WO2013045882A2

WO2013045882A2 - Fibre optic cable deployment, particularly for downhole distributed sensing

Info

Publication number: WO2013045882A2
Application number: PCT/GB2012/000759
Authority: WO
Inventors: Philip Head
Original assignee: Philip Head
Priority date: 2011-09-30
Filing date: 2012-10-01
Publication date: 2013-04-04
Also published as: WO2013045882A3

Abstract

A fibre optic cable is embedded in a slot or coating on the outer surface of a tubing as the tubing is deployed into a well. The cable may be mounted on a reel so as to be rotatable together with the tubing, and may be interrogated via a rotatable optical coupling while the tubing is rotated and deployed. The cable may be used to provide distributed sensing along a length of the tubing. Where the tubing comprises a drillstring, the cable may provide real-time broadband data telemetry from the drilling assembly while drilling.

Description

Fibre optic cable deployment, particularly for downhole distributed sensing

This invention relates to the deployment of fibre optic cables, particularly but not exclusively for distributed sensing in a borehole such as a hydrocarbon well.

In this specification, a fibre optic cable is a cable including at least one optical fibre. A tubing means any pipe, pipeline or other tubular member having a bore for conducting a fluid, including without limitation any drillstring, casing or other tubular member deployed in a wellbore, such as for the injection or extraction of water or hydrocarbons into or from a well. In this specification a tubing is referred to variously as a pipe, pipestring, drillstring or casing.

Oil and gas wells and other boreholes, are constructed generally by a cutting tool called a drill bit, which is deployed into the ground by connecting joints of tubing together above it, drilling fluid is pumped down the inside of the tubing to both lubricate the drill bit and circulate the drill cuttings back to surface via the tubing or drill pipe annulus. Another function of the drilling fluid is to apply sufficient hydrostatic pressure to prevent the uncontrolled entry into the borehole of hydrocarbons such as oil or gas. At certain depths it becomes necessary to seal the bore hole with a steel lining, generally referred to as casing. This is cemented in place to hydraulically seal the formations the open hole has been drilled into.

Eventually, the well reaches the reservoir, the well is completed with production tubing and various other components as required by the complexity of the completion requirements, i.e. sand screens, in flow control devices, packers, etc.

Conventionally, a fibre optic cable is strapped to the outside of production tubing and deployed into the borehole together with the production tubing from a rig, and then interrogated from the surface to provide distributed sensing of a measurable parameter such as ambient temperature or pressure as well known in the art. Fibre optics has become increasing used in the oil industry to monitor what is happening inside the wellbore, and across the reservoir. However the existing method of conveying it means it can only be installed at best with the completion.

It is a general object of this invention to provide a more convenient apparatus and method for deploying a fibre optic cable on tubing, particularly but not exclusively for distributed sensing in a well. In more specific aspects the invention sets out to enable the fibre optic cable to be conveyed into the well while the well is being drilled, cased or completed.

In accordance with the various aspects of the present invention there are provided an assembly and a method as defined in the claims.

In a preferred embodiment, a fibre optic cable is embedded in a slot or coating on the outer surface of the tubing as the tubing is deployed into a well. The cable may be mounted on a reel so as to be rotatable together with the tubing, and may be interrogated via an optical slip ring, i.e. a rotatable optical coupling, while the tubing is rotated and deployed. The cable may be used to provide distributed sensing along a length of the tubing. Where the tubing comprises a drillstring, the cable may provide real-time broadband data telemetry from the drilling assembly while drilling. Where the tubing comprises a casing of the well, the cable may be cemented into the annulus along with the casing and used to monitor the installation and subsequent integrity of the cement bond and to monitor any unwanted cross flow of fluid behind the casing. Where the cable is arranged across the reservoir it will provide valuable information relating to reservoir fluid movement and fluid types. Further features and advantages will become evident from the various illustrative embodiments which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which:

Figure 1 is a cross section through part of the wall of a casing;

Fig. 2 is a side view of the part of the casing wall shown in Fig. 1 ; Fig. 3 shows a casing according to an alternative embodiment having a strip affixed to its outer surface;

Fig. 4 is a side view of a casing coupling joining two pieces of casing. Fig. 5 is a section side view of Figure 6

Fig. 6 is an external view of a casing coupling illustrating another embodiment of the invention. Figs. 7 and 8 are enlarged sectional views of detail A in Fig. 8 taken respectively at lines A-A and B-B;

Fig. 9 is a sectional view of detail C in Fig. 8 ; Figs. 10A and I OB show a tubing in accordance with a further embodiment, respectively before and after spraying the tubing with a coating;

Figs. IOC and 10D are cross-sections respectively at C-C and D-D of Figures 10A and 10B; Fig. 10E is an enlarged view of part of Figure IOC;

Fig. 11 is a longitudinal section through a disconnected pin and box connection between two tubes forming a drill pipe;

