Patent application title: APPLYING WELLBORE LINING
John Cook (Cambridge, GB)
John Cook (Cambridge, GB)
Louise Bailey (St. Neots, GB)
Paul William Way (Cambridge, GB)
Gokturk Tunc (Cambridge, GB)
David Robert Evan Snoswell (Cambridge, GB)
SCHLUMBERGER TECHNOLOGY CORPORATION
IPC8 Class: AE21B3300FI
Class name: Processes placing or shifting well part with sealing feature (e.g., packer)
Publication date: 2014-10-09
Patent application number: 20140299333
Application of a photocurable lining to a downhole fixed surface
surrounding the axis of a wellbore is carried out by skimming an
applicator over the downhole surface while delivering a porous tape
impregnated with a photocurable liquid composition between the applicator
and the downhole surface so that the tape lies on the downhole surface
and directing light of wavelength in a range from 100 nm to 1500 nm onto
the curable composition impregnating the tape so as to initiate curing of
the composition and thereby attach the tape to the downhole surface.
1. A method of applying a photocurable lining to a downhole fixed surface
surrounding the axis of a wellbore containing a wellbore fluid, the
method comprising skimming at least one applicator over the downhole
surface while delivering a porous tape between the applicator and the
downhole surface so that the tape lies on the downhole surface; providing
a photocurable liquid composition impregnating at least part of the tape;
and directing light of wavelength in a range from 100 nm to 1500 nm onto
the curable composition impregnating the tape so as to initiate curing of
the composition and thereby attach the tape to the downhole surface.
2. A method according to claim 1 wherein the tape is preimpregnated with the photocurable liquid composition before it is delivered between the applicator and the downhole surface.
3. A method according to claim 1 comprising supplying a photocurable liquid composition between the applicator and the downhole surface to impregnate at least part of the tape.
4. A method according to claim 1 wherein the applicator comprises a light transmitting barrier between a light output and tape which is at least partially impregnated with the photocurable liquid composition, the light-transmitting barrier facing and being adjacent the impregnated tape on the downhole surface and the light output being configured to directing light of wavelength in the range from 100 nm to 1500 nm through the light-transmitting barrier onto the tape so as to initiate curing of the impregnating composition.
5. A method according to claim 4 wherein the tape plus photocurable composition impregnating the tape extend more than three quarters the distance between the barrier and the downhole surface.
6. A method according to claim 5 wherein the light-transmitting barrier is in direct contact with photocurable composition impregnating the tape.
7. A method according to claim 6 wherein the light-transmitting barrier has a fluorocarbon at its surface in contact with the curable layer.
8. A method according to claim 1 wherein the applicator comprises a laminar web of light-transmitting material and support means for positioning a portion of the web facing the downhole surface and in contact with impregnated tape between the portion of the web and the downhole surface wherein any movement of the said portion of the web relative to the tape and downhole surface is slower than the movement of other parts of the applicator assembly relative to the tape and downhole surface as the applicator is skimmed over the downhole surface; and wherein the light is directed through the said portion of the web onto the tape impregnated with curable composition so as to initiate curing of the composition between the said portion of the web and the downhole surface.
9. A method according to claim 8 wherein the support means comprises a pair of rotary elements spaced one from another and the laminar web is a belt running in a continuous loop over these rotary elements.
10. A method according to claim 9 wherein the assembly comprises a light output positioned between the rotary elements and within the loop of the belt so that it can direct light through the said portion of the belt.
11. A wellbore tool for applying a photocurable coating to a fixed downhole surface surrounding the axis of a wellbore wherein the tool comprises at least one applicator to face a downhole surface surrounding the wellbore tool; means to skim the applicator over the downhole surface; means to deliver a porous tape and photocurable liquid composition between the applicator and the downhole surface, and at least one light output configured to direct light of wavelength in a range from 100 nm to 1500 nm onto the photocurable composition.
12. A wellbore tool according to claim 11 further comprising a container with a supply of the tape therein, configured for delivery of tape from the supply to between the applicator and the downhole surface.
13. A wellbore tool according to claim 11 wherein the applicator further comprises a light transmitting barrier between the light output and the impregnated tape, with the barrier facing and adjacent the at least partly impregnated tape on the downhole surface and the light output configured to direct light through the barrier onto the impregnated tape.
14. A wellbore tool according to claim 11 wherein the applicator comprises a pair of rotary elements spaced one from another and a light transmitting belt running in a continuous loop over these rotary elements with a portion of the belt facing the downhole surface and at least one light output configured to direct light through the said portion of the light-transmitting belt towards the downhole surface.
15. A wellbore tool according to claim 14 wherein the light output is positioned between the rotary elements and within the loop of the belt so that it can direct light through the said stationary portion of the belt.
16. A wellbore tool according to claim 11 wherein the at least one applicator includes one or more light emitting diodes to provide the light output.
17. A wellbore tool according to claim 11 wherein the tool comprises a light source and a fibre optic cable to carry light to the at least one applicator.
18. A wellbore tool according to claim 11 comprising means to move the at least one applicator outwardly towards and inwardly from said fixed downhole surface.
CROSS-REFERENCE TO RELATED APPLICATION
 This application claims priority to British Patent Application No. GB1306103.1 filed Apr. 4, 2013, which is incorporated herein by reference in its entirety.
