METHOD FOR PREPARING A WELLBORE

Abstract

There is provided a method for preparing a wellbore for insertion of a barrier, the method comprising: providing a section of tubing or formation within the wellbore having a modified internal surface that is shaped such that a region adjacent the modified internal surface can be filled with barrier material and the barrier material can solidify to interlock with and be anchored by the modified internal surface.

Claims

1. A method for preparing a wellbore for insertion of a barrier, the method comprising: providing a section of tubing or formation within the wellbore having a modified internal surface that is shaped such that a region adjacent the modified internal surface can be filled with barrier material and the barrier material can solidify to interlock with and be anchored by the modified internal surface.

2. The method of claim 1, wherein the internal surface is modified such that it is shaped with a pattern of indents.

3. The method of claim 1, comprising filling the region adjacent the modified internal surface with the barrier material and allowing the barrier material to solidify such that it interlocks with and is anchored by the modified surface.

4. The method of claim 3, wherein the barrier material is a liquid during the filling stage.

5. The method of claim 1, wherein the modified internal surface comprises a region of the surface having a radial cross section which varies longitudinally, such that the barrier material can be or is anchored longitudinally.

6. The method of claim 1, wherein the method comprises modifying the shape of the internal surface of the downhole tubing or formation.

7. The method of claim 6, wherein modifying the shape of the internal surface of the downhole tubing or formation comprises removing material from the metal tubing or formation using a downhole tool.

8. The method of claim 7, wherein the internal surface is the internal surface of a section of electrically conductive tubing and modifying the shape of the internal surface comprises establishing an electrical connection between the electrically conductive tubing and at least one conductive element such that the selected portions of the internal surface are corroded via an electrolytic process.

9. The method of claim 8, wherein a surface of the at least one conductive element is shaped with patterns or grooves to control the eventual shape of the modified internal surface of the metal tubing.

10. The method of claim 8, wherein the at least one conductive element is centrally placed in the tool.

11. The method of claim 6, wherein modifying the shape of the internal surface of the downhole tubing or formation comprises adding material to the metal tubing or formation using a downhole tool.

12. The method of claim 1, wherein the modified surface is the internal surface of tubing within the wellbore and for at least a portion of the modified section of tubing the internal diameter of the tubing varies in a direction parallel to the central axis of the tubing while the external diameter or the tubing remains unmodified.

13. The method of claim 1, wherein the modified internal surface comprises a plurality of radial grooves formed in the surface.

14. The method of claim 12, wherein the profile of the grooves in a longitudinal cross section through the surface is sinusoidal.

15. The method of claim 1, wherein the modified internal surface comprises a length of the tubing or formation internal surface which has a larger diameter at a lower end and a smaller diameter at an upper end.

16. The method of claim 7, wherein the internal surface is the surface of a section of tubing and forming the modified surface comprises removing between 0.1% and 90%, preferably between 0.1% and 60%, and most preferably between 0.1% and 10% of the material in a length of the tubing.

17. The method of claim 12, wherein the internal surface is the surface of a section of tubing and forming the modified surface comprises removing between 0.1% and 90%, preferably between 0.1% and 60%, and most preferably between 0.1% and 10% of the material in a length of the tubing.

18. The method of claim 2, comprising filling the region adjacent the modified internal surface with the barrier material and allowing the barrier material to solidify such that it interlocks with and is anchored by the modified surface.

19. The method of claim 2, wherein the modified internal surface comprises a region of the surface having a radial cross section which varies longitudinally, such that the barrier material can be or is anchored longitudinally.

20. The method of claim 3, wherein the modified internal surface comprises a region of the surface having a radial cross section which varies longitudinally, such that the barrier material can be or is anchored longitudinally.

Description

[0051] FIG. 1 shows an improved barrier shaped with a sinusoidal pattern;

[0052] FIG. 2 shows an unmodified internal surface;

[0053] FIG. 3 shows a modified container with sinusoidal slots;

[0054] FIG. 4 shows a modified container with a modified internal surface comprising concentric slots separated by areas where material has not been removed;

[0055] FIGS. 5 to 7 show examples of different modified internal surfaces and corresponding barriers;

[0056] FIGS. 8 to 11 show examples of the preferred downhole tool to remove material from metal tubing;

[0057] FIG. 12 shows sinusoidal shaped cathodes with different frequencies and amplitudes;

[0058] FIG. 13 shows a barrier formed in as a long half cycle sinusoid;

[0059] FIG. 14 shows a frustoconical shaped barrier;

[0060] FIG. 15 shows the container with one sinusoidal anchoring slot;

[0061] FIG. 16 shows the container with 2 clusters of anchoring slots;

[0062] FIG. 17 shows different position of the anchoring places relative to the barrier;

[0063] FIG. 18 shows an unmodified internal surface;

[0064] FIG. 19 shows one of the preferred downhole tools to remove material from the metal tubing; and

[0065] FIG. 20 shows a modified internal surface.

