WO2016179677A1

WO2016179677A1 - Real-time monitoring of wellbore cleanout using distributed acoustic sensing

Info

Publication number: WO2016179677A1
Application number: PCT/CA2015/050429
Authority: WO
Inventors: Scott Sherman; Wenyin Jeff LI
Original assignee: Trican Well Service Ltd.
Priority date: 2015-05-12
Filing date: 2015-05-12
Publication date: 2016-11-17

Abstract

Described is a method of cleaning a wellbore using a fibre-optic distributed acoustic sensor (DAS) and temporary tubing deployed downhole. In real time, the DAS measures characteristics of the fluid along the entire length of the sensor/wellbore as the fluid is being pumped out of the wellbore, and operators can optionally adjust the wellbore cleanout operation in response to the characteristics determined from the DAS measurements.

Description

REAL-TIME MONITORING OF WELLBORE CLEANOUT USING DISTRIBUTED

ACOUSTIC SENSING

FIELD

[0001] The subject matter disclosed herein relates to an apparatus, system and methods for optimizing cleanout of a wellbore using temporary tubing, which combines real-time monitoring of downhole wellbore conditions during cleanout and adjustment of the cleanout process in response to the results obtained from the monitoring.

BACKGROUND

[0002] Coiled tubing ("CT") interventions are particularly adept at providing access to highly deviated or tortuous wells, where gravity alone fails to provide access to all regions of the well. During a CT intervention, a spool of pipe with a downhole tool at the end is forcibly pushed into the well. This may be achieved by running CT from the reel, on a truck, trailer or large skid mounted on the ground, a barge, ship or platform, through a gooseneck guide arm and injector which are positioned over the well at the oilfield. In this way, forces necessary to drive the CT through the deviated well may be employed, thereby advancing the tool through the well.

[0003] The accumulation of drill cuttings, produced sand, residual proppants, and other debris ("fill") in a wellbore can impair or stop the flow of hydrocarbons from a reservoir. This production-inhibiting problem is commonly dealt with by performing cleanouts with temporary tubing, to remove this fill. US 6,607,607 to Walker et al. is an example of a CT cleanout method. In cleanouts using coiled or jointed tubing, it is common to convey a cleanout tool, such as a jetting tool, via the tubing to the desired location downhole, such as for example the top of the fill. Fluid is driven down the bore of the tubing and through the cleanout tool, where because of the momentum of the fluid, the fill is picked up and transported to the surface via the annulus between the tubing and the production tubing, casing or edge of the wellbore. As fill is depleted from the bottom of the wellbore, the cleanout tool is moved upwards towards the surface, at a rate that in optimal conditions ensures that the fill remains above the tool, and is therefore transported to the surface. [0004] The movement of the cleanout tool into the wellbore is commonly known as running in hole or "R H" and movement upwards in the wellbore is commonly known as pulling out of hole, or "POOH". The speeds at which RIH and POOH occur are important to the cleanout process. The POOH speed must be slower than the linear velocity of the cleanout agents and entrained fill particles moving up the wellbore. If it is too fast fill will be left behind, and if too slow then time and materials are wasted.

[0005] A number of factors influence RIH and POOH speeds, including but not limited to: well configuration (deviated (e.g., horizontal), vertical), completion size, pump rate, equipment parameters, well conditions (e.g., temperature and pressure), the amount and type of fill being lifted, and the cleanout fluids/agents used. Engineers and unit operators generally rely on experience to determine the POOH speed and pump rates, based on perceived well parameters and equipment capabilities. Often several "wiper trips" are conducted, in which only short segments or "bites" of the wellbore are cleaned out at one time, followed by pumping a "bottoms up" of fluid to clear that segment of the wellbore of fill before moving further into the well to clean more fill. Often, pumping is continued until no more fill appears at which time operators may consider the well to be clean, but this can be misleading, as using another pump rate or fluid can produce more fill. Further, if the POOH speed is too fast, solids will be bypassed resulting in only a partial cleanout and the need for remedial treatment. It is desirable to remove all of the fill in one trip.

[0006] Known are computer models that simulate solids transport under various wellbore conditions. The well and equipment parameters are input into the model and ideal pump rates and corresponding POOH speeds that remove substantially all of the fill in one trip will be output from the model. The cleanout procedure is determined prior to mobilizing equipment to the field to perform the cleanout operations. One problem with this approach is that the amount of fill in the well is rarely known at the programming stage. The programmer must assume a fill volume that will be removed, and this assumption is based on experience not actual well conditions.

[0007] The ability to make real-time downhole measurements while pumping cleanout fluids is a valuable asset in controlling the quality and maximizing the effectiveness of cleanout operations, as it enables operators to expediently determine if additional cleanout efforts are needed while the cleanout equipment and personnel are at the well site. Walton et al. (Papers, Society of Petroleum Engineers; 6: 881-892 (SPE 37508) describe the use of real-time measurements for well-site monitoring and evaluation of CT cleanouts. These measurements are performed by a downhole sensor package (DSP) that is run on the end of a CT string, and data is transmitted to the surface via an electric line installed inside the CT.