 U.S. Pat. No. 7,931,091 disclosed a process for applying a fluid, photo curable, composition to the wall of an open hole wellbore, and curing the composition by illuminating it with electromagnetic radiation, referred to as actinic radiation and having a wavelength in a range spanning the ultra-violet, visible and near infra-red parts of the spectrum. Application of such coating can strengthen the wellbore wall. As pointed out in that document, by providing the radiation for solidifying or gelling the fluid composition in situ in the wellbore proximate to the region in which the solidified or gelled fluid composition is desired, more control over the lining of the wellbore is achievable. Thus, specific regions of a wellbore, e.g., cracked or fissured regions, can be lined or re-lined.
 US patent application 2010/0247794 disclosed a similar process used to apply a photo curable composition to the interior of tubing which has been inserted into the wellbore in order to repair the tubing and prevent leakage, possibly after corrosion or damage to the tubing.
 This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed.
 In a first aspect the present disclosure provides a method of applying a lining to a downhole surface surrounding the axis of a wellbore, the method comprising skimming at least one applicator over the downhole surface while delivering a porous tape between the applicator and the downhole surface so that the tape lies on the downhole surface; providing a photocurable liquid composition impregnating at least part of the tape; and directing light of wavelength in a range from 100 nm to 1500 nm onto the curable composition impregnating the tape so as to initiate curing of the composition and thereby attach the tape to the downhole surface.
 Traversing the applicator over the downhole surface may be accompanied by movement along the wellbore so that the tape is applied as a helix on the downhole surface. This helix may have adjacent turns in contact with each other or overlapping so that multiple turns of the helical pattern provide a continuous coating.
 The method may be carried out using a downhole tool which is placed in the wellbore and comprises at least one such applicator and a light source configured to direct light in the said wavelength range onto the curable composition impregnating the tape. Such a downhole tool may contain a supply of the porous tape.
 There are a number of possibilities for impregnation of the tape. One possibility is that the tape is manufactured as a pre-impregnating tape which is then transported to the site of the wellbore and this pre-impregnating tape is taken downhole and delivered between the applicator and downhole surface. Another possibility is that the tape is not pre-impregnated but photocurable composition is supplied separately from the tape between the applicator and downhole surface so as to impregnate at least part of the tape while the tape is located between the applicator and downhole surface. It will also be possible to use a tape which is at least partially pre-impregnated and supply additional photocurable liquid composition between the applicator and the downhole surface.
 The tape may be impregnated with photo curable composition over its entire area, either during manufacture as a pre-impregnating tape or during impregnation while located between the applicator and downhole surface stop it is also possible that the tape is impregnated over part only of its area, with the consequence that part of the tape is sealed with cured composition which also adheres it but wellbore wall. Any areas of the tape which were not impregnated with curable composition will still be held in place on the wall because they are attached in areas which have been secured to the wall. If drilling fluid is circulating in the wellbore, solids in the drilling fluid can form a filter cake on these areas of tape so that the whole area covered by tape becomes sealed, either with polymerised composition or with filter cake.
 Some forms of the method utilise a light output with a light transmitting barrier between the light output and the photocurable material which is being irradiated by the light. This barrier may be located facing and adjacent the tape and curable composition on the downhole surface.
 Initiating photocuring with the tape and curable composition located between a light transmitting barrier and the downhole surface assists in holding the tape along with the curable composition which impregnates the tape onto the downhole surface during curing or at least the first part of curing.
 The applicator and its operation may inhibit the presence of wellbore fluid (which may be opaque or strongly light absorbing) between this light transmitting barrier and the impregnated tape in order to reduce or avoid loss of light as a result of absorption by the wellbore fluid and so allow more of the light to reach the curable composition to bring about photocuring. The configuration and operation of the applicator may inhibit entry of wellbore fluid between the barrier and the impregnated tape or may expel wellbore fluid which does enter between the barrier and the curable composition. One possibility is that the tape, together with the curable composition impregnating it, bridges the distance from the barrier to the downhole surface so that the barrier is in contact with curable composition and wellbore fluid is excluded. We have then found that using a material with a low-friction non-stick characteristic at the surface of the light-transmitting barrier mitigates adhesion of the curable composition to the light transmitting barrier. Such a non-stick characteristic may be provided by a fluorocarbon at the surface of the light-transmitting barrier. The fluorocarbon may be a fluorocarbon polymer which may possibly be provided as a coating or may be used to form the barrier.
 Another possibility for inhibiting adhesion of curable composition to the light transmitting barrier is to introduce a layer of light-transmitting liquid, which does not undergo photocuring, between the barrier and the curable composition impregnating the tape. The combination of the tape, the curable composition impregnating the tape and the layer of light transmitting liquid may bridge the distance from barrier to the downhole surface.
 Even if a light transmitting liquid or a small amount of wellbore fluid is present between the barrier and the downhole surface, the tape with curable composition impregnating the tape may extend across more than half, possibly more than three quarters of the distance between a light transmitting barrier and the downhole surface.
 When the light output is arranged to direct light through a light transmitting barrier, it may be one or more light sources positioned to emit light directly onto and through the barrier, or it may be the output end of one or more fibre optic cables (also termed light guides), carrying light from a light source elsewhere, the output end of the light guide(s) then being positioned to direct light onto and through the barrier.