Fig. 12 corresponds to Fig. 11 after connection; Fig. 13 is an enlarged view of detail E of Fig. 12; Fig. 14 is a side view of the drill pipe of Fig. 12; Fig. 15 is an enlarged view of detail B of Fig. 14;

Fig. 16 is an illustration of a drilling rig showing the key components for the surface operation;

Fig. 17 is a plan view of a cable dispensing spool in the process of being clamped to a drill pipe; Fig. 18 corresponds to Fig. 17, with the cable spool fully clamped to the drill pipe;

Fig. 19 is a side view of the assembly of Fig. 18, showing the device in section view and the drill pipe not sectioned; Fig. 20 is a cross section through a tube having a steel strip in accordance with a further embodiment of the invention;

Figs. 21 and 22 are cross sections through a tube in accordance with a yet further embodiment of the invention, respectively during and after installation of the cable in a rubber strip; Fig. 23 is a half section view of a casing non return valve, with the cable penetrating the tubing wall so that it can sense internal as well as external pressure;

Fig. 24 is a longitudinal section through a wellhead and horizontal Christmas tree, showing various ways passing the fibre optic cable through the wellhead or tree;

Fig. 25 is a longitudinal section through a cementing plug containing a sound source;

Fig. 26 is an illustration of a well being drilled showing various sensing techniques; and Fig. 27 shows a casing being installed in a well having top and bottom cementing plugs with respective sound sources.

It will be understood that various features are illustrated by more than one of the specific embodiments and some corresponding features are indicated in each of those embodiments by corresponding reference numerals.

Referring to the figures and in particular to figures 1 and 2, an assembly includes a tubing 2 comprising a wall 120 made from metal, typically steel, having respectively inner 1202 and outer 1203 surfaces, the inner surface 1202 defining a bore 1204, and a fibre optic cable 4. The tubing includes a small slot 3 machined, forged or formed in the wall 1201. The slot extends substantially continuously along the tubing and opens externally of the tubing (which is to say, radially outwardly from the tubing) at an narrow opening 3' through which the cable may be inserted laterally into the slot. The cable is arranged in the slot. At least a part of the slot is formed in the wall. In the example shown, the tubing forms a casing and the slot is formed entirely in the wall, extending along the whole length of each rigid tube forming the casing and through the casing couplings in a manner explained in more detail below.

The slot has a cross section (Fig. 1) defining an inner region 3" which has a width D2 wider than the width Dl of the opening 3'.

Advantageously the slot has a T shaped cross section as shown, and preferably the cable has an outer diameter D3 as shown less than half the distance between the opposite sides 5, 6 of the slot, so that the cable extends sinuously along the slot so as to frictionally engage the opposite sides 5, 6 of the slot as shown, effectively transferring the weight of the cable to the tubing and holding the cable frictionally in place.

The metal clad cable has a residual wave form, and it is pulled tight (in the position 4' of Fig. 1) before freely inserting it into the slot 3 through the entrance 3'. The cable then relaxes and returns to its sinuous shape as shown, bringing it into contact with the either side of the slot 5, 6 so that it is retained in place beneath the narrow entrance 3'.

The slot can be formed in a section of regular wall thickness. In order not to compromise the burst pressure of the pipe, a heavier weight pipe could be used. Alternatively, the wall of the tubing may have a locally thickened portion 10 in the region of the slot so that the wall thickness of the casing is slightly eccentric, with the outer surface 1203 of the casing in the region of the slot extending radially outwardly slightly further than the outer surface at the opposite side of the casing, the relative radial position of which is indicated for comparison by the broken line 1203'. Referring to Fig. 3, in an alternative embodiment at least a part of the slot is formed in a strip attached to a surface of the tubing. In the example shown, an additional strip of steel 14 is seam welded 15 to the outside surface 1203 of the casing so as to not compromise the pressure integrity of the casing, and the slot 3 is formed for most or all of the length of the tubing in the strip.

The strip could be welded to continuous (coiled) tubing or alternatively to rigid tubes which are joined together to form a tubing string. Referring to figure 20 to 22 there are shown further embodiments of holding the fibre optic cable to the casing.

In Fig. 20 the wall 1201 of the casing 2 has a shallow slot 3 cut into it which is the depth of the metal clad fibre 4, the opening 3' to the slot having a slightly narrower width Dl than the external diameter D3 of the cable. The cable 4 is elastically deformable (comprising one or more optical fibres within an

elastomeric sheath and stainless steel outer casing) so as to pass between the inwardly projecting margins 3" of the opening during assembly. As the cable 4 is pushed through the slot, it deforms elastically until it enters fully into the slot when it recovers to its original round shape, after which it cannot freely leave the slot without sufficient over pull to deform it.