[0008] US 7,308,941 to Rolovic et al. describes an apparatus and method for measuring solids in a wellbore that includes a bottom hole assembly having a sensor for measuring a characteristic indicative of solids (settled or suspended) in the wellbore. The apparatus is stated to be useful for wellbore cleanouts. The bottomhole assembly (BHA) includes one or more sensors that may be acoustic sensors (sonic or ultrasonic), radiation sensors, electromagnetic sensors and optical sensors. Information from the sensors is transmitted uphole via a communication link such as a wireline, slickline or optic waveguide, or by wireless communication.

[0009] It would be desirable monitor the progress of a cleanout process, in real time, along the entire length of the wellbore. Problem areas could then be identified, not only at the bottom of the well, but at any position along the length of the wellbore. The cleanout process could then be modified to deal with these problem areas thus further ensuring the removal that all of the debris that might interfere with subsequent well service, completion or production operations. For example, if leak off of slurry could be detected, an operator would be able to adjust for this during the cleanout procedure.

[0010] Fibre-optics is a technology that uses glass (or plastic) waveguides to transmit information in the form of pulses of light that travel through the waveguide. A fibre-optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Fibre-optics has several advantages over traditional metal communications lines:

- they have a much greater bandwidth than metal cables, meaning that they can carry more data;

- they are less susceptible than metal cables to interference;

- they are much thinner and lighter than metal wires; data can be transmitted digitally rather than analogically;

- they can be used at higher temperatures;

- the fibre itself can be used as the sensor, and

they are chemically inert.

[0011] It is known to use fibre-optic cables with CT to transmit information during wellbore operations. For example, US 7,617,873 to Lovell et al. describes an apparatus having a fibre- optic tether disposed in CT for communicating information between downhole tools and sensors and surface equipment, and methods of operating this apparatus. The apparatus is purported to be useful for a variety of wellbore operations, such as matrix stimulation, fill cleanout and fracturing.

[0012] US 8,522,869 to Noya et al. describes a fibre-optic based downhole logging assembly that is deliverable via CT. The downhole portion of the assembly is directed to develop a logging profile of a well by way of the fibre-optic line, and this line transmits information from sensors on the downhole logging assembly to an uphole location. The downhole logging assembly may include treatment devices, so that the operator may direct a treatment application utilizing the logging assembly in response to the developing well profile.

[0013] Distributed acoustic sensing (DAS) is a specific type of fibre-optic technology, in which the optical waveguide can be used for sensing and/or telemetry. The optical waveguide itself responds to minute strains induced in the waveguide by external vibration, that is, sound (a pressure wave, also known as vibro-acoustic disturbance). Pulses of light are emitted from a light source at one end of the optical waveguide and this light is reflected at various points back along the optical waveguide to a detector at the light source. When external vibration causes a strain in the optical waveguide, this causes a change in the characteristics of the reflected light (Rayleigh and Brillouin backscattered light) which is measured by the detector. Knowing the speed of light and the moment of return of the reflected signal enables the point of origin along the waveguide to be determined. In this way the fibre-optic cable allows the measurement of the changes in the acoustic field at all positions along the length of the cable. DAS and the principles behind it is well known in the prior art. [0014] It is known to use DAS for pipeline monitoring. See e.g., Williams J. (2012) Distributed Acoustic Sensing for Pipeline Monitoring, Pipeline and Gas Journal Vol 239(7). US 2011/0088462 to Samson et al. describes distributed acoustic, vibration, density and/or strain sensing for downhole monitoring. US 2011/0280103 describes methods and apparatus for performing sonic well logging within a wellbore, based on DAS. US 2014/0036628 to Hill et al. describes using DAS to perform vertical seismic profiling in a wellbore.

SUMMARY

[0015] The instant disclosure provides a method for detecting solids that are contained in fluids along the entire length of a wellbore while a cleanout process is ongoing. The data received from the wellbore provides direct feedback in real time to the operator and the cleanout job parameters are adjusted on-the-fly based on actual downhole conditions and equipment capabilities.

[0016] In one aspect, described herein is a method of cleaning a wellbore comprising a fibre- optic distributed acoustic sensor and temporary tubing deployed downhole, the method comprising: a) pumping a fluid into the wellbore along: i) a bore of the temporary tubing, or ii) an annulus between the temporary tubing and the edge of the wellbore; b) displacing said fluid out of the wellbore: i) in the case of a(i), along the annulus between the temporary tubing and the edge of the wellbore, or ii) in the case of a(ii), along the bore of the temporary tubing; c) entraining solid fill material in the fluid as it is being pumped out of the wellbore; d) acquiring downhole distributed acoustic sensing (DAS) measurements from the fibre- optic distributed acoustic sensor as the fluid is being pumped out of the wellbore; e) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and f) optionally, adjusting the wellbore cleanout operation in response to the characteristic determined from the DAS measurements.