 The downhole surface encircling the wellbore axis, to which the tape is applied, may be the wall of an open hole section of a wellbore, or may be the interior of tubing such as wellbore casing inserted into the wellbore.
 In a second aspect there is disclosed here a wellbore tool for applying a photocurable coating to a downhole surface surrounding the axis of a wellbore wherein the tool comprises at least one applicator to face a downhole surface surrounding the wellbore tool; means to skim the applicator over the downhole surface; means to deliver a porous tape and photocurable liquid composition between the applicator and the downhole surface, and at least one light output configured to direct light of wavelength in a range from 100 nm to 1500 nm onto the photocurable composition.
 The wellbore tool may include a container for a supply of the tape. The applicator may comprise a light transmitting barrier between the light output and the tape impregnated with curable composition which is being irradiated by the light. This barrier may be located facing and adjacent the tape and curable composition on the downhole surface. The downhole tool may then be configured to inhibit the presence of wellbore fluid between the light transmitting barrier and the downhole surface, as mentioned above.
 The downhole tool may have a plurality of applicators. It may comprise means to move the applicator or each one of the applicators radially outwardly to an operative position proximate the downhole surface and means to retract the applicator or each applicator radially inwardly when not required, such as when the tool is not in use and is being transported up or down the well. An applicator may be shaped so that spacing between the downhole surface and a confronting surface of the applicator narrows to a minimum as the applicator skims over the downhole surface.
 It is possible that an opaque portion of an applicator contacts the tape and/or curable composition which impregnates the tape and defines one boundary of a gap between the applicator and the downhole surface while the light transmitting barrier is positioned to follow the opaque portion across the downhole surface. A surface of the barrier may be aligned with a surface of the opaque portion and in some embodiments a surface of the barrier may be a smooth (and possibly an uninterrupted) continuation of a surface of the opaque portion. The light-transmitting barrier and the opaque portion of the applicator may both have a fluorocarbon at a surface, facing the surrounding downhole surface, in order to mitigate adhesion of the curable composition. This surface may be provided by a single piece of fluorocarbon polymer. It may be a fluorocarbon, which may be a fluorocarbon polymer, as a coating on another material. In some other embodiments there is a brief discontinuity between an opaque portion of the applicator and a following portion of the applicator which comprises the light transmitting barrier.
 Another possibility is that the applicator comprises a body part which is formed of fluorocarbon polymer and includes a region which is sufficiently thin to allow light transmission through it and so provides the barrier.
 A fluorocarbon polymer used in an applicator as above may be a polymer or copolymer of one or more monomers which are fluorocarbons. One possibility is polytetrafluoroethylene (PTFE). Another is fluorinated ethylene propylene (FEP) which is a copolymer of hexafluoropropylene and tetrafluoroethylene. A further possibility is perfluoroalkoxy polymer resin (PFA) which is a copolymer of tetrafluoroethylene and trifluoromethoxy trifluoroethylene.
 Possibilities other than fluorocarbons include clear silicone rubbers, ultrahigh molecular weight polyethylene and other polyolefins.
 A further possibility is that a light-transmitting barrier is such that it remains stationary or slow moving relative to the impregnated tape on the downhole surface while light is directed through the barrier. This can be achieved if the material of the barrier moves relative to the applicator. It may be provided by a laminar web of light transmitting material which is moving relative to other structure of the applicator and so moves more slowly than other structure of the applicator relative to the downhole surface and may be stationary relative to this surface. The web may have a surface to give low adhesion to the curable composition with which the tape is impregnated. This surface may be fluorocarbon or silicone.
 The applicator may then be an assembly comprising a laminar web of light-transmitting material and support means for positioning a portion of the web facing the downhole surface. The support means may comprise a pair of rotary elements spaced one from another and the laminar web may be a belt running in a continuous loop over these rotary elements so that a portion of the belt moves over the impregnated tape on the downhole surface more slowly than the support means and may be stationary relative to the downhole surface. Separation of the belt from the composition after exposure to the light which initiates curing will then be a peeling action and the belt surface may be chosen to have low adhesion to the layer of tape and cured composition. The assembly may comprise a light output positioned between the rotary elements and within the loop of the belt so that it can direct light through the said portion of the belt.
 Curing of the composition may be a polymerisation reaction, possibly involving crosslinking and/or chain elongation of a pre-polymer. The photocurable composition and the light used to initiate curing may be as disclosed in U.S. Pat. No. 7,931,091 or US2010/0247794. In some embodiments the light lies in a wavelength range with a lower limit of 200 nm or 250 nm. Independently of the lower limit the wavelength range may extend up to 1200, 900 or 800 nm. It will be appreciated that the term "light" is being used here to include ultra-violet and infra-red electromagnetic radiation as well as the visible range which is generally taken to be from about 380 nm to about 750 nm. "Actinic radiation" is another term for light in a range which includes and extends beyond the visible range.
 The photocurable composition used as disclosed herein may be formulated to have a viscosity which is greater than the viscosity of the wellbore fluid, perhaps at least 10 times the viscosity of the wellbore fluid, at the place of application to the downhole surface. The composition may possibly have a viscosity at ambient temperature of at least 1 Pa.sec.