A slightly different approach is shown in figures 21 and 22. Since in some circumstances it might not be advisable to machine a slot in the casing or drill pipe, in this embodiment, a strip of elastomer 101 is bonded to the outer surface of the casing or drill pipe 2. A small slot 102 with an opening comprising a slit 103 form part of this bonded elastomer strip. The metal clad cable 4 is installed into the slot 102 by using an installation tool 104, this is pushed into the slit 103 until it contacts the casing 2, as it is moved along the casing pipe the cable is dropped into the opening the installation tool has made. As the installation tool continues along the casing, the slit closes behind it and retains the fibre cable snugly in the slot 102.

Referring now to Figs. 4 - 9 and Figs. 11 - 15, the tubing may comprise a plurality of rigid tubes 20, 22, 1240, 1241 connected together at joint regions 1221, wherein the slot 3 extends along each of the rigid tubes. Each of the joint regions has at least one annular recess 1 13, 48, 27 extending around the tubing and communicating with the slot; and the cable 4 extends along the slot and is arranged in the annular recess. This allows the cable to travel smoothly and continuously through the portions of the slot extending along each of the rigid tubes and across the joint region, without requiring those portions to be aligned around the circumference of the tubing.

The slot includes portions 23, 26, 44, 45, 114, 116, 110, which are radiused relative to a length of the slot, the radiused portions of the slot communicating with the annular recess. The annular recess 48 may be defined between opposed surfaces 401, 41 1 of two adjacent tubular portions (Figs. 11 - 13). Alternatively, the annular recess may be defined in a collar 21 which is threadedly connected to the respective adjacent tubular portions 20, 22 (Figs. 6 - 9).

Each joint region may include at least one ring 24, 25 (Figs. 4 - 5), the ring including a first portion 26 of the slot 3 communicating with the annular recess 27, the ring being rotatable about the tubing so as to align the first portion 26 of the slot with a further portion 3 of the slot.

These features will now be described in more detail.

Referring to Figs. 4 and 5 there is shown a typical casing connection. This consists of a rigid tube comprising a lower joint of casing 20, a coupling 21 and another rigid tube comprising an upper joint of casing 22. When these are torqued together on the rig floor their alignment is impossible to guarantee.

The slot 3 is machined as a straight line in the joints of casing. A straight slot 3 is machined in the coupling at each end it has a radius entrance 23. Two further sleeves or rings 24, 25 fit onto each casing joint, in each ring is a continuous portion of the slot 3, consisting of a curved entrance 26 which goes into an annular recess 27. Once the coupling is torqued up the curved entrance 26 is aligned with the slot 3 in the casing and tapered dowels 28 are inserted to lock the ring 24 in this position. This is done on each side of the coupling. There is now formed a smooth continuous path for the fibre optic cable to pass over the coupling with maximum protection. To prevent the fibre optic cable being pulled out of the slot, a pin 30 is installed over the cable and ensures the cable is retained at that point. This is duplicated at the entrance to the slot on the coupling 31, at slot exit of the coupling 32 and the slot exit 33 of the upper ring 25.

In the illustrated embodiment, two rotatable rings are provided, one on either side of the threaded collar. It will be understood that the threaded collar could alternatively be a fixed or integral part of one of the two tubing portions, in which case only one rotatable ring might be provided.

Referring to figures 6 to 9 there is shown another embodiment of the invention to enable the cable to be retained in a slot 3 in one casing 20, traverse a casing coupling 21, and enter the slot 3 of the next section of casing 22. At the end of each casing the slot 3 forms a radius 110 in the clockwise direction which terminates under the coupling 21, as the fibre optic cable 4 is installed into the radius, a small clip 111 is inserted into a matching slot 1 12 to retain the fibre in place. The clip 1 11 is shown in plan view in Fig. 6 and respectively in end and side view in Figs. 7 and 8. The cable 4 is directed into the annular recess 113 under the coupling; for maximum protection the cable follows the coupling circumference until it can go into the slot 11 which has a radius anti clockwise, a similar clip 1 1 1 holds the cable below the surface at this point, where the clip 11 1 is retained in snap-fit relation in a recess 112. The cable 4 is installed in the portion 1 15 of the slot extending along the coupling and exits via a clockwise radius 116, where the same pattern is repeated until the cable enters the slot 3 of the next rigid tube forming the casing. It will be understood that each of the clips 111 and recesses 112 as indicated for example in the respective detail portions A in Fig. 6 are similar. Referring to figures 1 1 to 15, there is shown a drill pipe type connection. This is a simpler connection as it only consists of two parts: a lower box connection 40 and an upper pin connection 41 , with upsets 42 and 43 for the pipe tongs to grip and make the connection tight. Again, it is impossible to determine where the final position of the couplings will be after being made tight, so in this case, the slot 3 is terminated at its box end with a curved exit 44 turning to the right and its entrance to the pin connection 41 is curved 45 rising from left to right. No matter where the exit 46 is relative to entrance to the next joint 47, once the fibre exits at 46 it follows the annular recess 48 formed as an undercut between the two couplings until it reaches the entrance 47 to the next joint.