[0017] In one embodiment, the fibre-optic cable is deployed inside the bore of the temporary tubing. In another embodiment the fibre-optic cable is deployed on an outside surface of the temporary tubing. In one embodiment the fibre-optic cable is permanently installed in the wellbore. In one embodiment the wellbore is cased and the fibre-optic cable is deployed between the casing and the formation.

[0018] In one embodiment the temporary tubing is coiled tubing. The fluid may be pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore. In other embodiments the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

[0019] In one embodiment the acquiring of the downhole DAS measurements means detecting Rayleigh, Brillouin and/or Raman backscatter. In one embodiment the fibre-optic distributed acoustic sensor comprises a single mode optical waveguide.

[0020] In one embodiment the characteristic being determined is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

[0021] In one embodiment the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

[0022] In another aspect described herein is a method of cleaning a wellbore comprising: a) deploying a fibre-optic cable distributed acoustic sensor comprising at least one optical waveguide into a temporary tubing or onto the surface of a temporary tubing; b) deploying the temporary tubing and the fibre-optic cable into the wellbore; c) pumping a fluid into the wellbore along: i) the bore of the coiled tubing, or ii) the annulus between the coiled tubing and the edge of the wellbore; d) displacing said fluid out of the wellbore; i) in the case of c(i), along the annulus between the temporary tubing and the edge of the wellbore, or ii) in the case of c(ii), along the bore of the temporary tubing; e) entraining solid fill material in the fluid as it is being pumped out of the wellbore; f) acquiring downhole distributed acoustic sensing (DAS) measurements from the at least one optical waveguide as the fluid is being pumped out of the wellbore; g) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and h) optionally, adjusting the wellbore cleanout operation in response to the characteristic determined from the DAS measurements.

[0023] In one embodiment the fibre-optic cable is deployed inside the bore of the temporary tubing. In one embodiment the temporary tubing is coiled tubing.

[0024] In one embodiment the fluid is pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore. In another embodiment the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

[0025] In one embodiment the acquiring of the downhole DAS measurements means detecting Rayleigh, Brillouin and/or Raman backscatter. In one embodiment the at least one optical waveguide is a single mode optical waveguide. [0026] In one embodiment the characteristic being determined is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

[0027] In one embodiment the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

[0028] In another aspect, described herein is a method of cleaning a wellbore having a fibre- optic cable distributed acoustic sensor with at least one optical waveguide permanently installed in the wellbore comprising: a) pumping a fluid into the wellbore along: i) the bore of the temporary tubing, or ii) the annulus between the temporary tubing and the edge of the wellbore; b) pumping said fluid out of the wellbore: i) in the case of a(i), along the annulus between the temporary tubing and the edge of the wellbore, or ii) in the case of a(ii), along the bore of the temporary tubing; c) entraining solid fill material in the fluid as it is being pumped out of the wellbore; d) acquiring downhole distributed acoustic sensing (DAS) measurements from the at least one optical waveguide as the fluid is being pumped out of the wellbore; e) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and f) optionally, adjusting the wellbore cleanout operation in response to the characteristic determined from the DAS measurements.

[0029] In one embodiment the wellbore is cased and the fibre-optic cable is installed between the casing and the formation. In one embodiment the fluid is pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore.

[0030] In one embodiment the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

[0031] In one embodiment the acquiring of the downhole DAS measurements means detecting Rayleigh, Brillouin and/or Raman backscatter. In one embodiment the at least one optical waveguide is a single mode optical waveguide.

[0032] The method of any one of claims 20 to 24, wherein the characteristic is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

[0033] The method of any one of claims 20 to 25, wherein the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A shows an embodiment of a wellbore cleanout operation performed using the system and methods described herein. Figure IB shows a cross section of an embodiment of a typical fibre-optic cable, used in the system and methods described herein.

[0035] Figure 2 shows another embodiment of a wellbore cleanout operation performed using the system and methods described herein.

[0036] Figure 3A shows another embodiment of a wellbore cleanout operation performed using the system and methods described herein. Figure 3B shows a cross section taken along line B-B of Figure 3 A.

[0037] Figure 4 shows another embodiment of a wellbore cleanout operation performed using the system and methods described herein. [0038] Figure 5 shows a schematic of the basic components of a fibre-optic distributed acoustic sensor.

