METHOD FOR PREPARING A WELLBORE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patent application Ser. No. 17/927,690, filed on Nov. 23, 2022, which is a 371 National Stage entry of Patent Cooperation Treaty Application No. PCT/NO2021/050134, filed on May 26, 2021, which claims priority from Norwegian patent application No. 20200624, filed on May 27, 2020, the disclosures of which are hereby incorporated by specific reference thereto.

DESCRIPTION OF THE INVENTION

The present invention relates to a method for preparing a wellbore by providing a section of piping with at least a part of its external surface shaped to include a pattern of indents and/or protrusions. The present invention also relates to a section of piping having a shaped external surface and to a method for producing a section of piping having a shaped external surface.

The preparation of a wellbore begins with the setup of a rig above a drilling site. A bore is then created in the formation below the rig using a drill bit at the end of a drill string which is lowered downhole to the desired depth. Once drilling is complete, the resulting bore is lined using connected sections of piping referred to as casing in order to both protect the surrounding formation and stabilize the wellbore. Several concentric layers of piping may be present (casing within casing). The drilling and the placement of the casing may be carried out in stages, progressing to greater and greater depths at each stage. Another piped structure, referred to as tubing, is lowered into the lined wellbore and it is through this tubing that hydrocarbons and/or other substances to be extracted are carried from the formation to the rig and the surface.

It is common to fill any gaps between the casing and the formation, between nested and adjacent sections of casing, and in some cases between the casing and the tubing, with a filler such as concrete or a similar material. Particularly in sections of the wellbore that are horizontal or inclined, eccentricity of the casing or tubing can be an issue. This occurs when parts of the piping structure sag, bend, or drop so that they are no longer centered within the wellbore. An example of eccentricity of concentric casing strings and a tubing string is shown in FIGS. 1A and 1B. In FIG. 1A the casing strings 1 and the innermost tubing string are centered within a wellbore which has been drilled through formation 7 (the piping sections are coaxial in that the longitudinal axes of the casing string, tubing string, and wellbore are aligned). In FIG. 1B, the tubing and casing sections 1 have dropped so as to sit against a lower wall of the wellbore in contact with the formation 7 and with adjacent casing sections. Where casing and tubing strings are in contact as shown in FIG. 1B, it is difficult to produce a sufficient seal when filling regions between adjacent parts of the casing strings, tubing, and formation. Filler 21 is not able to access these regions.

The integrity of the final wellbore structure, including the borehole, casing, tubing, and filler, is crucial to maximizing the efficiency of the extraction process. Methods to improve this are therefore desirable.

According to a first aspect of the present invention, there is provided a method for preparing a wellbore comprising: positioning one or more sections of piping within the wellbore as part of a piping string by attaching the one or more sections to additional similar sections of piping, wherein at least a region of an external surface of the one or more sections of piping is shaped to include a pattern of indents and/or protrusions, and wherein the shaped region of the external surface is located so as to be exposed to a surrounding external environment when the section of piping is coupled to the additional similar sections of piping as part of the piping string.

The patterning on the external surface of the piping section ensures that filler material is able to enter a space created by the indents and/or protrusions between the piping external surface and an adjacent structure to provide an effective seal. This is possible even if the piping is not centered within the wellbore and is in contact with or sits close to the adjacent structure. The adjacent structure may, for example, be a section of casing external to the piping section or a region of the surrounding formation. Using a section of piping having a modified external surface in this way improves the integrity of the final wellbore structure. The additional similar sections of piping may or may not also have a shaped external surface.

The modification can be applied downhole, but a simple way to achieve the associated advantages is to provide sections of piping for which the modification is already present and install these prepared sections in a wellbore, for example as part of a casing or tubing string. The modification may therefore be achieved using a shaped mould, by way of a specialized tool, or in another manner during production, or by adapting the piping in a factory, at a storage or service facility, or on a rig before placing it downhole. Reference to a section of piping can be to any type of piping, including at least a section of piping, such as casing and/or tubing, which can be used to form a piping string within a wellbore by coupling a plurality of similar sections together end to end. One, some, or all of the sections of piping within the string can have an external surface that is shaped to include a pattern of indents and/or protrusions.

The external surface refers to a surface facing outwards towards the surrounding formation or structure within a wellbore. For a section of piping, such as tubing or casing, the external surface will be the convex surface facing outwards away from the longitudinal axis of the piping, with the internal surface referring to the piping surface within the pipe bore and facing towards the longitudinal axis. In embodiments, at least a part of the external surface which will be positioned so as to contact a filler material during use is modified to include one or more indents or protrusions. In embodiments, at least a part of the external surface which does not form part of a coupling element for connection to similar piping sections is modified.