Referring to Figs. 16 - 19, a drilling assembly 66 including downhole equipment (i.e. equipment such as sensors capable of transmitting data) 66' may be arranged at a lower end of the tubing 310 which is configured as a drill pipe. An upper end portion 4' of the cable 4 may be arranged on a spool or reel 60, the reel being mounted for rotation together with the tubing 310. The upper end portion of the cable is connected with surface equipment 500 via an optical slip ring 130, the surface equipment being arranged to remain stationary and to interrogate the cable via the optical slip ring during rotation of the tubing. In this way the cable is arranged to transmit data from the instrumentation to surface as drilling proceeds. In more detail, the fibre spool 60 sits in a cradle 61 which runs on rollers 62, 63, rollers 62 allow the device to move up and down the pipe, and rollers 63 allow the device to traverse around the pipe. An anvil 64 aligns the cradle to the slot 3, and sensors (not shown) also monitor its position. A jetting tool can also be included with the anvil to clean any accumulated dirt from the slot. Included in the drilling assembly at the lower end of the cable 4 is downhole equipment 66' including a battery powered telemetry package, which can both talk to a communications device at the drilling assembly 66 and send wireless data 67 to a surface computer 68.

Alternatively, if the fibre optic instrumentation has to be connected directly to the cable embedded in the casing, an optical slip ring 130 can achieve this. A fibre optic connector 131 can connect the cradle via a fibre optic cable 132 to the appropriate instrumentation box in the surface equipment 500. These

instrumentation boxes can be housed in a clean instrumentation room away from the dirty rig floor.

It will be understood that in the various embodiments of the invention, the tubing may be suspended within a borehole 600 (forming for example a production tubing or a drillstring) or may be a casing, which optionally may be used as a drillstring, but is ultimately cemented into a borehole. In each case, the cable may be configured for distributed sensing along at least a portion of the tubing.

This may be accomplished as described in more detail herein by connecting together a plurality of rigid tubes 20, 22, 1240, 1241, 1251, 1252 at joint regions 1221 to form a tubing 2, 310, 401, the tubing comprising a wall having

respectively inner and outer surfaces, the inner surface defining a bore; providing at least one slot 3 extending along the tubing at least in each of the joint regions 1221 and opening externally of the tubing at an opening 3'; and providing a fibre optic cable configured (for example, by including Bragg gratings, or a perforated casing, or otherwise being arranged to provide a measurable response to a sensed parameter at positions along its length) for distributed sensing. The cable is arranged along the tubing and inserted laterally into the or each slot 3 via the respective opening, before deploying the tubing and cable together into the borehole. The cable is then interrogated so as to sense at least one measurable parameter along at least a portion of the tubing. Advantageously, the cable is protected by insertion into the slot from damage by contact with the wall of the borehole where it traverses each joint region, which typically comprises a relatively enlarged diameter portion of the tubing.

Referring to Figs. 10A - 10E, in a further embodiment an assembly includes a tubing 1201, the tubing comprising a wall 1202 having respectively inner 1203 and outer 1204 surfaces, the inner surface defining a bore 1205; and a fibre optic cable 4; wherein the outer surface of the wall of the tubing is coated with a coating material 1260, and the cable is arranged on (i.e. adjacent) the outer surface 1204 of the wall and embedded in the coating material.

In the example shown, the tubing comprises a plurality of rigid tubular portions 1251, 1252 connected together at joint regions 1221 by collars 1253; and a respective slot 3 is formed in each of the joint regions, and the cable 4 is arranged in the slot.

The tubing may be assembled by coating the outer surface of the wall of a first one 1251 of the rigid tubes with a coating material 1260 so as to embed the cable in the coating material; and then lowering the first one of the rigid tubes; and then connecting a further one 1252 of the rigid tubes to the said first one of the rigid tubes, arranging the cable along the further one 1252 of the rigid tubes, and coating the outer surface of the further one 1252 of the rigid tubes with the coating material 1260 so as to embed the cable in the coating material. The coating material is preferably a rapid drying or rapid hardening coating material, for example, a 2-part cementitious grout which hardens instantaneously by chemical reaction between the two parts as it is sprayed from a nozzle array 1270 onto the tubing; for example, a grout including epoxy or other reactive components; or a polymer which is rapidly cured by exposure to UV (ultraviolet) light after being applied (for example, sprayed or extruded) onto the tubing.

Alternatively, the tubing may be continuous (i.e. coiled) tubing rather than an assembly of jointed rigid tubes, with the cable being laid along the outer surface of the tubing and the coating applied in the same way.