DETAILED DESCRIPTION

[0039] Described herein is a system and method for monitoring the condition of a wellbore substantially along the entire length of the wellbore, in real-time, during a wellbore cleanout operation. More particularly, fibre-optic distributed acoustic sensing is used to create a dynamic acoustic profile of the fluids and suspended solids disposed along the entire length of the wellbore during the cleanout operation, and this profile is used by operators to determine whether there is a need to adjust the clean-out operation for optimization thereof. The method thus provides a real-time profile of the wellbore during a cleanout operation, allowing operators to assess and adjust the cleanout operation as needed.

[0040] The method may be performed using a temporary tubular, such as coiled tubing (CT), jointed tubing, or both. Embodiments are described herein in the context of a method using CT, however they are applicable as well to jointed tubing, or a combination of CT and jointed tubing.

[0041] Figure 1 A shows a typical procedure used for wellbore cleanout. A deviated wellbore 10 comprises fill 12, which needs to be removed from the wellbore. A fluid (represented by the arrows) is pumped down the bore of coiled tubing 16 and through a washing or jetting nozzle 18 attached to the end of the CT string, then up out of the wellbore via the annulus between the CT and the edge of wellbore 14. Examples of useful jetting nozzles are high pressure jetting nozzles such as the HydroClean™ and Hydrostim™ tools available from Trican, or the SpinCat™ tool available from StoneAge Inc. Upon exiting the nozzle 18, the fluid penetrates the fill material 12, and the fill material becomes entrained in the circulating fluid flow, to be transported with it out of the wellbore via the annulus. If the fill 12 is consolidated, the cleanout procedure may require the use of other tools, such as a downhole motor and mill or tricone bit.

[0042] The wellbore may be an injection well or a production well. The wellbore may have production tubing/lining, casing, or it may be an openhole wellbore. Accordingly, the annulus between the coiled tubing and the edge of the wellbore may comprise production tubing/lining, casing or other type of tubular conduit or it could be open hole. [0043] As used in this specification the term "distributed acoustic sensor" means a sensor comprising an optical waveguide which is interrogated optically to provide a plurality of discrete acoustic sensing portions distributed longitudinally along the waveguide and which can detect mechanical vibration or incident pressure waves.

[0044] The system and method use an optical waveguide cable 22 (referred to herein as a fibre- optic cable), installed along the length of the wellbore 10, and more particularly along the length of the CT 16 that is used to perform the cleanout operation. The fibre-optic cable includes one or more optical waveguides (such as optical fibres, optical ribbons or other types of optical waveguides) that are useful in detecting strain and pressure.

[0045] A cross section of an exemplary fibre-optic cable 22 is shown in Fig. IB. The fibre- optic cable 22 is designed to protect the optical waveguides 24 disposed therein from corrosive wellbore fluids and elevated pressures while allowing for direct mechanical coupling and optical coupling to an interrogator unit, which will be described more fully below. The cable may comprise single mode and/or multi-mode optical waveguides. The exemplary fibre-optic cable 22 shown in Fig. IB has two glass or plastic optical waveguides 24 within a cladding layer 26 that has a lower refractive index than the waveguide core(s). The cladding maintains light within the optical waveguide by reflecting it internally at the boundary between the two. The fibre-optic cable may further comprise a protective buffer layer and/or inert gel 27 which may provide mechanical protection to the optical waveguides by cushioning and protecting them when the cable 22 is bent or spooled. The buffer and cladding layers may further be covered with one or more layers 28 of a plastic coating, a protective material, or other materials that would provide strength as protection (e.g., a metal casing), from the downhole environment, when appropriate. Acoustic signatures of fluids flowing inside the wellbore during the cleanout process attenuate the signal.

[0046] The fibre-optic cable 22 and more specifically the waveguides 24 disposed therein are optically connected to the interrogator at the surface, but are not optically connected to the BHA assembly, sensors, or any other devices downhole. Thus, the fibre-optic cable functions only for DAS and not for transmission of information to or from the BHA assembly. [0047] The fibre-optic cable 22 may also include at least one electrical conduit 30 for transmitting information between BHA tools and the surface. The electrical conduit 30 may be used to send control signals to the jetting nozzle 18, to send signals to activate a downhole motor and bit or an impact drill, and to otherwise send signals to implement changes in the cleanout process that might be occasioned by the information received from the DAS. Further, downhole sensors connected to the BHA, such as mechanical sensors, radiation sensors, densitometers and optical sensors, may transmit information to the surface using the electrical conduit. If not disposed in the same cable as the optical waveguide, the electrical conduit 30 may be in a cable that is separate from the fibre-optic cable, which has the advantage that the two cables, the fibre- optic cable and the electrical cable, may be separately introduced to the wellbore - e.g., one inside the CT and one outside.

[0048] The fibre-optic cable 22 can be many kilometres in length and can be at least as long as the temporary tubing used in the cleanout operation. The optical waveguide 24 in the fibre-optic cable may be a standard, unmodified single mode optical waveguide without the need for deliberately introduced reflection sites such as Fibre Bragg Gratings or the like. Or it may be a multimode optical waveguide. Fibre-optic cables which are suitable for use in the methods described herein are commercially available, for example from American Fibertek.