The shaped part of the external surface is exposed to a surrounding external environment when the section or sections of piping are coupled to other sections as part of a longer piped structure (a longer section of tubing or casing, for example). Use of the term exposed to a surrounding external environment in this context refers to the fact that it can directly face other structures, such as the surrounding formation, additional larger diameter casing sections, or concrete filler, and is not covered by parts of adjacent piping sections within the same piping string, for example within a coupling structure. The exposed parts of the external surface are those that would be visible if the compound piping string were to be assembled at the surface from the component piping sections by joining them together end to end either directly or using an additional attachment member. When filler, such as concrete, is used to seal against the external surface of the pipe section, after the piping string has been assembled and placed downhole, the filler is able to reach the shaped sections of the external surface. Where these are in contact with adjacent structures due to eccentricity of the piping string, filler can still reach the shaped regions by filling the indents in the surface. This is a great advantage in terms of improving wellbore integrity.

In embodiments, the method comprises drilling a borehole through a formation prior to locating the piping section within the borehole.

In embodiments, the at least one section of piping comprises coupling structures at either end for attachment to the additional similar sections of piping, and the external surface extends between these two coupling structures. The external surface does not, therefore, include the outer surface of any coupling structure (although this can also be modified in some examples). The coupling structures may be screw threads in some examples. If screw threads are present, threads can be provided internal to the piping at one end and external to the piping at the other end for coupling to a similar section of piping at either end. These screw threads will not be exposed to a surrounding external environment when the piping section is coupled at either end to another piping section within a string, but rather will directly face another part of the same string. Other types of coupling structure can be used, such as mating pins and holes, clips, and so on. An additional attachment member can be used to mate to the coupling structure at either end of adjacent pipes within the wellbore to hold the two together in some cases. These coupling structures will then also form a part of the piping string. The end regions of the piping will usually then be contained within the attachment member once the larger piping structure is assembled from a number of smaller sections, and will not be exposed to a surrounding external environment in use.

In embodiments, an external surface of one or both of the coupling structures that will be exposed to a surrounding external environment in use is also shaped to include a pattern of indents and/or protrusions. This helps to provide optimal sealing also where coupling points are present. If the coupling structures simply comprise an internal threaded connection at one end and an external threaded connection at another end, only the structure with the internal threaded connection will include a part that will be exposed to the surrounding external environment in use.

In embodiments, at least a region of the external surface midway between the two ends of the one or more sections of piping is shaped to include a pattern of indents and/or protrusions. The central regions of the piping may be most likely to sag, so that surface shaping in these regions can more effectively reduce leak rate in these areas.

In embodiments, the method comprises filling a region directly adjacent the shaped region of the external surface with a filler material so that the filler material fills one or more spaces formed by the indents and/or protrusions. The spaces may be formed in between the shaped external surface and an adjacent structure, for example another casing string contacted by the piping string. The filler material will, on filling the spaces, contact at least some of the shaped region. In embodiments, the filler material is concrete. Traditional wellbores use concrete to fill between casing and formation, between sections of casing that are nested or contained within one another, and/or in some cases also between tubing and casing. Other types of filler material can also be used. As compared to a flat pipe surface, leak rate is greatly reduced due to improved sealing in areas that this filler would otherwise be unable to reach. The surface modification reduces contact area between the piping external surface and any adjacent structure in areas where the two are directly adjacent, allowing the filler to access spaces created between the contacting structures.

In embodiments, the method comprises modifying the region of the external surface of the one or more sections of piping.

In embodiments, the modifying comprises positioning one or more conductive elements of a tool around the one or more sections of piping, connecting the piping to the tool, and connecting the conductive elements to a power source to remove parts of the external surface by a process of electrolysis. Other methods, such as milling, etching, engraving, adding material, smelting, molding, and so on, can also be used to achieve the modified or shaped external surface as desired. Use of electrolysis, however, allows for a high level of control in terms of achieving the desired shape. It is also easy to implement on a rig or downhole using a reasonably compact tool.

In embodiments, the method comprises modifying the region of the external surface of the one or more sections of piping prior to inserting the one or more sections in the wellbore. Modification may be carried out in a factory, storage facility, service facility, or on a rig before installation downhole. Standard sections of casing or tubing can be modified easily and as needed.

In embodiments, the method comprises modifying the region of the external surface downhole after positioning the one or more sections of piping within the wellbore.

In embodiments, the section of piping is a section of wellbore casing or tubing.