 This photocurable composition may contain one or more materials capable of undergoing polymerisation, together with a photoinitiator such that exposure of the composition to light from the light output causes the photo initiator to liberate reactive species which react with the polymerisable material and cause polymerisation to begin. Depending on the nature of the polymerisable material which is employed, the mechanical properties of the layer of cured composition may range from hard and rigid to flexible.
 In some embodiments of this invention, external energy supplied to the downhole location is used to initiate the reaction, but not to sustain it. The chemistry of the polymerisation reaction may be chosen such that once it has begun, the polymerization reaction propagates at the temperature of the downhole location where it takes place. This may be a higher temperature than the ambient temperature prevailing on the surface. The polymerisation reaction may be exothermic and may accelerate as it proceeds (so-called auto-acceleration) until the rate of reaction is restrained by consumption of polymerisable material and decreasing mobility of the polymer molecules within the composition as their size grows. The amount of energy supplied as light may be less than would be required for a polymerisation brought about by electrical heating. By using light energy rather than temperature to initiate polymerisation, the beginning of polymerisation will not be coupled to wellbore temperature, even though wellbore temperature will have an effect on the rate of polymerisation after it has begun.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a schematic view of the lowest portion of a wellbore and a drill string bottom hole assembly which includes an applicator for preimpregnated tape to line the wellbore;
 FIG. 2 is a schematic view of a portion of a cased wellbore and a wireline tool for applying preimpregnated tape as a lining to the casing tubing;
 FIGS. 3 and 4 are schematic cross-sectional views of embodiments of applicator for applying preimpregnated tape;
 FIG. 5 diagrammatically illustrates a tape impregnated over only part of its area;
 FIG. 6 is a schematic cross-sectional view of an embodiment of applicator for applying tape and impregnating the tape as it is applied;
 FIG. 7 is a detail of a part of FIG. 6 illustrating a space for a liquid film between a layer of curable composition and a light transmitting sheet extending over it;
 FIGS. 8 and 9 are schematic cross-sectional views of further embodiments of applicator for applying a preimpregnated tape;
 FIG. 1 shows part of a wellbore in which there is the lower end portion of a drill string terminating in a drill bit 10. The drill string is positioned in the wellbore, shown here as vertical but which could be deviated to extend at an inclined angle or horizontally. It will be appreciated that at this stage when the drill string is present in the wellbore, the lowest portion of the borehole, shown here, is an open hole without casing. Connected above the drill bit 10 is a bottom hole assembly 11 which may include measuring equipment. In this embodiment it is also a tool with an applicator 12 for applying a porous tape preimpregnated with a photo curable viscous liquid composition to the wall of the wellbore. The porous tape is manufactured and preimpregnated with the curable composition at a factory. It is then transported to a well site and a quantity of the preimpregnated tape 14 spooled on a pair of rollers 15 is placed within the bottom hole assembly 11 before the drill string is inserted in the wellbore.
 As is conventional while drilling, drilling mud is supplied down drill pipe 18. It passes through apertures (not shown) in drill bit 10 and returns carrying cuttings upwardly along the annulus around the bottom hole assembly 11 and drill pipe 18, as indicated by arrows 19. Thus, the applicator 12 operates whilst submerged in drilling mud which is returning to the surface. The applicator 12 may be used to apply preimpregnated tape to the wellbore wall as drilling proceeds, or alternatively drilling may be stopped and the drill string drawn upwardly by a short distance before utilising the applicator 12 to apply tape to the wellbore wall.
 With both of these possibilities the tape 14 is drawn off the spool on the rollers 15, run over an inclined roller 16 to turn it through a right angle and delivered between the applicator 12 and the well bore wall as will be explained more fully below. The spool of tape 14 on rollers 15 may be enclosed within a container inside the bottom hole assembly, but the applicator 12 must be exposed at the exterior of the bottom hole assembly so as to be able to skim over the wellbore wall. It is envisaged that the preimpregnated tape 14 will emerge from within the bottom hole assembly 11 between the inclined roller 16 and the applicator 12.
 FIG. 2 shows a portion of a wellbore which has been cased with steel tubing 20 with cement 21 filling the space between the tubing 20 and the surrounding geological formation. In order to apply a lining to the interior surface of the tubing 20, for example to repair a portion of the tubing, a downhole tool having upper and lower parts 24, 26 is lowered into the wellbore by means of wireline 22. This wireline provides (as is normal for wireline operations) an electrical power supply from the surface to the tool and data and control communication between the tool and the surface.
 The tool's upper body part 24 is centred within the tubing 20 and constrained against rotation by centering devices 28 pressed outwardly against the tubing 20. Below this upper part 24 is a lower body part 26 which can rotate relatively to the upper part 24, around the longitudinal axis of the tool, driven by a motor which is not shown. The lower body part 26 carries an applicator 12 for applying a preimpregnated tape 14 to the inside face of tubing 20. A supply of the tape 14 is spooled on rollers 15 and supplied to the applicator 12 over inclined roller 16, when required.
 FIGS. 1 and 2 both show a single applicator 12. However, the bottom hole assembly in FIG. 1 and the lower part 26 of the tool in FIG. 2 could possibly carry more than one applicator, for example two similar applicators at diametrically opposite positions with a separate supply of tape 14 to each applicator.