[0049] The fibre-optic cable 22 may be disposed inside the temporary tubing, as shown in Fig. 1, using methods that are well known in the art. For example, CT is made by forming a flat steel strip into a round tube and then fusing the edges by welding. The fibre-optic cable may be disposed along what will be the inside of the CT, before the edges are sealed together. It may be, but need not be, attached to the inside surface of the CT. Methods using vibration and/or fluid flow may be used to insert the cable into jointed tubing or into CT that has already been spooled onto a reel - see for example US2014/0151030.

[0050] The fibre-optic cable 22 may be attached on the outside surface of the temporary tubing using straps 32 (see Fig. 2) or other mounting mechanisms, as described for example in US 2014018592 for the mounting of fibre-optic cable to a liner. The insertion of the temporary tubing can be paused to allow workers to attach the cable to the temporary tubing with straps. Cable protectors may optionally be employed to protect the cable from getting pinched, for example between the temporary tubing and the edge of wellbore. Alternatively, CT can be manufactured with the fibre-optic cable 22 disposed thereon. Figure 3B shows the mounting of the fibre-optic cable 22 onto the outside surface of a CT string, for example in a longitudinal or helical groove 34 that is machined or impressed into the outside surface of the CT string 16 so as to encapsulate the cable 22 at least flush with or slightly below the external surface of the CT. The cable 22 can be bonded into the groove using a special epoxy such as that available from US resins.

[0051] Alternately, the fibre-optic cable 22 may be delivered downhole in a conduit/string that is distinct from the injection conduit/string, for example in a carbon fibre rod (see WO

2011043768). This combination could be used with either CT or jointed tubing.

[0052] The removal of sand or fill from a wellbore has several names, including sand washing, sand jetting, sand cleanout, and fill removal. The objective of this process is to remove an accumulation of fill in the wellbore, as these materials impede fluid flow and reduce well productivity.

[0053] The typical procedure used for wellbore cleanout is shown in Fig. 1 A. As described above, a fluid (represented by the arrows) is pumped down the bore of CT and through a high pressure jetting nozzle 18, then up out of the wellbore via the annulus between the CT and the edge of wellbore or between the CT and another tubing. Upon exiting the jetting nozzle, the fluid penetrates the fill material, and the fill material becomes entrained in the circulating fluid flow, to be transported with the fluid out of the wellbore via the annulus.

[0054] An alternative approach to wellbore cleanout, known as reverse cleanout, is shown in Figs. 2 and 3. In this procedure, a fluid (represented by the arrows) is pumped down the annulus between the edge of the wellbore 14 and CT 16 and returns to the surface via the bore of the CT string. This procedure can be very useful for removing large quantities of particulate, such as proppants, from the wellbore. It may also be applied when a particular wellbore configuration precludes annular velocities sufficient to lift the fill material. Other less-used methods of wellbore cleanout are also known, and are intended to be included herein. [0055] The method described herein is distinguishable from prior art methods - for example US 7,308,941 - that use a sensor that is connected to a BHA to measure wellbore properties as the BHA is moved axially down into or up out of the wellbore. In the prior art methods each measurement is taken at a different time point, and it is not possible to obtain the profile of the entire wellbore along its axial length, at a single point in time. The instant method allows an axial profile of the wellbore, at a single point in time, to be obtained.

[0056] The use of DAS in wellbores has been proposed for monitoring various steps in well formation and operation. However, the skilled person would not have, prior to the present disclosure, considered DAS to be suitable for use in real-time monitoring of well cleanout operations. This is because a significant amount of noise is generated during wellbore cleanouts, from the fluid that is forcibly pumped into the wellbore, jetted into the fill, and carried out of the wellbore. A person of skill in the art would have expected that the signal to noise ratio would be too high to provide DAS measurements that enable an assessment of the type and quality of the fluid that is flowing out of the wellbore.

[0057] When considering a method in which the fibre-optic cable 22 is separated, by the wall of a temporary tubing, from the fluid for which DAS measurements are being obtained, a person of skill would likely have even further reservations about the method disclosed herein. The cable 22 may be inside the temporary tubing 16 whereas the fluid flowing out of the wellbore is in the annulus between the temporary tubing and the edge of the wellbore (see e.g. Fig. 1 A); or the cable may be outside the temporary tubing and the fluid flowing out of the wellbore is in the bore of the temporary tubing (see e.g. Fig. 2). For the optical waveguide to detect an acoustic signal from the fluid flowing out of the wellbore, there must be a strain on the waveguide itself. This requires the acoustic wave in the fluid being measured to couple to the optical waveguide, which is disposed on the other side of the tubing wall from the fluid for which DAS measurements are being obtained. There is therefore a significant opportunity for loss of the acoustic signal in this arrangement by the damping of the signal by the wall of the temporary tubing.