In embodiments, at least a region of the internal surface of the one or more sections of piping that is exposed to a surrounding external environment when the section of piping is coupled to additional similar sections of piping within a wellbore is also provided with a pattern of indents and/or protrusions. In embodiments, the section of piping comprises coupling structures at either end for attachment to the additional similar sections of piping, and the internal surface extends between these two coupling structures. In embodiments, at least a region of the internal surface midway between the two ends of the one or more sections of piping is shaped to include a pattern of indents and/or protrusions. The whole of the internal surface of the piping section may be shaped to include a pattern of indents and/or protrusions.

Where the internal surface sits against a section of piping for which the external surface has also been modified, sealing is particularly effective. The shapes of the facing internal and external surfaces can be selected to provide optimal sealing by creating spaces of a desired size between the two surfaces, as well as maximizing support by ensuring that any contact points are well distributed across the facing surfaces. Piping can be produced as standard with a modified internal and external surface, meaning that it can be used to improve sealing as part of a larger piping string within a compound system of nested casing and tubing strings.

In embodiments, the method comprises positioning the at least one piping section in the wellbore within another piping section of larger diameter, wherein the piping section of larger diameter is modified to include a pattern of indents and/or protrusions on at least a part of its internal surface that will be exposed to a surrounding external environment in use. In embodiments, the method comprises filling a space between the at least one piping section and the piping section of larger diameter using a filler.

In embodiments, the pattern of indents and/or protrusions on the external and internal surfaces are different. This ensures that voids are present between contacting surfaces when the piping sits either within or outside of another similar section of piping having a different diameter. The two patterns can be selected to optimize void space while providing well-distributed contact points, as discussed above.

In embodiments, the pattern of indents and/or protrusions on the external and internal surfaces comprise a plurality of radial grooves, slanting ringed grooves, and/or one or more helical grooves, and the distance between adjacent grooves in a direction along the length of the one or more sections of piping is different for the internal and the external surfaces. The distance between adjacent grooves refers to the tightness of the spiral or helix for a spiral or helical groove, and the number of rings along a particular length of the pipe for radial and slanting ringed grooves. Longitudinal grooves can also be used, or a combination of longitudinal and radial grooves, in some embodiments. Where longitudinal grooves are used on the internal and external surfaces, the distance between longitudinal grooves can be different for the internal and external surfaces. It is also possible to use one type of groove for the internal surface and another type for the external surface (helical grooves for one and longitudinal grooves for the other, for example).

In embodiments, the method comprises modifying the region of the internal surface of the piping section. The modification of the internal and external surfaces can be carried out simultaneously. In a similar manner as for the external surface, the modifying to the internal surface can comprise positioning one or more conductive elements of a tool within the one or more sections of piping, connecting the piping to the tool, and connecting the conductive elements to a power source to remove parts of the internal surface by a process of electrolysis. Other methods, such as milling, etching, engraving, adding material, smelting, molding, and so on, can also be used to achieve the modified or shaped internal surface as desired. Use of electrolysis, however, allows for a high level of control in terms of achieving the desired shape. It is also easy to implement on a rig or downhole using a reasonably compact tool. In embodiments, modifying the external and internal surfaces of the at least one section of piping comprises inserting the piping section into a tool comprising a first cathode internal to the inserted piping section and a second cathode external to the inserted piping section, coupling the cathodes and the piping section to a power source as part of the same circuit, and providing the patterns of indents and/or protrusions via a process of electrolysis.

In embodiments, the tool comprises two separate fluid circulation systems for passing electrolytes around the first cathode and the second cathode respectively. The fluid systems can share the same fluid tank, from which fluid travels into the area inside the respective cathode and back. A separate tank and separate fluid can also be provided for each of the circulation systems, so that the fluid in the two systems doesn't mix in use. Including separate fluid circulation systems helps to prevent hotspots and uneven corrosion of the piping section and ensures that differing pressures around the first and second cathode does not lead to a lack of fluid around one cathode. In embodiments, fins are positioned within the fluid system to ensure turbulent flow and ample mixing of the fluid. The fins can be positioned on or near to the cathodes. These fins also improve mixing and reduce localized heating. Each of the fluid systems passes fluid through a region between a respective cathode and the piping surface when the tool is in use. The fluid may be an electrolyte.

In embodiments, substantially the whole of the external surface of the one or more sections of piping is provided with a pattern of indents and/or protrusions. This configuration provides the best protection in that the improved sealing can be achieved wherever an eccentric casing or tubing section may be present.

In embodiments, the pattern comprises a series of radial grooves, slanted ringed grooves, or one or more helical grooves. Radial grooves refer to circular grooves extending around the piping outer surface in a plane substantially perpendicular to the longitudinal axis of the piping. Slanted ringed grooves refer to circular grooves extending around the piping outer surface in a plane that is angled relative to a plane perpendicular to the longitudinal axis of the piping. Helical grooves refer to one or more grooves that wrap around the piping several times to form a spiral-shaped indent on the outer surface. These specific patterns are easy to produce, and provide for a good balance between maintaining piping strength and integrity and allowing for an optimal seal between the piping and any surrounding structure.