 FIGS. 3, 4, 6, 8 and 9 show possible forms of applicator 12 which may be used in the bottom hole assembly of FIG. 1 or in the wireline tool of FIG. 2. In each case the applicator is seen looking in the direction of the wellbore axis, that is looking downwardly in FIGS. 1 and 2. For convenience, these forms of applicator will be described as carried on the bottom hole assembly of FIG. 1 and used to coat a wellbore wall 31. However, the manner of operation when the applicator is carried on the lower part 26 of the wireline tool of FIG. 2 is the same as when it is carried on the bottom hole assembly of FIG. 1 except that the inside of tubing 20 takes the place of the wall 31 of the wellbore as the downhole fixed surface to which curable composition is applied. It should also be appreciated that the wireline tool of FIG. 2 could be used to line an openhole wellbore.
 Referring to FIG. 3, the applicator is carried on a support 30 which is used to move applicator outwardly from the bottom hole assembly of FIG. 1 for operation and also to retract it when not in use. In this embodiment, the support 30 carries the applicator bodily outwardly or inwardly. A possible alternative is for the applicator to be pivotally attached to the bottom hole assembly (or wireline tool) and swung outwardly around the pivot as illustrated by the applicator shown in FIG. 8 below. The support 30 includes a spring or other compliant element so that when the applicator has been moved out to the position shown in FIG. 3, it is being pushed gently towards the wall 31 of the wellbore.
 The applicator has a main body 32 with a sheet 34 of polytetrafluoroethylene (PTFE) at its outer surface facing the wellbore. This sheet is translucent, allowing light to pass through it, with some diffusion. The applicator is traversed across the surface of the wall 31 of the wellbore, in the direction indicated by arrow 36 as the drill string rotates. Consequently the outer surface of sheet 34 is carried across the wellbore wall 31 which it faces. As this movement takes place, preimpregnated tape 14 is delivered into the gap between sheet 34 of the applicator and the wellbore wall 31.
 The applicator has a light source 42, which in this embodiment is one or more light emitting diodes (LEDs) emitting light at one or more wavelengths in a range from 200 to 600 nm. This range is ultra-violet through to green light. These LEDs are contained within a housing 44 attached to the applicator body 32 and sealed to the sheet 34. The light source 42 is positioned to direct light onto and through the sheet 34.
 The applicator is shaped so that within the arc 46 the spacing between the sheet 34 and the wall 31 of the wellbore progressively narrows as the applicator advances over the wellbore wall 31, pressing the preimpregnated tape onto the wellbore wall 31. The curable layer which is provided by the preimpregnated tape (i.e., the porous tape itself plus the photocurable composition impregnating it) is in contact with both the sheet 34 and the wall 31 and so bridges the spacing between them. Next, as the applicator travels in the direction 36, the light source 42 travels over the tape 14 and photo curing of the impregnating composition is initiated by the light from source 42 passing through the translucent sheet 34 to reach the tape. The photocuring causes the impregnated tape to become a solid layer adhering to the wellbore wall 31 as indicated at 47.
 The sheet 34 provides a light transmitting barrier between the light source 42 and the tape, protecting the light source 42 from contact with the curable composition impregnating the porous tape and hence preventing this composition becoming attached to the light source by curing while in contact with it. We have found that a sheet 34 of a fluorine containing polymer such as PTFE avoids wetting by the photocurable composition, thus minimising attachment of the impregnating composition to the sheet 34 as the composition cures.
 Because the tape with its impregnating composition fills the gap between the sheet 34 and the wellbore wall 31, drilling mud is largely prevented from entering the path of light from the light source 42 to the tape 14 on the wellbore wall 31 thereby mitigating light attenuation by the opaque drilling mud.
 FIG. 4 shows an applicator which has a number of features similar to those in FIG. 3 and these are indicated by the same reference numerals. There are two main differences from the embodiment of FIG. 3 and it should be appreciated that these features could be used separately from each other if desired. Firstly, instead of light emitting diodes, the light output which directs light onto the composition is the outlet end of a light guide 60 carrying light from a lamp which may for example be a mercury vapour discharge lamp elsewhere in the bottom hole assembly. Such a lamp emits at several wavelengths between 200 and 600 nm. The light guide 60 is a bundle of optical fibres within a surrounding sheath. Each optical fibre has an elongate core filament of glass or of organic polymer surrounded by one or more layers of cladding, with the core having a higher refractive index than the cladding, so that light introduced at one end of the fibre will be internally reflected for transmission longitudinally within the core to the other end of the fibre.
 A light guide may also be formed from a liquid core within an enclosing tube where the refractive index of the liquid core exceeds that of the tube. Use of a liquid core or use of a bundle of separate optical fibres within a sheath allows such light guides to be flexible. Light guides are available from various manufacturers including Universal Fibre Optics Ltd, Coldstream, Scotland.
 The embodiment in FIG. 4 also differs from FIG. 3 in that the applicator has a main body 62 which is made of PTFE. The end portion of the light guide 60 is sealed into a cavity in the body 62 so that the outlet end of the light guide 60 directs light onto and through a portion 63 of the body 62 which is sufficiently thin to have good light transmission. This portion 63 of the body 62 provides a light transmitting barrier between the tape 14 preimpregnated with photocurable composition and the light guide 60.