[0058] In a preferred arrangement of the method described herein the fibre-optic cable 22 is separated from the fluid that is flowing out of the wellbore (that is being assessed by DAS) by the temporary tubing wall. In this arrangement the cable is protected from the debris that is entrained in the fluid that is flowing out of the wellbore, as this debris may damage the optical waveguide in the fibre-optic cable.

[0059] In another preferred arrangement of the method described herein the fibre-optic cable 22 is permanently installed in the wellbore, for example in the cement surrounding the casing (e.g., Fig. 4) and thus again is separated from the fluid that is flowing out of the wellbore (that is being assessed by DAS). Thus, the cable is protected from the debris that is entrained in the fluid that is flowing out of the wellbore, as this debris may damage the optical waveguide in the fibre- optic cable.

[0060] It is also preferred that the fluid being pumped into the wellbore, either down the bore of the CT, or via the annulus between the CT and the edge of the wellbore, be in laminar flow, to avoid turbulence and the associated noise generated by that turbulence. To this end, a friction reducer such as a gel or surfactant may be added to the fluid that is pumped into the wellbore. Fluid flowing out of the wellbore, which contains the entrained fill, should be turbulent, to maintain entrainment of the fill.

[0061] Figure 5 shows a schematic of a distributed fibre-optic sensing arrangement used herein. A length of fibre-optic cable 22 is optically connected at one end to an interrogating unit 36. The output from the interrogating unit 36 is passed to a processing unit 38, which may be co- located with the interrogating unit or may be remote therefrom, and an operator

interface/graphical display unit 40, which in practice may be an appropriately specified PC. The operator interface 40 may be co-located with the processor 38 or may be remote therefrom. A controlling unit 42 controls the interrogator. Interrogators useful herein include the OptaSense® Interrogator Unit available from QinetiQ Group Pic, or the Helios™ DAS system available from Fotech Solutions Ltd.

[0062] In operation, the interrogating unit 36 launches interrogating electromagnetic radiation, which may for example comprise a series of optical pulses having a selected frequency pattern, into the optical waveguide. The optical pulses may be in the visible spectrum or they may be infrared radiation and ultraviolet radiation. The properties of returning light ("backscatter") which comprises absorption and retransmission of light energy, is recorded. The phenomenon of backscattering results in some fraction of the light input into the waveguide being reflected back to the interrogating unit where it is detected. An altered backscatter indicates acoustic disturbances in the vicinity of the fibre.

[0063] The backscattered light includes different spectral components, e.g., Rayleigh,

Brillouin, and Raman bands. Brillouin or Rayleigh backscatter sensing may be used for DAS monitoring, with preferably Brillouin backscatter gain or coherent Rayleigh backscatter being sensed. Coherent Rayleigh backscatter is preferably used to monitor dynamic strain (e.g., acoustic pressure and vibration) and Brillouin backscatter detection is preferably used to monitor static strain.

[0064] The interrogating unit therefore conveniently comprises at least one laser 44 and at least one optical modulator 46 for producing a plurality of optical pulses separated by a known optical frequency difference. The interrogating unit 36 also comprises at least one photodetector 48 arranged to detect radiation which is backscattered from intrinsic sites within the optical waveguide. The at least one photodetector 48 is able to detect Rayleigh and/or Brillouin backscattered light, and in some embodiments also Raman backscattered light.

[0065] The signal from the photodetector 48 is processed by the processing unit 38, which demodulates the returned signals based on the frequency difference between the optical pulses. The phase of the backscattered light from various sections of the optical waveguide can therefore be monitored. Any changes in the effective path length from a given section of waveguide, such as would be due to incident pressure waves causing strain on the waveguide, can therefore be detected. Thus, by analyzing reflections from within the optical waveguide and measuring the time between the optical signal being launched and the signal being received, the fibre-optic sensing arrangement is able to measure the effect of the acoustic reflections on the optical signal at all points along the waveguide, limited only by the spatial resolution.

[0066] The processing unit 38 is run by software that collects and processes the data from the photodetector 48 and sends the processed results to a display unit 40 in real time. The display unit provides these results to the operator to enable the operator to determine whether adjustment to the cleanout process is needed, and may prompt the operator via an audible or visual alert to speed up or slow down the RIH or POOH speed, increase pumping rate, or otherwise alter the cleanout process. [0067] During a cleanout operation careful attention must be paid in particular to the POOH speed. While it is important to POOH as quickly as possible, to optimize the cleanout it is also important to ensure that all particulate solids are maintained uphole of the end of the temporary tubing assembly. The POOH speed should not exceed the speed at which this can be maintained. The system described herein enables the operator to monitor the DAS measurements in real time which enables operators to quickly recognize changing or unforeseen conditions in the well, and the method parameters can be altered in accordance with these conditions, ensuring continued safe and efficient operations.