In embodiments, the density of the indents and/or protrusions changes in a direction along the external surface parallel to the longitudinal axis of the piping. The density of the indents refers to the distance between grooves in a direction along the external surface parallel to the longitudinal axis of the piping. A smaller distance between adjacent indents or grooves on the surface will mean a higher density of indents. The patterning can, in this way, maximise sealing effectively while also minimizing the cost of providing the adaptation. Denser regions of patterning can be applied in regions where sagging or eccentricity of the piping is most likely, such as in sections of a wellbore which are not vertically orientated, in wells that are highly deviated or have a high level of tortuosity, or further from connections between piping sections. A plurality of piping sections can be assembled end to end to form a longer piped structure (a piping string). In such a case, the density of patterning on the external surface can vary from pipe section to pipe section (the density may or may not be constant for any particular section). The sections can be arranged to account for the conditions within a particular wellbore in this way, which makes the system particularly adaptable. Some pipe sections can include radial, ringed, or helical grooves which are closer together, and some with grooves that are further apart, for example. The depth of the grooves can also be adapted to suit the conditions within a specific wellbore. Similar considerations will apply to longitudinal grooves, dents, or other types of patterning that can result in the creation of indents and/or protrusions in a surface of the piping section.

Simulation may be used to determine the optimal patterning in a particular case, such as by optimizing the density and/or depth of the indents and/or protrusions.

According to a second aspect of the present invention, there is provided a section of piping for use as part of a wellbore structure, wherein at least part of an external surface of the section of piping that is exposed to a surrounding external environment when the piping is coupled to additional sections of piping within a pipe string and positioned downhole is provided with a pattern of indents and/or protrusions. The additional sections of piping may be similar to or the same as the at least one section of piping.

In embodiments, the section of piping is a section of wellbore casing or tubing.

In embodiments, the section of piping comprising coupling structures at either end for attachment to another similar section of piping, and the external surface extends between these two coupling structures.

In embodiments, at least a region of the external surface midway between the two ends of the one or more sections of piping is shaped to include a pattern of indents and/or protrusions.

In embodiments, a region of the internal surface of the one or more sections of piping that is exposed to a surrounding external environment when the piping is coupled to additional sections of piping within a pipe string and positioned downhole is also provided with a pattern of indents and/or protrusions.

In embodiments, substantially the whole extent of the external surface of the one or more sections of piping is provided with a pattern of indents and/or protrusions.

In embodiments, the pattern comprises a series of radial grooves, slanted rings grooves, or one or more helical grooves.

In embodiments, the density of the indents and/or protrusions changes in a direction along the external surface parallel to a longitudinal axis of the piping.

According to a third aspect of the present invention, there is provided a method for producing a section of piping for use as part of a wellbore structure, the method comprising: providing a pattern of indents and/or protrusions on at least a part of the external surface of the section of piping, wherein the at least a part of the external surface is located on the piping so as to be exposed to a surrounding external environment when the piping is coupled to similar piping sections as part of a piping string and positioned downhole.

According to a fourth aspect of the present invention, there is provided a tool for modifying an internal and/or external surface of a piping section, the tool comprising: an internal conductive element shaped to fit inside the piping section; an external conductive element shaped to surround the piping section; a coupling apparatus for coupling one or both of the internal and external conductive elements to a power source; and a connection device for electrical coupling of the piping section to the power source. In embodiments, the tool comprises the power source.

In embodiments, the tool comprises at least one fluid circulation system for moving fluid through a space between each of the external and internal conductive elements and a facing surface of the piping section.

In embodiments, the tool comprises a first circulation system for moving fluid through a space between the external conductive element and an external surface of the piping section and a second circulation system for moving fluid through a space between the internal conductive element and the internal surface of the piping section. Two separate circulation systems can be helpful because a higher pressure in the internal cathode can result in a reduction in the amount of fluid flowing around this cathode. Separate systems mean that the amount of fluid applied to each cathode can be properly controlled. The two circulation systems can draw fluid from (and in most cases also send fluid back to) a fluid storage unit, which can be the same or different for the two systems.

In embodiments, the tool comprises a masking device for covering a part of an outer surface of the internal conductive element.

In embodiments, the position of the external conductive element and the internal conductive element are fixed relative to one another, so that the piping section can be inserted into the tool in between the two conductive elements.