[0066] The method described herein improves the sealing capabilities and stability of barriers in contact with a downhole surface. Barriers are anchored to help to prevent shifting position of the barrier once installed. This is achieved by the modification of the downhole surface to produce anchoring points for the barrier material. Generally, the surface against which the barrier will sit once set will be the surface of metal well tubing or casing or the internal surface of the wellbore itself (the formation surface). The formation or the tubing forms a container which is open at one end and into which barrier material can be melted, poured, or placed. A plug may be placed into the well before inserting the barrier material to control the level of the barrier within the wellbore. The formation surface or the internal surface 30 of a tube or casing 2 is shown in FIG. 1. In this case the surface is shaped to form areas of larger and smaller diameter. For a cross section of the surface taken in a longitudinal direction, the grooves in the internal surface form sinusoidal surface features 3.

[0067] An interface 3, which in the embodiment shown in FIG. 1 has a sinusoidal shape in a longitudinal cross-section, is formed between the barrier 1 and the internal surface 30 of the container 2. The barrier is held in position due to the presence of wider regions which sit against wider regions of the internal surface located above and below. The contact surface area between the barrier and the internal surface is increased by the modification to the surface, which improves sealing. Sealing may be improved by the expansion of the barrier if a material such as bismuth, which expands on cooling, is used. Radial forces due to any expansion of or pressure from the barrier material are distributed both axially 5 and radially 6 instead of only radial forces bearing on the internal surface, which may cause damage to casing. Forces originating from the well pressure below the barrier are also distributed both axially 5 and radially 6 instead of only axially. These forces increase the sealing capacity of the barrier 1 by increasing the pressure in the interface 3 between the mating and interlocking surfaces of the barrier 1 and the tubing or formation (internal surface 30). Increasing the sealing capacity in this way means that a smaller barrier may be required in order to support similar pressures compared to larger barriers where surfaces are unmodified.

[0068] FIG. 2 shows an unmodified container comprising a section of tubing 2. In FIGS. 3 and 4 material has been removed from or added to parts of the inner surface of the tubing to form annular grooves 22. The spacing between grooves and the depth and width of the grooves can be varied as shown in FIGS. 5 to 7. Grooves may have a sinusoidal profile as shown in FIG. 1, or other profile type as shown in FIG. 4, among many. In general, a smooth profile is preferable (avoiding sharp edges). The grooves may be spaced close together (FIG. 3) which may improve the anchoring properties of the surface or further apart (FIG. 4) which may reduce any potential weakening of the surface structure while still helping to anchor the barrier.

[0069] When installing a barrier 1, such as the barrier shown in FIGS. 1 and 5 to 7, the barrier will initially be in its liquid form. The liquid barrier will fill the voids or grooves 22 left by modification of the surface. Once the liquid barrier has solidified, the barrier 1 will have a shape which corresponds to (is the inverse of) and interlocks with or mates with the shape of the internal surface 30 as shown in FIGS. 5 to 7.

[0070] The integrity of a structure forming the internal surface 30 (such as metal tubing or casing 2) might be weakened when material is removed. Therefore, the amount of material to be removed and the remaining surface shape of the structure must be optimized in order to increase the barrier 1 performance while minimizing the effect on the integrity of the metal tubing 2. There are several ways in which to achieve this optimization.

[0071] Increasing the number of grooves for a grooved structure will increase the number of seals as wells as anchoring places, however it will also remove more material from the structure forming the internal surface 30. A choice of how many anchoring points to include and how closely spaced these should be will depend on the material used to form the barrier, as well as the material of the internal surface itself. The surface may be shaped with one anchoring point 23, 26, or 27 as shown in FIGS. 13 to 15, or may contain a plurality of anchoring points as shown in at least FIGS. 4, 5, 6, and 7. Anchoring points formed by the modified surface may extend along the whole length of the barrier as shown in at least FIGS. 3, 6, and 7.