[0068] Certain patterns in the DAS measurements of the fluid flowing out of the wellbore are indicative of certain material properties in the fluid. In the methods provided herein DAS measures a characteristic that informs the operator about the level of solids entrainment in the fluid that is flowing out of the wellbore. For example, if the fluid contains particulate matter (such as sand, proppants, fines) greater vibration of the cable will be produced as the fluid flows along the wellbore, as compared to when a clear fluid surrounds the cable. Suspended particles may rub on each other, on the wellbore and on the coiled tubing, to generate frictional noise. Different fluid compositions may have different levels of turbulent flow because of their solids content. Different fluid compositions may attenuate, or fail to attenuate, acoustic energy propagating from external or ambient sources differently because of their solids content. Or, different fluid compositions may have different densities and/or viscosities, to alter a resonance frequency of a surface or vibrating element because of their solids content. Or, elements and particles suspended in the fluid may actively generate acoustic energy via the imposition of elevated temperatures, pressures or other downhole conditions. A DAS may also be used to infer temperature at a point along the length of the fiber-optic cable, provided that there is a reference for measuring the temperature, which enables the inference to be made.

[0069] In addition, the leakoff of fluids into uphole perforations of flowports could also be detected by DAS, as the velocity of the fluid in the annulus will decreased above this point.

[0070] Fluids contemplated for use herein may be one phase (liquid or gas), two phase (liquid/gas; liquid/solid or gas/solid) or three phase (liquid/gas/solid). Each of these fluids has a different acoustic signature. Potentially useful fluids are disclosed in US 1,123,21 '4 and US 8, 105,986.

[0071] In one embodiment the fibre-optic cable 22 is installed down the wellbore inside the temporary tubing, as shown in Fig. 1 A. In another embodiment the fibre-optic cable 22 is installed down the wellbore on the outside surface of the temporary tubing, as shown in Fig. 2 or Fig. 3 A. In some embodiments the fibre-optic cable 22 may be installed down the wellbore in a conduit/string that is distinct from the injection conduit/string, for example in a carbon fibre rod (see WO 2011043768). This distinct conduit/string could be installed inside or outside of the temporary tubing.

[0072] Typically producing or injection wells are formed by drilling a borehole and then forcing sections of metallic casing down the borehole. However, in some wellbores casing is not used in the producing section of the well ("openhole"). In some embodiments the fibre-optic cable is permanently installed in the wellbore, that is, it is not intended to be removed from the wellbore after installation. Figure 4 shows one embodiment of the method in which the fibre- optic cable 22 is installed between the casing 20 and the formation 50, likely in a layer of cement. The fibre-optic cable may also be installed within or on the outside of another tubular that is inside the wellbore.

[0073] During a cleanout operation, coiled tubing is inserted down the borehole. As shown in FIG. 1 A, 2 or 3 the fibre-optic cable may be disposed inside the bore of the temporary tubing or on the outside surface of the temporary tubing. When on the inside of the tubing, the fibre-optic cable may be deployed in a linear path along the entire length of the coiled tubing and protected therein. Or, the cable can be deployed on the outer surface of the temporary tubing. The fiberoptic cable protrudes from the well head and is connected to interrogator, which may operate as described above.

[0074] As noted above, a significant amount of noise is generated during wellbore cleanouts. It is important to distinguish background noise from noise generated by the fluids and/or solids moving out of the wellbore. The method described herein contemplates at least two means of accomplishing this that may be used alone or in combination. In the first embodiment, in which the fibre-optic cable is disposed inside or outside of the temporary tubing, cleanout fluid may be introduced into the tubing above the wellhead and the acoustic signature of this fluid is measured. The acoustic signature of the fluid above the wellhead is then subtracted (as background) from the DAS data of the fluids in the wellbore below the wellhead. The difference represents the acoustic signature of the fluids and/or solids moving out of the wellbore.

[0075] In the second embodiment, which may be used when the fiber-optic cable is temporarily or permanently installed in the wellbore, a frequency spectrum in the 200 to 3,000 Hz range is used, as this spectrum represents sand/particulate noise. In this higher frequency spectrum, there is no need to remove background noise, as there is less background noise at this frequency.

[0076] While the method has been described in conjunction with the disclosed embodiments which are set forth in detail, it should be understood that this is by illustration only and the method is not intended to be limited to these embodiments. On the contrary, this disclosure is intended to cover alternatives, modifications, and equivalents which will become apparent to those skilled in the art in view of this disclosure.