 A pre-impregnated tape 14 used with an applicator, as disclosed here, may be impregnated with curable composition across its entire area. However, FIG. 5 illustrates the possibility that tape is impregnated over only part of its area. FIG. 5 shows a length of porous tape which has two bands 65 of curable composition applied to it at a factory. The tape is dispensed from a spool as in FIG. 1 or FIG. 2 and supplied between an applicator and a wellbore surface (or correspondingly between an applicator and tubing as in FIG. 2). As the applicator presses the tape onto the wellbore wall the bands 65 spread somewhat as indicated at 67 but they do not spread across the entire width. Consequently when the impregnating composition is cured, the tape is held onto the wellbore wall by the cured bands 65. The portions 68 outside and between the bands 65 are not directly adhered to the wall but are nevertheless held in place against it. If fluid flows out of the wellbore through these portions 68, solids in the fluid may form a filter cake on these portions 68 reducing their porosity and so reducing continuing leakage of fluid.
 FIG. 6 shows an applicator with some features similar to those in FIG. 3 and these are indicated by the same reference numerals. The features which differ from FIG. 3 could be employed individually, if desired.
 This applicator uses a tape 54 supplied from a spool in the bottom hole assembly as in FIG. 1 (or wireline tool as in FIG. 2) where the tape 54 is porous but is not preimpregnated. The tape 54 is delivered into the space between the main body 32 of the applicator and the wellbore wall 31. Liquid photocurable composition 39 is supplied along a pipe 38 extending through the applicator body and discharged between the applicator body 32 and the tape 54. The main body 32 of this applicator is made of steel and is in contact with the curable composition 39. The applicator is shaped so that within the arc 46 the spacing between the applicator body 32 and the wall 31 of the wellbore progressively narrows as the applicator advances over the wellbore wall 31, forcing the liquid photo curable composition 39 into the porous tape 54 and so creating a curable layer 53 comprising the porous tape impregnated with the curable composition.
 The housing 44 containing LEDs as the light source 42 is sealed to a light transmitting sheet 58 which attached to the applicator body 32 and trails behind it. A transparent liquid is supplied in small quantity along pipe 55 so as to form a thin film of this liquid over the curable layer 53 and filling a narrow space 56 between the curable layer 53 and the light-transmitting sheet 58 as indicated in the enlarged view in FIG. 7. In consequence, unwanted curing of the composition 39 onto the surface of the sheet 58 is here prevented by the presence of the film of clear liquid between the layer 53 and the sheet 58 rather than by requiring the sheet 58 to have a non-stick property. This allows a wider range of light-transmitting materials to be used for the sheet 58 than for the sheet 34 of FIG. 3.
 FIG. 8 shows an applicator which has a pair of freely rotatable rollers 70, 71 with a belt 72 running on these rollers 70, 71. This belt 72 is made of flexible light-transmitting polyurethane with a coating of PTFE lubricant particles on its surface. Other possible materials for this belt include silicone rubber, PTFE and FEP. A light source 73 is positioned between the rollers 70. This light source comprises LEDs, similar to the LEDs 42 in FIG. 3, partially embedded in a block 74 of light transmitting silicone rubber which is in sliding contact with the inside face of the belt 72.
 This assembly of parts 70-74 is supported by a pair of arms 76 (the lower one of these arms 76 is shown in FIG. 8) pivoted at 77 on the bottom hole assembly of FIG. 1. The arms 76 and the assembly of parts 70-74 carried on them can be swung outwardly towards the wellbore wall 31 when required by a rod 78 extensible from the bottom hole assembly. A similar arrangement of arms 76 and rod 78 could likewise be used if this applicator was carried on the lower part 26 of the wellbore tool of FIG. 2.
 When required for use the applicator is positioned as shown in FIG. 8. The extensible rod 78 incorporates a spring or other compliant element, so that the applicator is being pushed onto the wellbore wall. As the applicator is advanced across the wellbore wall 31 in the direction indicated by arrow 36, preimpregnated tape 14 is supplied as shown so as to enter between a portion 82 of the belt which extends between the rollers 70, 71 and is adjacent to the wellbore wall 31. This portion 82 of the belt 72 remains stationary relative to the wall 31 and the tape 14. The rollers 70, 71 turn as indicated by arrows. This manner of motion is analogous to that of a tracked vehicle in which the lowest part of the track is stationary on the ground.
 It is possible that the portion 82 of the belt is not completely stationary relative to the wellbore wall 31 but instead slides slowly over the tape 14. Whether the portion 82 of the belt is stationary or is moving slowly relative to the tape 14 and wellbore wall 31, the belt is travelling over the rollers 70, 71 and in consequence the portion 82 of the belt is moving over the tape 14 and the wellbore wall 31 more slowly than the light source 73 and other structure of the applicator.
 Light from source 73 is directed through the portion 82 of the belt onto the preimpregnated tape 14 and initiates curing onto the wellbore wall. The cured layer on the wellbore wall is indicated at 47.
 In this embodiment the portion 82 of the light transmitting belt 72 provides a barrier between the light source 73 and curable composition impregnating the tape 14 but when this portion 82 of the belt separates from the cured tape layer at the trailing roller 71, the separation is a peeling action rather than a sliding motion. The block 74 of silicone rubber in contact with the inside face of the belt 72 inhibits entry of drilling mud into the light path from the light source 73 to the impregnated tape 14. However, if the wellbore fluid contains suspended solids, as is the case with drilling mud, we have found that it is desirable to structure the surface of the block 74 where it contacts the belt 72 so that the block has shallow grooves between ribs which contact the belt. These grooves are dimensioned to provide an exit path for any solid particles which do enter between the block 74 and the belt 72.