Alternatively, the cable can simply be fed off a reel 60 into a slot 3 which extends continuously along the tubing, with the rotatable rings 24, 25 (if any) being rotated and locked at each joint region so as to provide a smooth, continuous path in which the cable is received.

In each case, the tubing can be assembled on the rig and then immediately lowered section by section into the borehole (or for a subsea well, into the sea), with the cable being fed off the rotatable reel 60 and continuously interrogated so as to feed back data from the wellbore as the tubing string is rotated to deepen the borehole and as each new tube is added and deployed.

Referring to Figure 23, there is shown a half section view of a casing non return valve, commonly called a float collar. It consists of a drillable spring loaded non return valve 140, cemented 141 into a collar 142. To assist in the casing

installation and monitor the non-return valve integrity it would be very useful to measure both internal and external pressure.

In order to achieve this, a hole 143 is formed in the wall of the tubing, and a portion of the cable 4 is arranged to extend through the hole into the bore of the tubing. Specifically, the hole is drilled in the collar 143 before inserting the metal clad cable 144 into the hole. The hole is sealed with a permanent sealing material and the metal clad cable is bonded to the inner surface 145 of the collar 142. Two small perforations 146, 147 are made in the metal casing of the cable so that the DTSS fibre inside the tube can be exposed to the wellbore fluids and measure the pressure at these points.

Referring to figure 24, this is a section through a wellhead and horizontal

Christmas tree. The main components are as follows. There is a low pressure wellhead housing 200 connected to a conductor pipe 201. Installed into this is a high pressure wellhead housing 202 attached to the surface casing 203. As the well is drilled deeper, subsequent casings are installed, such as intermediate casing 204 which is suspended in the wellhead by a hanger 205. The production casing 206 is the last casing in the well and this is again suspended in the wellhead housing by a hanger 207. The Christmas tree 208 is then connected to the wellhead and retained using the wellhead connector 230. A completion string 209 is run and suspended in the Christmas tree by a tubing hanger 210. Production is directed to a side port 21 1 and horizontal valves 212, 213. Access to the

completion can be achieved by removing the debris cap 214, and crown plugs 215, 216.

The casing embedded fibre 231 is terminated in the hanger with either an optical wet connector, either facing upwards 220 or to the side 221, if it is facing to the side, it has to be aligned using an orientation means during the landing process so that it is aligned with the optical wet connector 222 installed in the wellhead housing. The optical cable 223 is fed of to a control panel and then back to the rig using an umbilical not shown. If the optical wet connector is looking upwards through the casing hanger, it has to align with the optical wet connector 224 fitted to the Christmas tree, again orientation means is required during the mating of these two parts. The fibre optic cable 225 exiting from the wet connector 223 again goes to a control panel and then back to a surface facility 500.

The well may be monitored by providing a sound source; pumping the sound source down the bore of the tubing; and interrogating the cable to determine the position of the sound source. The sound source may be a battery powered sound emitter encapsulated in a cement plug slidably received in the bore of the tubing, or may be carried by a fluid comprising a sound emission means which is pumped down the tubing so as to flow back up an annulus externally of the tubing, with the cable in each case being configured to track the position of the sound source in the well.

Referring to figure 25, there is shown a section through a bottom cementing plug. This consists of a rubber exterior 250 with fins 251 which fit inside the casing it is being pumped down. Its upper surface 252 is a burstable membrane and when it lands on its landing collar, the fluid will burst this membrane and pass through the bore 253. Built into the drillable annular ring 254 is a sound source 255 which is wired to a battery 256. This sound source can be tracked inside the casing by the acoustic sensing embedded fibre optic cable.

Referring to figure 26, this is an illustration of drilling a well, and will be used to explain the benefits of embedding the fibre to the outside of the drill pipe 310.

There is shown a drilling assembly 300 which is terminated with a drill bit 301, the last casing is terminated at 302, the remainder of the hole is open to the formations the drilling assembly is passing through. The first thing that is possible with embedded fibre optics is bi directional telemetry from surface to the instrumentation in the drilling assembly 303. Next with acoustic fibre optics sensing it offers simultaneously measurements of the true acoustic amplitude, frequency and phase at every point along the optical fibre. All measurements along the fibre are independent with no crosstalk between simultaneous events at different locations. With the correct fibre and

instrumentation a large dynamic range of 90 dB is possible enabling detection performance to be maintained along the entire length of the fibre optic cable. So as each new joint of drill pipe is added a seismic 304 source can be fired and reflections 305 from different formations can be measured and a drilling model updated to determine where it is necessary to steer the drilling assembly, and when it is necessary to stop drilling and set the next casing string. Similarly, the noise 306 from the drill bit 301. itself can be used to again acquire look ahead of the bit seismic while drilling. By incorporating more than one fibre it is also possible to measure distributed strain and temperature. These would be very helpful for managed pressure drilling as accurate control of wellbore pressure is essential for this process to work.