[0072] The shape of the surface, and in particular of the longitudinal variation in width of the tubing or formation, may also be optimized. Possible configurations of the longitudinal cross sectional shape of the grooves are triangular, square, metric, ACME, buttress or a combination of the above. Grooves may extend in a helical path around the internal surface or may extend as a plurality of annular grooves as described above. One of the preferred shapes for the grooves is the sinusoidal shape, as it provides good debris tolerance and reduces the stress on the container 2. It is also one of the easiest shapes to form using downhole electrolytic cells to remove material, which is a convenient method for modifying the internal surface and which will be described in more detail below. The sinusoidal anchor cluster is shown in FIG. 1.

[0073] The optimal amplitude and frequency of the sinusoidal shape is dependent on the size of the metal tubing, properties of the barrier material and downhole pressures to mention a few variables. The surface 30 may therefore be shaped with high frequency and high amplitude sinusoidal longitudinal cross section, with a low frequency and low amplitude sinusoidal longitudinal cross section, or a combination thereof. The sinusoidal shape of the surface 30 may have a high frequency and low amplitude as shown in FIG. 12 (left side) or a higher amplitude and lower frequency sinusoidal shape as shown in FIG. 12 (right side). The amplitude may be formed in an example by removing between 0.1 to 90%, preferably between 0.1% and 60%, and most preferably between 0.1% and 10%, of the wall thickness of the metal tubing 2 over the length of one quarter sinusoid. The frequency may be for example between one quarter sinusoid over the entire anchoring point, or the entire length of the modified surface, to 10 entire sinusoids over 1 centimeter of longitudinal cross section. In an example, as shown in FIG. 13, the surface 30 of container 2 is shaped so that the barrier includes an anchor point shaped as a half sinusoid 26 (low frequency).

[0074] An alternative preferred shape is shown in FIG. 14. This frustoconical shaped barrier 27 has larger outside diameter in the downhole end as compared to the upper end. This allows for the downhole pressure applied to the barrier to be evenly distributed over a wide surface area, increasing the sealing capacity of the barrier while preserving the integrity of the container, formation, or tubing. Any combination of the different shapes for the modified surface can be applied. As specific examples of combinations which may be applied, the frustoconical shape or the half sinusoid shown in FIGS. 13 and 14 can include one or more additional radial or helical grooves on their surfaces of the types described above. Alternatively, the modified surface may include a length modified to include grooves and an adjacent length modified as in FIG. 13 or 14.

[0075] The anchoring points may be ring shaped, however they may also be in the form of a helix extending around the surface 30, If the grooves cut into the surface are ring shaped or helical then they will extend all of the way around the cylindrical surface. In some embodiments, however, grooves may extend only part of the way around the surface in a radial direction.

[0076] Anchoring points, here in the form of grooves, may also be separated into clusters 24 spaced along the length of the barrier. As an example, while FIG. 15 shows a surface modification in the form of a single groove 23 cut into the internal surface 30 of downhole tubing 2, FIG. 16 shows two clusters 24 each comprising two sinusoidal grooves in two different positions along the surface 30. Between the two clusters no material is removed or added from or to the surface. Each cluster 24 increases the sealing capacity of the barrier but the spaced configuration helps to reduce the amount of material removed from the surface 30. The number of clusters, and the shapes of anchoring points or grooves within each cluster, the positions of the clusters as well as the distance between the clusters can vary as necessary in order to optimize the barrier performance.

[0077] Downhole pressures applied axially (from below) to an anchored barrier may cause the barrier to balloon below the anchoring point. The axial force may deform the barrier radially, increasing the radial forces between the barrier and the container and therefore the sealing capacity of the barrier. The radial deformation of the barrier is dependent on the properties of the barrier material and the length of barrier below the anchoring point. An anchoring point is shown as point 23 on barrier 1 which sits within casing or tube 2 in FIG. 17. The region of the barrier 25 below the anchoring point may be caused to contract axially and expand radially by pressure from below. The position of the anchoring point can therefore be adjusted to provide a high sealing capacity while reducing the risk of damaging the casing due to the radial forces caused by the barrier ballooning effect. The single anchoring point or anchoring clusters may be placed at the top, bottom, or between the top and bottom of the barrier, as shown in FIG. 17. As mentioned, some material below the anchoring point is preferable to provide a tighter seal due to pressure forces, however this should be balanced with the possibility of damage to the tubing if the barrier expands too far.

[0078] There are a number of means by which to modify the internal surface of a formation or downhole tubing in order to obtain the benefits described above. A downhole tool may be used that is configured to mill, ream, drill, grind, erode or cut material. Such tools can be deployed using wireline, coil tubing or drill pipe and may include commercially available reamers, underreamers and wireline or coiltubing operated cutting tools to mention a few alternatives.