 FIG. 9 shows an arrangement in which the applicator has both a body part 80 which is somewhat similar to the body 32 shown in FIG. 3 and an assembly of parts 70-74 as in FIG. 8. The body part 82 and the assembly 70-74 are carried by a support (not shown) which is able to carry them bodily outwardly and inwardly analogously to the support 30 in FIG. 3.
 The tape 14 is supplied from a spool as in FIG. 1 or FIG. 2. It passes through a slot in the applicator body 82 and is gripped between a pair of rollers 84 which can be turned by a motor, not shown, to initiate the supply of preimpregnated tape between the applicator and the wellbore wall 31. As the body 80 of the applicator travels over the wellbore wall 31, it presses the preimpregnated tape 14 onto the wellbore wall. Curing of the impregnated tape which has been applied is then carried out with the assembly of parts 70-74 as in FIG. 8. The body part 80 has a short piece 86 of PTFE attached to it, which wipes the exterior of the belt as it comes off the roller 70 onto the impregnated tape 14 thus largely excluding drilling mud from entering between the belt 72 and the impregnated tape 14. A shear mechanism is provided at 84 and used to cut the tape 14 after sufficient tape has been applied.
 Use of a tool as in FIG. 1 or FIG. 2 provided with one or more applicators as in any of FIG. 3, 4, 6, 8 or 9 when required to deliver tape onto a downhole surface (which could be the wellbore wall or the interior of tubing) may comprise rotating the bottom hole assembly 11 of FIG. 1 or the lower body part 26 of FIG. 2 in order to skim the applicator over the downhole surface while also moving the tool linearly in the wellbore so that the tape is applied in a helical pattern.
 When the tool is the bottom hole assembly of FIG. 1, application of the tape may take place while drilling. It could also be done with drilling stopped and the drill string moved slowly up the wellbore as tape is applied and cured. This would of course be an interruption to drilling, but the interruption would be relatively short because there would be no need to trip the drill string out of the wellbore. An analogous possibility would be to draw the drill string back from the end of the wellbore by short distance and then apply tape while advancing the drill string forwardly towards the end of the wellbore.
 The wireline tool of FIG. 2 can be used to apply tape whilst being lowered or raised within the wellbore.
 Both with the bottom hole assembly of FIG. 1 and the tool of FIG. 2 the rates of linear and rotary motion may be chosen so that there is no separation between successive turns of the helix which is applied and the tape forms a complete covering of the downhole surface over the length of wellbore to which tape is applied. The rates of motion may be chosen such that successive turns of the helix overlap.
 Tape utilised in accordance with the present disclosure may be a woven tape or may be a non-woven material. It may be made from a natural or synthetic textile fibre such as cotton or nylon or it may be made from glass fibre. The photocurable composition used to impregnate the tape may comprise one or more compounds which are able to participate in a polymerization reaction and thereby extend a growing polymer chain. Such compounds may provide at least 50% probably at least 80% or 85% of the liquid components of the polymerizable composition. The composition may also contain a minor proportion of one or more compounds with a greater number of groups able to participate in the polymerization reaction and so create branching of polymer chains or cross-linking between polymer chains. Such a crosslinking or branching agent may be present as up to 15%, possibly 1 to 10% by weight of the liquid components of the curable composition.
 Polymerizable compounds contain at least one reactive group to enable polymerisation to occur. In some embodiments envisaged for use as disclosed here, the polymerizable compounds may be linear molecules with a reactive group at each end, such as esters of an olefinically unsaturated acid and a dihydroxy compound (although such esters may be manufactured using other starting materials such as an acid chloride, of course). The acid moiety may be an olefinically unsaturated acid containing 2 to 5 carbon atoms notably acrylic or methacrylic acid.
 Some examples of such monomer compounds are:--
bisphenol A ethoxylate diacrylates, having the general formula
bisphenol A ethoxylate dimethacrylates, having the general formula
and poly(ethylene glycol) diacrylates having general formula:
 In the above three general formulae, m and n are average values and may vary. Possibly m and n will each lie in a range up to 15, such as 1 or 1.5 up to 6. We have found that monomers containing ethylene oxide residues improve flexibility of the cured polymer but reduce its strength.
 A compound able to act as a crosslinker may have more than two olefinically unsaturated groups, to create branched or cross-linked polymer chains. Such compounds may be acrylate or methacrylate esters of poly hydroxy compounds.
 Some examples are as follows:
TABLE-US-00001 MW Name Formula (g/mol) trimethylolpropane triacrylate ##STR00004## 296 trimethylolpropane ethoxylate triacrylate ##STR00005## The average value of n in the above formula may be chosen so that the mean molecular weight is about 430, about 600 or about 900 pentaerythritol tetraacrylate ##STR00006## 352 di(trimethylolpropane) tetraacrylate ##STR00007## 466
 Monomer compounds with two olefinically unsaturated groups may also be vinyl ethers such as 1,6-hexane diol divinyl ether, poly(ethylene glycol) divinyl ether, bis-(4-vinyl oxy butyl)hexamethylenediurethane, and vinyl ether terminated esters such as bis-(4-vinyl oxy butyl) adipate and bis-(4-vinyl oxy butyl) isophthalate.