In embodiments, the masking device represents a separate part which is removable from the tool for replacement of this part. The masking device may be a frame which sits around the internal cathode, for example, or inside the external cathode. The device can be easily removed from the tool for replacement to produce different surface patterns. Two masking devices may be present or a compound masking device for masking parts of the internal and the external cathode.

In embodiments, the tool comprises fins for creating a turbulent flow in the fluid passing between the external and internal conductive elements.

Embodiments of the present invention will now be described, by way of example only, with reference to the following diagrams wherein:

FIG. 1A illustrates a wellbore include multiple casing sections which are centered in the wellbore;

FIG. 1B shows a similar multiple casing string in which the casings are eccentric;

FIGS. 2A to 2J show alternative pattern configurations for sections of piping with a modified external surface;

FIG. 3 shows a section of piping with a modified external surface next to a section of formation within a wellbore;

FIG. 4 shows a close-up of a section of the piping wall and formation of FIG. 3;

FIG. 5A shows a section of piping having a spiral groove pattern applied to the internal (and external) surface;

FIG. 5B shows a section of piping having a spiral groove pattern applied to the internal (and external) surface that is in the opposite sense to that shown in FIG. 5A;

FIG. 5C shows a cross-section of a section of piping having a modification applied to the outer and inner surfaces;

FIG. 6 illustrates a section of piping with a modified external surface sitting inside a section of piping with a modified internal (and external) surface;

FIG. 7 shows a cross-section through the piping sections of FIG. 6;

FIG. 8A illustrates a section of piping with an external surface that is unmodified;

FIGS. 8B to 8D show some examples of piping sections with modified external surfaces, where the piping sections include coupling structures at either end;

FIG. 9 illustrates a tool for use in modifying the external and internal surface of a section of piping to be placed in a wellbore;

FIG. 10 shows an internal conductive element of the tool of FIG. 9, including a masking device;

FIG. 11 shows a close-up of an end section of the internal conductive element of FIG. 10, including a connection to a power source;

FIG. 12 shows the internal conductive element of FIGS. 10 and 11 in place within a section of piping to be modified;

FIG. 13A shows the modified external surface of a section of piping modified by the tool of FIG. 9; and

FIG. 13B shows the modified internal surface of a section of piping modified by the tool of FIG. 9.

FIGS. 2A to 2J shows some examples of a section of piping 1 where the external surface 2 of the casing has been modified to include a pattern of protrusions 4 and indents 5. In most cases, the piping 1 will be modified across the whole or substantially the whole of its external surface 2 to provide the most effective sealing. The internal surface may or may not be modified. Any modification which reduces an area of contact with a surrounding structure will enable filler to enter a region or regions between the piping and the surrounding structure and will improve sealing capability at that surface. In general, the aim of the surface modification is to minimize contact with an adjacent external structure and to provide a space or spaces between the two surfaces, whilst maintaining a desired piping integrity and strength, as well as wellbore integrity.

The optimal modification will depend on a specific situation, but FIGS. 2A to 2J illustrate some preferred patterns. Spiral/helical or radial grooves, such as those shown, provide indents or grooves which are well distributed across the modified area to allow filler material to access a region between the piping and a contacting structure to provide a good seal. Contact between the surface and an adjacent structure is also well distributed across the surface when this type of patterning is used. Filler material will fill the spiral or ring-shaped regions within the grooves when applied.

FIGS. 3 and 4 illustrate the function of the surface modification when filler 21 is used during the installation of a section of piping 1 having a modified external surface 2 and an unmodified internal surface 3 sitting next to and contacting a region of the surrounding formation 7. The section shown zoomed-in in FIG. 4 illustrates indents 5 and protrusions 4 providing spaces 6 containing filler.

Patterning can include flat regions 8 between grooves (shown in FIGS. 2D to 2J), which can provide an area of contact with an adjacent structure if piping eccentricity is present. This can be advantageous if stability and pipe strength is a particular issue, since pipe integrity is increased whilst still allowing filler to access a region between two adjacent structures where there is contact. The width, spacing, and regularity of the grooves can be adapted as desired.

The grooves forming the surface modification may have a sinusoidal surface profile or may be configured as flat grooves to provide a ribbon-shaped indent running around the piping as a spiral or a series of radial or slanted ring-shaped grooves. This can help filler to enter the area between the grooves more easily.

Surface patterning can be selected to account for a specific structure of the piping string or piping section. For example, if the piping further away from the ends of a particular section is more likely to contact adjacent structures, then the surface modification can be selected to be more undulating in the center of the piping, away from the ends. Several radial grooves may be present in the surface for example, or a helical groove, and the grooves may be deeper and/or closer together, or the turns of the helix closer together in the center of the piping section than at either end. Similarly, entire sections of piping with a modification including a denser patterning can be selected for regions of the piping string where eccentricity is likely to be more of a problem, for example when sections of a horizontal or inclined wellbore are being lined.