Finally, other acoustic events can be monitored along the length of the fibre such as but not limited too, gas influx 307, loss circulation 308, cutting bed build up 309.

Referring to figure 27, this is an illustration of cementing a casing that has been installed in the well, and again will be used to explain the benefits of embedding the fibre to the outside of the casing. There is shown an open hole 400, inside the open hole is a casing 401 with a float collar (i.e. a non-return valve) 402 that can measure pressure above the float collar inside the casing and outside the float collar. When the casing reaches total depth, it is normal to circulate drilling mud to condition the well before cementing. If a small volume of fluid 406 is pumped which contains a sound emission means, such as an additive which makes a noise (for example, comprising two chemical components which react to produce an audible signal) or a quantity of sound emitting devices which are small enough to pass through the non-return valve at the lower end of the casing, it would be possible to gauge the open hole and determine the cement volume required to achieve the required top of cement.

Each device could comprise for example a frangible element, e.g. a glass sphere, which is arranged to fracture at a given pressure, the fracture producing a recognisable sound; each sphere may be arranged to fracture at a different pressure, so that certain ones of the spheres fracture at different depths by exposure to the hydrostatic pressure in the well. Alternatively each device could comprise a body (for example, a reactive material within a shell) which reacts chemically in contact with wellbore fluid so as to produce a noise (for example, by expanding the reactive material through an orifice or by fracturing the shell), and having a coating which dissolves slowly when exposed to wellbore fluid. Each device may have a coating of a different thickness, so that the devices react at different times when deployed in the well, producing a continuous sound signal which can be tracked by interrogating the cable.

If cement plugs 403, 404 had a sound source installed, as described in figure 25, it would be possible to track their position on their entire journey to their landing position at the end of the casing. This would ensure the cement is fully displaced into the annulus around the casing, but not risk over displacing.

Once the cementing process was completed, the integrity of the non-return valves could be confirmed; the top of cement determined by the geothermal setting process, and the annulus external to the casing could be monitored for potential migration of fluids (i.e gas channelling) during the cement setting process.

The fibre optic cable may thus be arranged for distributed sensing or data telemetry or both, and may be used to measure local variations in ambient temperature, pressure, flow rate, seismic or acoustic vibrations, or other parameters at different points in a borehole as well known in the art. Depending on the sensing methodology, variations in the inherent optical characteristics of the optical fibre corresponding to local variations in the measured parameter may be sensed continuously along its length. Alternatively, discrete optical

discontinuities may be formed at specific points along the length of the fibre, for example as Bragg gratings. Measurable variations in the optical characteristics of the fibre may result from strain induced locally in the fibre by variations in the measured parameter.

Referring again to Figs. 10A - 10E, the cable may include a metal casing 1280 comprising a single wall, or alternatively a plurality of thin, seam welded concentric metal layers which are swaged to form a thick walled tube with high tensile load carrying capability, as disclosed for example in WO2006059158 and WO2006/097772. The optical fibre 1281 or bundle of optical fibres may be enclosed within the casing 1280 in an elastomeric sheath 1282. The metal casing 1280 may be perforated with a plurality of perforations 146 arranged at intervals along its length so that local variations in pressure or seismic or sonic vibrations are applied to the fibre, e.g. via the sheath, causing local discontinuities in strain in the fibre at each perforation relative to the portion of the fibre between the respective perforations. These local discontinuities are sensed from the surface so as to measure the respective parameter at those points.

In such embodiments a strip of stainless steel may be fed off a drum to a set of rollers which form it into a round tube. While it is still flat, a series of small holes are formed in the stainless steel strip by a laser at pre determined intervals. As the strip is formed into a tube, the encapsulated optical fibre is fed in a relaxed condition into the centre of the tube, followed by a pourable, flexible sealing material which fills the area around the fibre, before the tube is hermetically sealed by laser seam welding the abutting edges of the strip. Each subsequent stainless steel layer added is perforated with a series of holes in corresponding positions to those in the previous strips, so that the corresponding holes in each of the concentric strips are aligned to form continuous passages extending from the outside of the tube to the surface of the settable flexible material, through which the fibre is subjected to direct temperature and strain from the wellbore environment at that point.

Alternatively, the optical fibre may be surrounded by a nonmetal casing, such as an elastomeric or polymeric casing.

The fibre may also be mechanically coupled at intervals along its length to an elastic structure such as a bellows or diaphragm or other element configured to induce strain in the fibre in response to variations in the measured parameter as known in the art.

The fibre can be interrogated (i.e. sensing can be carried out) from the surface using a laser so as to sense local variations in the optical characteristics of the fibre at any desired position along the whole or a selected part of the length of the fibre, which may extend for many kilometres, from which the local value of the sensed parameter may be derived with a positional resolution of as little as lm or even less.

