casing;
• Figure 1e shows the separate portions of the well casing following the reaction of the chemical heating mixture within the well bore hole.

Detailed Description of a Preferred Embodiment

So that the general concept of the present invention might be better understood an exemplary process of the well casing/tubing disposal method of the present invention will be described with reference to Figures 1a-1e.

The skilled person will appreciate from the following description, that certain steps shown in the drawings may be omitted without departing from the general inventive concept. Further, the skilled person will also appreciate that additional steps to those shown may also be used to achieve additional benefits.

Turning now to Figures 1a, 1b, 1c, 1d and 1e, which show in order the steps involved in removing/disposing of a portion of a well casing/tubing so as to create an exposed region capable of receiving a well plug that can be used to form a plug across the entire cross-section of a well bore hole (i.e. from the rock formation on one side of the borehole to the rock formation on the other side of the borehole).

It will be appreciated that although Figures 1a, 1b, 1c, 1d and 1e depict the application of the clearance method of the present invention to remove a well casing and expose the surrounding rock formation the described method can also be employed to remove tubing other than well casing.

One example of alternative tubing that can be removed using the described methods is production tubing. In cases where only the production tubing is to be removed the surrounding rock formation does not necessarily need to be exposed.

Figure 1a shows a well 1 provided in a rock formation 2. The well comprises a well casing or other form of tubing 3 formed within a bore hole in the rock formation 2. In the region between the rock formation and the casing/tubing 3 is provided an annulus 4, which may be filled with cement.

Figure 1b shows the first stage of the casing/tubing removal method, wherein a plurality of perforations 6 are formed in the casing/tubing 3 by way of a perforating device 5 that is deliverable down the well 1 using existing delivery means.

Preferably the perforating device 5 is capable of delivering a controlled explosion within the region of the casing that is to be perforated. The device 5 is preferably capable of perforating the casing in a 360° target region so that perforations are provided around the entire circumference of the casing.

Figure 1c shows a possible next stage in the method of the present invention, wherein a pressure washing or water jet washing device 7 is delivered down the well 1 to the region of the casing in which the perforations 6 were formed. Once again existing delivery means can be utilised to deliver the washing device 7 to the target region within the well (e.g. cable wire line).

Once the pressure washing or water jet washing device 7 is in position the device can be focused towards the perforations 6 in the casing. In this way the washing device 7 can be used to clean out or erode the annulus material 4 adjacent to the perforated region of the well casing/tubing.

The step of the clearing away a region of annulus material 4 from area surrounding the perforated casing is considered to be advantageous because it provides additional space into which the active chemical agent 9 (see Figure 1d) can be received. In this way the level of heating applied from the outer surface of the casing is enhanced.

It is envisaged that as an alternative, insulating material or a heat drawing material (such as DOWTHERM™) may be received in the space formed by clearing away the annulus material with pressure jet washing.

However it is envisaged that, although beneficial, the step of washing out the region of annulus material 4 may not be essential in all circumstances; for example when the preceding perforating step itself causes the formation of space in the annulus material 4 surrounding the casing 3, which further helps to expose the surrounding rock formation.

Figure 1d shows the step of deploying the active chemical agent 9 to the perforated region of the well casing/tubing 3. The active chemical agent 9 is delivered to the target region using a delivery tool 8, which is connectable to existing delivery means; such as cable wireline.

The delivery tool 8 is capable of carrying the active chemical agent 9 down the well to the target region. Once in position the delivery tool 8 can then be operated to force the active chemical agent 9 through the perforations 6 in the casing and in to the cleared region in the annulus material 4.

In this way both the inside and the outside of the casing/tubing 3 are placed in contact with the active chemical agent 9 thereby allowing a more uniform treatment of the casing/tubing 3 to be achieved.

It is envisioned that in the case of the chemical heating mixture the active chemical agent 9 might advantageously be provided in the form of paste or gel of a material such as thermite or thermate so that it can more readily be squeezed through the casing perforations 6.

Once the active chemical agent 9 is suitably distributed in and around the casing/tubing 3 the chemical reaction can be initiated. Depending on the nature of the active chemical agent being used this may be done remotely or by way of timing device.

As explained above, depending on the type of active chemical agent 9 used the method of the present invention might facilitate the removal/disposal of the well casing by way of melting the well casing in the target region.

Consumption of the well casing

In an aspect not part of the present invention the well casing/tubing is broken down by using chemicals that react with materials from which the well casing is formed.

In its broadest sense any chemicals that are capable of reacting with the well casing in such a way that produces material which is readily cleared from the target region (e.g. in the form of gases or powders) to expose the rock formation are considered applicable.

One appropriate chemical reaction is considered to be oxidation, wherein the casing is effectively consumed or burnt (i.e. like a fuel) rather than melted (i.e. turned from a solid state to a liquid state).

In situations where the well casing/tubing has a steel component it is envisaged that a process of Iron oxidisation might be employed.

Embrittlement or softening and subsequent removal of the well casing

In another aspect not part of the method of the present invention the well casing/tubing is again subjected to high temperatures; whether as a consequence of the high temperatures already present in the down-hole environment or as a result of chemical heating means delivered to the target region.

However unlike the melting approach adopted in the present invention the well casing is subjected to a rapid cooling before melting occurs.

It is envisaged that the rapid cooling of the well casing (possible using cooling means such as liquid nitrogen or cold water) results in a change the structural orientation of the metal from which the casing is formed. This can make the casing more brittle and susceptible to shattering. Although in the case of well casings/tubings that are formed from alloys that have low carbon content the casing/tubing has a tendency to soften rather than embrittle. In both cases the rapid temperature changes transform the well casing/tubing and make it easier to clear/remove.

Once the casing has been embrittled the target region can be subjected to mechanical stresses, such as physical attack or sonic attack. Thus the weakened casing is removed by shattering the casing in the targeted region.

Alternatively, once the casing has been softened the target region can be milled/drilled out using standard milling/drilling equipment. The softened casing is much easier, and thus quicker, to remove. An added benefit is achieve by the softening of the well casing/tubing, in that the sward formed during the milling/drilling process is created in smaller more manageable pieces.

It is envisioned that providing the perforations in the well casing enables the cooling medium to access both the inside and the outside of the well casing, thus providing uniform cooling.

Melting of the well casing

In the method of the present invention the well casing/tubing is subjected to high temperatures which melt the target region of the casing. It is envisaged that thermite and thermate mixes would be particularly suitable to achieve the high melting temperatures of over 1800°C that are required.

Unlike in the first and second aspect described above, where it is considered merely an beneficial additional step, the step of perforating the well casing/tubing is considered essential to this aspect of the invention as it allows the heat to be applied not only from within the casing but also from outside - thereby maximising the destruction of the targeted casing region by melting.

In each of the above approaches a region of the casing/tubing 3 is destroyed and a cleared region 10 is created within the well bore hole. Figure 1e shows the well hole 1 following one of the above mentioned casing removal stages (i.e. melting; embrittlement/shattering; burning).

Once created within the well hole the cleared region 10, which extends to the rock formation within which the borehole is formed, facilitates a well abandonment plug to be deployed therein. The removal of the casing/tubing means that a plug can be formed which extends across the entire cross-section of the well hole (i.e. from the rock formation on one side of the borehole to the rock formation on the other side of the borehole), thus providing a substantial and effective seal.

It is envisioned that the cleared region 10 provided using the method of the claimed invention would be suitable for both cement plugs and plugs formed using eutectic alloys.