The surface patterning or modification will in most cases comprise a regular pattern which is applied across the whole extent of the external surface 2 of the piping (possibly excluding the exposed surface of any coupling structure 11a; 11b to be used to join two adjacent sections of the piping although this can also include a pattern of indents and/or protrusions as mentioned above). The density of the regular pattern (for example the distance between adjacent grooves along the piping section) may change in a direction along the piping section in some cases, or may be the same along the whole length of the modified region of the external surface.

As mentioned above, it is common when lining a wellbore to include nested sections of casing having increasingly larger diameter in at least in some sections of the wellbore. A tubing string will also be installed within the casing string or strings. All of these structures can be anchored in place and sealed by pumping the space adjacent the piping outer surface(s) with a filler such as concrete. For this reason, it can be advantageous to modify both an internal and an external surface of at least some of the piping sections to be used in a particular wellbore. A standard modification can be applied to casing strings, and this modification can be applied on a rig or at a production, storage or service facility for the piping sections, for example, such that all casing strings installed downhole have modified both at least a part of their internal surface and at least a part of their external surface that will be exposed to a surrounding external environment in use.

In order that the same pattern of indents and/or protrusions can be applied for a number of piping sections within a wellbore, which improves efficiency of the overall process, the internal and external surfaces of a single piping section can be provided with patterning which is different. Different refers to the fact that the modifications to the internal and external surfaces do not correspond in the sense that they cannot interlock if placed directly adjacent one another. This way, two similar sections of piping which are installed one within another downhole, and which contact one another, will not contact one another across the whole extent of the adjacent surfaces. This effect can be achieved by, for example, either providing spiral grooves on the internal and external surfaces which have a different sense, or a different size of spacing between the grooves can be used for the internal as compared to the external surfaces. FIGS. 5A and 5B illustrate a modification to an internal surface of a piping section including a helical groove in a first sense (FIG. 5A) and in the opposite sense (FIG. 5B). Such patterning can also be provided on an external surface. Surfaces of casing or tubing which will face one another after installation can be provided with helical grooves in opposite senses, so that the two cannot interlock on contact between the two surfaces.

In the example shown in FIG. 5C, spiral patterning is provided on both the inner and outer surface, but the indents and protrusions are such that they line up to maintain a given wall thickness along the piping section. This helps to ensure that weak points are not produced in the pipe section walls. If pipe sections are to be placed within one another it can be advantageous to ensure that these are positioned so that any contacting surfaces do not interlock, or to ensure that pipe sections having a different patterning are used for the outer and inner casing/tubing sections.

In the example shown in FIGS. 6 and 7, two nested sections of piping (in this case one section of tubing 9 surrounded by a larger diameter section of casing 10) are shown. The tubing 9 has a modified external surface 2, and the casing has 10 patterning provided on both the internal and external surfaces. Indents 5 on all modified surfaces are in the form of helical grooves in the example shown, although this need not necessarily be the case, and any type of patterning can be used. There is some eccentricity present in the section of the wellbore shown, meaning that the casing and the tubing sections contact one another along at least a portion of their extent. The casing 10 is provided with a spiral groove pattern which has a groove spacing L1 which is larger (in this case twice the size) of the spacing L2 between the grooves provided on the tubing outer surface. This ensures that contact between the two surfaces is minimal, provides well dispersed contact where this is present between the two structures for good stability, and provides spaces or voids 6 in between the two surfaces into which filler can be pumped for providing an effective seal. It can be advantageous to ensure that grooves on an internal surface are separated by a distance that is an integral multiple of the distance between the grooves on the external surface, or vice versa.

FIG. 8A illustrates a section of standard piping for use as part of the casing or tubing string. The external surface 2 of the section of piping shown in FIG. 8A is unmodified, and the piping includes a coupling structure 11a at one end having an external thread, and a coupling structure 11b including an internal thread on an opposite end for coupling to a similar piping section via its external threaded coupling. The external thread 11a will be contained within an adjacent piping section when the section shown is installed as part of a piping string, and does not therefore form part of the external surface which is exposed a surrounding environment (an environment external to the piping string) in use. It is this latter external surface to which a pattern of indents and/or protrusions can be applied in order to improve sealing around the piping section.