[0079] If the surface modification is to be performed in metal tubing, or any electrically conductive surface, the preferred method for modifying the surface is to remove portions of the casing material using a downhole tool comprising an electrolytic cell to accelerate the corrosion of the metal tubing. An example of such a tool is shown in FIGS. 8 to 10, 11, and 19.

[0080] The downhole tool may comprise at least one conductive element 8 arranged to corrode selected portions of the surrounding tubing 2 using an electrolytic process, said conductive element 8 being made of electric conductive material, an apparatus 9 to establish a connection to the metal tubing 2, and a source of electrical power.

[0081] In order to operate said downhole tool, the brine contained in the well may be conditioned to be of the preferred conductivity. This brine creates a conductive path which allows the electrical current to flow between the conductive element 8 and the conductive tubing 2.

[0082] In order to modify the internal surface of the tubing, the downhole tool is lowered into the well as a conventional wireline or coil tubing tool. It is positioned at the desired depth and clamps 12 and connector 9 for coupling the downhole tool to the metal tubing are activated.

[0083] If the downhole tool is fitted with a milling apparatus 13 as shown in FIG. 11, said apparatus can be used to clean scale or other material depositions from the surface of the casing.

[0084] The conductive elements 8,11 are then provided with electrical current either by a downhole power unit 16 or directly from the surface through the wire 10. Accelerated corrosion of the metal tubing will then begin.

[0085] The brine contained in the well may be circulated around the conductive element 8,11 and the metal tubing 2 in order to avoid the formation of by-products which could reduce the efficiency or the electrolytic process. Circulation may be achieved using an apparatus 15 (shown in FIG. 11).

[0086] Expandable rails may be used in order to set the one or more conductive elements at the desired distance from the tubing. The distance is, however, limited by the presence of non-conductive spacers 14 in order to avoid shorting. Once set at the optimal distance, the electrical current will be provided.

[0087] The conductive elements may be configured to rotate and/or to move in an axial direction within the borehole. Rotation may be continuous or intermittent (may rotate for a period of time in a direction, stop rotating for a period, and then start again in the opposite direction, and so on). If the downhole tool is fitted with rotating conductive elements 11 then the continuous or periodic rotation may be used in order to even out the corrosion of the internal surface of the metal tubing. Spacers 14 can also be used to remove any by-product from the metal tubing 2 or aid the circulation of the electrolyte surrounding the conductive elements 11.

[0088] The shape of the conductive elements can be configurable or can be set in order to form particular shapes. Conductive elements may be shaped to achieve the desired surface modification. A possible shape for the conductive elements is shown in FIG. 12, and this will result in an internal surface of the tubing shaped as shown in FIG. 1. Where the conductive element is wider, material will be corroded from the internal surface faster, so that the shape of the modified surface will mirror that of the conductive element. The downhole tool can also be fitted with one or more elements together forming a frustoconical shape in order to shape the internal surface of the tubing as shown in FIG. 14, where more material has been removed adjacent the bottom end of the conductive element 8,11 than adjacent the top end.

[0089] The variation in distance between the conductive elements 8,11 and the metal tubing 2 will force more electrical current to be diverted towards the zones where this distance is shorter. Higher current will result in more material being removed and therefore the shape of the conductive element 8,11 would be mirrored in the metal tubing internal surface.

[0090] An alternative method, which can be used to create the grooves shown in FIGS. 4 and 20, is to cover areas of the conductive elements with non-conductive material in order to isolate zones 20 where material from the metal tubing 2 does not need to be removed. The uncovered portions of the conductive elements will allow the current to remove material from the metal tubing 2 in regions 22 of the internal surface 30 that are located adjacent to these portions.

[0091] The amount of material removed from the surface is proportional to the electrical current provided. The amount of material to be removed can be calculated and controlled by a measurement of the current applied between the conductive elements and the tubing over time. Once the desired amount of material is removed and the desired surface configuration has been achieved, the electrolytic process is stopped. The shaped surface 30 of the metal tubing 2 is cleaned using the rotating conductive elements 8,11 and the spacers 14 or by any other method. The downhole tool is then pulled out of the hole so that the barrier material can be inserted.

[0092] In order to install the barrier, a plug may need to be placed downhole of the modified surface in order to prevent the barrier material from travelling further down into the borehole. Once the plug is inserted, the barrier material is placed above the level of the plug. This may be achieved by pouring the material into the borehole or by melting the material once already inserted into the borehole. The barrier material fills the area adjacent to the shaped surface such that it conforms with the surface and is left to solidify at which point a barrier is formed. The barrier will be anchored to the shaped or modified surface wherever an indent is formed in the surface as described above.