The position of each perforation or other sensing location with respect to the tubing string may be recorded during assembly or deployment or subsequently by reference to another remotely sensed parameter so as to identify the position of each sensing point in the borehole after deployment.

The fibre optic cable may be configured as a distributed sensing system to sense a measurable parameter as a continuous variable at any selected points over the length of the fibre, for example, by means of optical time domain refiectometry or optical frequency domain refiectometry.

The optical fibre may be used for example for distributed temperature sensing at any selected points along the length of the fibre using optical time-domain refiectometry by applying a pulsed laser light source to the fibre, calculating the time of flight of a selected sample of backscattered light from the instant of the initial pulse to correlate the sample with the position along the length of the optical fibre from which it originates, and analysing the spectrum of the backscattered light to obtain a measurement of thermally induced molecular vibrations in the respective portion of the optical fibre corresponding to its local temperature. The position of the reading along the length of the fibre is correlated with its position in the wellbore. The cable may include a series of Bragg gratings as taught for example in

US2008181555, the Bragg gratings being subjected to strain which may be induced mechanically, hydraulically, electrically, or magnetically.

The optical fibre may be a microstructured fibre (photonic-crystal fibre) useable for example for distributed pressure sensing.

The fibre optic cable may be used for data transmission as well as sensing, for example, to carry signals between surface monitoring or control equipment and a downhole sensor or other equipment deployed in the borehole.

Where the cable is arranged in a slot in the tubing, the pressure, temperature or other variable to be measured the slot is applied to the cable via the opening of the slot, which may be left open after the cable is inserted. Alternatively the slot may be filled with an elastomer or polymer or other suitable filler material which may be selected to transmit pressure, vibration, heat or another measurable ambient parameter to the cable where the cable is arranged for distributed sensing.

In embodiments of the invention, the slot extends substantially continuously along the tubing, which is to say that it extends for most or all of the length of the tubing. Of course, the tubing could communicate with another tubing, perhaps of a different diameter, not having a slot.

Where the slot is arranged in a strip attached to the outer surface of the tubing, the strip could be made for example from a metal or an elastomer, and could be applied to rigid tubes to make a jointed tubing string, or alternatively to

continuous or coiled tubing (i.e. to semirigid tubing deployed from a reel).

Where the tubing forms a jointed string, each of the joint regions will typically have an enlarged diameter portion as shown, the enlarged diameter portion having an external diameter greater than that of the tubular portions between the joint regions; and a portion of the slot is then formed in each of the enlarged diameter portions. The enlarged diameter portions may be integral with the respective rigid tubular portions, or may comprise a collar threadedly engaged with the rigid tubular portions. An annular recess may be formed in the ring, the collar, and/or the rigid tubular portions.

In alternative embodiments, the tubing need not be used in a well, but could for example form a pipeline (on the land surface or seabed) and may be used for distributed sensing (e.g. of flow rate, the integrity of the tubing, or the like) along the length of the pipeline.

Many further adaptations may be made within the scope of the claims.

Claims

1. An assembly including a tubing,

the tubing comprising a wall having respectively inner and outer surfaces, the inner surface defining a bore; and

a fibre optic cable;

wherein the tubing includes a slot extending substantially continuously along the tubing,

the slot opening externally of the tubing at an opening through which the cable may be inserted laterally into the slot,

and the cable is arranged in the slot.

2. An assembly according to claim 1, wherein the wall of the tubing is made from metal, and at least a part of the slot is formed in the wall.

3. An assembly according to claim 2, wherein the wall of the tubing has a locally thickened portion in the region of the slot.

4. An assembly according to claim 1, wherein at least a part of the slot is formed in a strip attached to a surface of the tubing.

5. An assembly according to claim 1, wherein the slot has a cross section defining an inner region, and the inner region is wider than the opening.

6. An assembly according to claim 5, wherein the slot has a T shaped cross section.

7. An assembly according to claim 5, wherein the opening is narrower than an outer diameter of the cable, and the cable is elastically deformable so as to pass through the opening during assembly.

8. An assembly according to claim 1, wherein the cable extends sinuously along the slot so as to frictionally engage opposite sides of the slot.

9. An assembly according to claim 8, wherein the cable has an outer diameter less than half of a distance between the opposite sides of the slot.

10. An assembly according to claim 1 , wherein the tubing comprises a plurality of rigid tubes connected together at joint regions, and the slot extends along each of the rigid tubes.

11. An assembly according to claim 10, wherein each of the joint regions has at least one annular recess extending around the tubing and communicating with the slot; and the cable is arranged in the annular recess.

12. An assembly according to claim 1 1, wherein the slot includes portions which are radiused relative to a length of the slot, the radiused portions of the slot communicating with the annular recess.

13. An assembly according to claim 1 1, wherein the annular recess is defined between opposed surfaces of two adjacent tubular portions.