Any method can be used to produce the indent(s) 5 and/or protrusion(s) 4 on the external surface 2 of the piping string, as well as the internal surface in cases where this is also modified. The surface structure can be introduced during production of a section of piping (which may be casing or tubing), for example, by shaping a mould used to manufacture the section. A section of classical piping, as shown in FIG. 8A for example, can be adapted by milling the surface, by adding material to the surface to produce protrusions, by etching, by printing, or in any other way. Examples of different modifications to the external surfaces of a section of piping with coupling structures at either end are shown in FIGS. 8B-8D. The distance between consecutive grooves or consecutive turns of a helical groove can vary along the length of the pipe section as in FIG. 8D, or can be regular as shown in FIG. 8C. The surface modification can be continued across the surface of one or both coupling structures if this surface will be exposed to a surrounding external environment once the piping string is assembled and positioned downhole, as shown for coupling structure 11b in FIGS. 8B to 8D. The external surface of the coupling structures 11b having an internal thread will be exposed after insulation, and a modification to this external surface will improve sealing. Only a region of the piping section midway between the ends can be modified in some cases, as shown in FIG. 8B. In the example shown this region comprises less than half (around a third) of the whole extent of the external surface, but the modified region may be larger or smaller.

One preferred method for modifying metal or electrically conductive structures makes use of a specific tool, an example of which is described below with reference to FIGS. 9 to 12. The tool corrodes parts of the surface of a piping section 24 by a process of electrolysis. The tool 12 contains one or more conductive elements 13a; 13b which serve as the cathode of an electrical cell while the external surface to be modified functions as the anode. In order for current to flow through the cell between the cathode and anode via an electrolyte which may be already present where the tool is used (i.e. brine within a wellbore if the modification is carried out downhole), or which may be introduced. A power supply 14 provides power to the tool, and this can be collocated with the tool 12 or separate, with a cable or wireline transporting power from the supply to the tool. In some cases, the same tool can simultaneously provide modifications on both the external and the internal surface of a section of piping for use in lining a wellbore. This is possible using the tool shown in FIG. 9.

The tool can be used downhole in some cases, or can be used at the surface in a factory or on a rig, and can be placed such that the conductive element or elements of the tool encircle and surround a section of downhole piping to be adapted. One or both of the conductive element(s) 13a; 13b can be shaped or masked so as to produce the desired shaping in the external surface of the piping when power is applied to them. A mask may be applied either to the conductive element or to the piping section itself in some cases. Where shaping is used, an undulating internal surface for the conductive element or elements will, for example, produce a corresponding undulating surface on the external surface of the piping because of the different speeds of corrosion with different distances between the conductive element and the exterior surface of the object to be eroded.

The tool 12 may comprise at least one conductive element 13b which is shaped to surround the object 24 of which the external surface is to be modified so that the elements face the external surface of that object, a connector 15 to provide the electrical connection between the positive pole of the power source 14 and the piping to be modified. The power source is coupled to the conductive elements of the tool via connections 16, in order to close the circuit together with the electrolyte. The conductive elements are made of an electrically conductive material. It may be necessary to include means for regulation of the current applied to the outer and inner cathodes in order to enable an even dissolution between the outer and inner surfaces of the piping. In one example, one or more resistors could be included in the circuit to adjust the amount of current flowing to each of the cathodes. Other methods for current regulation can be used in place of or as well as the resistors. As an alternative or in addition, the outer surface can be connected up and modified first, and the inner surface afterwards, or vice-versa. It is also possible to provide two completely separate circuits with their own power supplies for modification of the inner and outer surfaces.

The object to be modified may be a section of piping 24, such as a section of casing or tubing to be used as part of a wellbore structure, an example of which is the piping section shown in FIG. 8A.

For use in modifying a piped structure, the one or more conductive elements will include a conductive surface that faces inwards, and that is shaped to surround, and to form an internal surface of, a cylindrically shaped borehole. One conductive element 13b providing an internal borehole can be provided, or a plurality of elements each providing a section of a wall of the borehole can be present. These can be adjustable inwards and outwards in order to adjust the effective size of the borehole to be adaptable to adapt the external surfaces of different sizes of piping. Where a number of conductive elements are present, it can be useful to include a rotation function so that any gaps between elements will not be evident in the modified external surface, however this is not a necessity and will depend on the desired patterning on the surface. Where rotation is used, radial grooves are a preferred choice of shaping for the conductive elements and the resultant external surface. Non-conductive spacers can, in some cases, be included in order to maintain a desired gap between the conductive element(s) and the facing piping surface. These are indicated as elements 17 in the example shown in FIG. 9.

The amount of material removed from the surface is substantially 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.

Where the external surface is the external surface of a section of casing or tubing, it is particularly efficient to provide sections which are already modified prior to being installed downhole. The modification can occur in a factory, a storage or service facility, or on a rig using any method, but preferably using a tool for corroding parts of a metallic piping external surface using electrolysis, as mentioned above. If the tool is installed on a rig or in a factory, sections of casing or tubing can simply be inserted into the tool prior to installation for modification. The tool can be adapted depending on the exact patterning desired, and whether this needs to be applied to the external surface only, the internal surface only, or both the internal and external surfaces of the pipe section to be modified. The tool may include a housing or structure for containing electrolyte between the object to be modified and the conductive element or elements of the tool.