 Another possibility is that the groups able to participate in the polymerization reaction are epoxide groups. A suitable category of monomer compounds containing epoxide groups are glycidyl ethers of dihydroxy compounds, some specific possibilities being 1,6-hexanediol diglycidyl ether, bisphenol A diglycidyl ether and poly(ethylene glycol) diglycidyl ether.
 The photocurable composition may comprise a mixture of monomers. Notably a mixture of monomers may be used in order to obtain a desired combination of mechanical properties of the polymer lining on the tubing. The monomers will generally provide at least 50 wt % of the composition and preferably from 70 to 99.5 wt % of it.
 In addition to the polymerizable compounds and photoinitiator the photocurable liquid composition may include various other materials. One possibility is a leveling agent or a wetting agent to aid adhesion to the downhole surface. Such an agent may be a surfactant to displace any film of wellbore fluid on the surface of the tubing which is about to have the photocurable composition applied to it. Such a surfactant may be monomeric or polymeric and may include a reactive moiety such as an acrylate group to enable it to copolymerize with the main monomers of the composition. More specifically it may be a silicone polymer with pendant acrylate groups. Examples are available as TEGO RAD from Evonic Tego Chemie, Essen, Germany and EFKA 3883 from Ciba Inc. The amount (if any) of such additives are likely to be no more than 5 wt % of the composition.
 The photocurable composition may include an additive to increase its viscosity. Examples of rheology modifiers which may be added to the composition to enhance viscosity are fumed silica, clays and high molecular weight organic polymers. The amount (if any) of a material added solely to enhance viscosity may be no more than 5 wt % of the composition.
 The photocurable composition may include one or more suspended solids serving to reinforce it after polymerisation such as bentonite clay particles, or short fibres such as chopped glass fibres. These materials may have an additional effect of enhancing viscosity. The amount of suspended solids in a photocurable composition may possibly range from 0 to 60 volume % of the composition and in some embodiments may lie in a range from 0 to 20 volume % of such solids.
 A photoinitiator in the curable composition is a compound that it is capable of generating a reactive species effective to initiate polymerisation upon absorption of light. The initiating species which is generated may be a cation or a free radical. A photo initiator may therefore be referred to as a cation photo initiator or a radical photo initiator respectively.
 A radical photo initiator may be a type I (cleavage type) or a type II (H-abstraction and electron donor) initiator. A type I initiator undergoes a unimolecular bond cleavage (α-cleavage) upon irradiation to yield the free radical. A type II initiator undergoes a bimolecular reaction where the triplet excited state of the photoinitiator interacts with a second molecule, which may be another initiator molecule, to generate a free radical. Typically, the second molecule is a hydrogen donor. Where the second molecule is not another initiator molecule, it may be referred to as coinitiator. The coinitiator may be an amine, alcohol or ether. Preferably, the coinitiator is an amine, most preferably a tertiary amine. Where the second molecule is another initiator molecule, the initiator may contain amine, alcohol or ether functionality.
 Type I cleavable photo-initators include benzoin ethers, dialkoxy acetophenones, phosphine oxide derivatives, amino ketones, e.g., 2-dimethyl, 2-hydroxyacetophenone, and bis(2,4,6-trimethyl benzoyl) phenyl phosphine oxide.
 Type II initiator systems (photoinitiator and synergist) include aromatic ketones, e.g., camphorquinone, thioxanthone, anthraquinone, 1-phenyl 1,2-propanedione, combined with H donors such as alcohols, or electron donors such as amines.
 A cation photo-initiator is preferably a photoacid generator, typically a diazonium or onium salt, e.g., diaryliodonium or triarylsulphonium hexafluorophosphate.
 Photo initiator will generally be a small percentage of the photocurable composition. The percentage of photo initiator in the composition is likely to be a least 0.5% by weight and may extend up to 3% or even 10% by weight of the liquid components of the composition.
 An example of a curable composition had the following formulation.
TABLE-US-00002 Function Material Wt % polymerisable diacrylate Bisphenol A ethoxylate diacrylate 68.23 polymerisable diacrylate PEG Diacrylate 22.75 crosslinking agent Pentaerythritol triacrylate (PETA) 2.27 photoinitiator Irgacure 784 0.19 photoenhancer Methylene Blue 0.04 clay particles Bentone SD-2 1.87 polymer chain terminator Isobornyl acrylate (IBA) 4.66
 This composition was observed to be immiscible with water and also to be immiscible with hydrocarbon oil.
 It will be appreciated that the example embodiments described in detail above can be modified and varied within the scope of the concepts which they exemplify. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.
Patent applications by David Robert Evan Snoswell, Cambridge GB
Patent applications by Gokturk Tunc, Cambridge GB
Patent applications by John Cook, Cambridge GB
Patent applications by Louise Bailey, St. Neots GB
Patent applications by SCHLUMBERGER TECHNOLOGY CORPORATION
Patent applications in class With sealing feature (e.g., packer)
Patent applications in all subclasses With sealing feature (e.g., packer)