14. An assembly according to claim 11, wherein the annular recess is defined in a collar which is threadedly connected to the respective adjacent tubular portions.

15. An assembly according to claim 1 1 , wherein each joint region includes at least one ring, the ring including a first portion of the slot communicating with the annular recess, the ring being rotatable about the tubing so as to align the first portion of the slot with a further portion of the slot.

16. An assembly according to claim 1, wherein the cable is configured for distributed sensing along at least a portion of the tubing.

17. An assembly according to claim 1, wherein the tubing is suspended within a borehole.

18. An assembly according to claim 1, wherein the tubing is a casing cemented into a borehole.

19. An assembly according to claim 1 , wherein a drilling assembly is arranged at a lower end of the tubing, and an upper end portion of the cable is arranged on a reel, the reel being mounted for rotation together with the tubing.

20. An assembly according to claim 19, wherein the upper end portion of the cable is connected with surface equipment via an optical slip ring, the surface equipment being arranged to remain stationary and to interrogate the cable via the optical slip ring during rotation of the tubing.

21. An assembly according to claim 20, wherein the cable is arranged to transmit data from the drilling assembly to surface.

22. An assembly according to claim 1, wherein a hole is formed in the wall of the tubing, and a portion of the cable extends through the hole into the bore.

23. An assembly according to claim 1, wherein a plug is slidably received in the bore, and a sound source is provided in the plug, and the cable is configured to track the position of the sound source.

24. An assembly according to claim 1, wherein a fluid comprising a sound emission means is provided, and the cable is configured to track the position of the sound emission means.

25. A method of carrying out distributed sensing in a borehole, comprising: connecting together a plurality of rigid tubes at joint regions to form a tubing, the tubing comprising a wall having respectively inner and outer surfaces, the inner surface defining a bore;

providing at least one slot extending along the tubing at least in each of the joint regions and opening externally of the tubing at an opening;

providing a fibre optic cable configured for distributed sensing;

arranging the cable along the tubing and inserting the cable laterally into the or each slot via the respective opening;

deploying the tubing and cable together into the borehole; and then interrogating the cable so as to sense at least one measurable parameter along at least a portion of the tubing.

26. A method according to claim 25, including coating the outer surface of the wall of a first one of the rigid tubes with a coating material so as to embed the cable in the coating material; and then lowering the first one of the rigid tubes; and then connecting a further one of the rigid tubes to the said first one of the rigid tubes, arranging the cable along the further one of the rigid tubes, and coating the outer surface of the further one of the rigid tubes with the coating material so as to embed the cable in the coating material.

27. A method according to claim 25, including arranging a drilling assembly at a lower end of the tubing, and interrogating the cable while rotating the tubing so as to operate the drilling assembly to deepen the borehole.

28. A method according to claim 27, including transmitting data from the drilling assembly to surface via the cable.

29. A method according to claim 25, including providing a sound source;

pumping the sound source down the bore of the tubing; and interrogating the cable to determine the position of the sound source.

30. A method according to claim 29, wherein the sound source is arranged in a cement plug.

31. A method according to claim 29, wherein the sound source is carried by a fluid and pumped down the tubing so as to flow back up an annulus externally of the tubing.

32. An assembly including a tubing,

a fibre optic cable;

wherein the outer surface of the wall of the tubing is coated with a coating material, and the cable is arranged on the outer surface of the wall and embedded in the coating material.

33. An assembly according to claim 32, wherein the tubing comprises a plurality of rigid tubular portions connected together at joint regions; and a respective slot is formed in each of the joint regions, and the cable is arranged in the slot.

34. An assembly according to claim 32, wherein the cable is configured for distributed sensing along at least a portion of the tubing.

35. An assembly according to claim 32, wherein the tubing is a casing cemented into a borehole.

36. An assembly according to claim 32, wherein the tubing is suspended within a borehole.

37. An assembly according to claim 32, wherein a hole is formed in the wall of the tubing, and a portion of the cable extends through the hole into the bore.

38. An assembly according to claim 32, wherein a plug is slidably received in the bore, and a sound source is provided in the plug, and the cable is configured to track the position of the sound source.

39. An assembly according to claim 32, wherein a fluid including a sound source is pumped down the bore, and the cable is configured to track the position of the sound source.

40. An assembly according to claim 32, wherein a drilling assembly is arranged at a lower end of the tubing, and an upper end portion of the cable is arranged on a reel, the reel being mounted for rotation together with the tubing.

41. An assembly according to claim 40, wherein the upper end portion of the cable is connected with surface equipment via an optical slip ring, the surface equipment being arranged to remain stationary and to interrogate the cable via the optical slip ring during rotation of the tubing.

42. An assembly according to claim 40, wherein the cable is arranged to transmit data from the drilling assembly to surface.

43. An assembly substantially as described with reference to the accompanying drawings.

44. A method substantially as described with reference to the accompanying drawings.