FIGS. 9 to 12 illustrate some details of a preferred example of a tool for use in modifying the external and/or internal surface of a section of piping for use in lining a wellbore. The tool includes two cathodes, one (13a) internal to the piping to be modified, and one (13b) external to the piping to be modified. These can be connected up as desired in order to modify one or both surfaces.

In the example shown in FIG. 9, only the external 13b cathodes is visible, and piping section 24 has been inserted into the tool for modification. Two separate circulation systems 18a and 18b are included, for providing electrolyte to the internal and the external cathodes respectively. In the example shown, each circulation system includes a pump 22 for circulating the fluid. One circulation system can be used for both conductive elements, but including a fluid circulation system dedicated to each cathode reduces localized heating and improves performance of the tool. A fluid storage chamber 19 is present, from which electrolyte is taken and returned by both circulation systems. One passes fluid from chamber 19 inside the internal cathode 13a, and the other passes fluid from chamber 19 inside the external cathode 13b. Fins 20 may be present within the fluid system(s), as shown, in order to provide a turbulent flow in the circulating fluid. This prevents localized heating or the concentration of corrosive activity in small areas. The external surface of the internal cathode is taped or masked in order to leave exposed an area corresponding to the area to be removed on the piping inner surface. In this case the internal surface of a piping will be modified to include a spiral shaped groove which is fairly wide in comparison to the protrusions left between the grooves. This leaves ample space within the spiral groove or indent for filling material to enter an area directly adjacent the inner surface.

On either or both of the internal and external surfaces of the piping, either the piping itself or the facing cathode can be masked to provide the desired patterning. The cathode(s) 13a; 13b can alternatively be shaped to achieve a similar effect, or a combination of the different methods can be used. The internal cathode 13a is shown in FIG. 10, including masking 23 for modification of the entire internal surface of a piping section. In FIG. 11, an end of the same cathode is illustrated, including the electrical connection 16 to the negative pole of a power supply. The connection between the positive pole and the piping is provided as connection 15 shown in the figures. FIG. 11 shows the fins 20 for circulation of fluid entering the tool at one end as part of fluid circulation system 18a.

It can be advantageous to use a masking device formed of a material that is not electrically conductive, such as plastic. The masking device can take the form of taping adhered to the surface to be masked, or it can be configured to hold its shape when the piping is not present and can form a part of the tool into which the section of piping is inserted prior to electrolysis. This makes use of the tool more efficient and flexible, since the piping can simply be inserted within or around the masking device. Two such masking devices can be present, one for inserting inside the piping section for use in modifying the internal surface, and one into which the piping section is inserted for modification of the external surfaces. A section of piping placed over the internal conductive element 13a of FIG. 11 is shown in FIG. 12.

The external conductive element 13b of the tool is visible in FIG. 9 as a cylindrical shaped structure into which a section of piping can be inserted. The part of the external surface of the piping 24 to be provided with a pattern of indents and/or protrusions is masked in the example shown in a similar manner to the internal conductive element described above, although as mentioned the internal surface of the piping or the external cathode may be masked as an alternative, or one-piece masking device may be used. The one-piece device functions particularly well where a helical groove pattern is desired.

A section of piping 1 modified by the tool shown in FIG. 9 is illustrated in FIG. 13A (external surface) and FIG. 13B (internal surface). In this case, the whole of the internal surface of the piping section is modified to include a spiral groove pattern and a central portion of the external surface of the piping section is modified to include a spiral groove pattern having a different sense to the spiral groove on the interior surface. The piping section in this case has a coupling section 11a at either end both having external threads, however this is obviously not necessarily the case, and the type of coupling provided can be different. In general, providing a pattern on the internal surface that is different from the pattern on the external surface is beneficial in that when piping sections are placed one inside the other, any contacting regions of the two piping sections cannot interlock and will always include spaces into which filler material can be pumped.

Modification of the tool to accommodate larger or smaller piping sections is straightforward. In many cases, the tool may be able to be used without reconfiguration, although the space between the interior/exterior cathodes and the piping external/internal surface will change and the process of corrosion may take less or more time. In some cases, the cathodes may need to be replaced with larger or smaller cathodes. Provision of a different pattern on the external or internal surfaces of the piping requires replacement of the masking device or retaping/covering of the cathode and/or piping section surface. Although not a necessity, ultrasound can be used to improve efficiency of the electrolysis when modifying the piping surface.