Claims

1. A method of cleaning a wellbore comprising a fibre-optic distributed acoustic sensor and temporary tubing deployed downhole, the method comprising: a) pumping a fluid into the wellbore along: i) a bore of the temporary tubing, or ii) an annulus between the temporary tubing and the edge of the wellbore; b) displacing said fluid out of the wellbore: i) in the case of a(i), along the annulus between the temporary tubing and the edge of the wellbore, or ii) in the case of a(ii), along the bore of the temporary tubing; c) entraining solid fill material in the fluid as it is being pumped out of the wellbore; d) acquiring downhole distributed acoustic sensing (DAS) measurements from the fibre-optic distributed acoustic sensor as the fluid is being pumped out of the wellbore; e) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and f) optionally, adjusting the wellbore cleanout operation in response to the

characteristic determined from the DAS measurements.

2. The method of claim 1, wherein the fibre-optic cable is deployed inside the bore of the temporary tubing.

3. The method of claim 1, wherein the fibre-optic cable is deployed on an outside surface of the temporary tubing.

4. The method of claim 1, wherein the fibre-optic cable is permanently installed in the wellbore.

5. The method of claim 4, wherein the wellbore is cased and the fibre-optic cable is deployed between the casing and the formation.

6. The method of any one of claims 1 to 5, wherein the temporary tubing is coiled tubing.

7. The method of any one of claims 1 to 6, wherein the fluid is pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore.

8. The method of any one of claims 1 to 6, wherein the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

9. The method of any one of claims 1 to 8, wherein the acquiring of the downhole DAS measurements means detecting Rayleigh backscatter.

10. The method of any one of claims 1 to 9, wherein the acquiring of the downhole DAS measurements means detecting Brillouin backscatter.

11. The method of any one of claims 1 to 10, wherein the acquiring of the downhole DAS measurements means detecting Raman backscatter.

12. The method of any one of claims 1 to 11 wherein the fibre-optic distributed acoustic sensor comprises a single mode optical waveguide.

13. The method of any one of claims 1 to 12, wherein the characteristic is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

14. The method of any one of claims 1 to 13, wherein the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

15. A method of cleaning a wellbore comprising: a) deploying a fibre-optic cable distributed acoustic sensor comprising at least one optical waveguide into a temporary tubing or onto the surface of a temporary tubing; b) deploying the temporary tubing and the fibre-optic cable into the wellbore; c) pumping a fluid into the wellbore along: i) the bore of the coiled tubing, or ii) the annulus between the coiled tubing and the edge of the wellbore; d) displacing said fluid out of the wellbore: iii) in the case of c(i), along the annulus between the temporary tubing and the edge of the wellbore, or iv) in the case of c(ii), along the bore of the temporary tubing; e) entraining solid fill material in the fluid as it is being pumped out of the wellbore; f) acquiring downhole distributed acoustic sensing (DAS) measurements from the at least one optical waveguide as the fluid is being pumped out of the wellbore; g) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and h) optionally, adjusting the wellbore cleanout operation in response to the

characteristic determined from the DAS measurements.

16. The method of claim 15, wherein the fibre-optic cable is deployed inside the bore of the temporary tubing.

17. The method of claim 15 or 16, wherein the temporary tubing is coiled tubing.

18. The method of any one of claims 15 to 17, wherein the fluid is pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore.

19. The method of any one of claims 15 to 17, wherein the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

20. The method of any one of claims 15 to 19, wherein the acquiring of the downhole DAS measurements means detecting Rayleigh backscatter.

21. The method of any one of claims 15 to 19, wherein the acquiring of the downhole DAS measurements means detecting Brillouin backscatter.

22. The method of any one of claims 15 to 19, wherein the acquiring of the downhole DAS measurements means detecting Raman backscatter.

23. The method of any one of claims 15 to 22 wherein the at least one optical waveguide is a single mode optical waveguide.

24. The method of any one of claims 15 to 23, wherein the characteristic is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

25. The method of any one of claims 15 to 24, wherein the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

26. A method of cleaning a wellbore having a fibre-optic distributed acoustic sensor comprising at least one optical waveguide permanently installed in the wellbore comprising: a) pumping a fluid into the wellbore along: i) the bore of the temporary tubing, or ii) the annulus between the temporary tubing and the edge of the wellbore; b) pumping said fluid out of the wellbore: i) in the case of a(i), along the annulus between the temporary tubing and the edge of the wellbore, or ii) in the case of a(ii), along the bore of the temporary tubing; c) entraining solid fill material in the fluid as it is being pumped out of the wellbore; d) acquiring downhole distributed acoustic sensing (DAS) measurements from the at least one optical waveguide as the fluid is being pumped out of the wellbore; e) processing the DAS measurements to determine a characteristic, along the length of the sensor, of the fluid being pumped out of the wellbore; and f) optionally, adjusting the wellbore cleanout operation in response to the

characteristic determined from the DAS measurements.

27. The method of claim 26, wherein the wellbore is cased and the fibre-optic cable is installed between the casing and the formation.

28. The method of claim 26 or 27, wherein the fluid is pumped into the wellbore along the bore of the temporary tubing and out of the wellbore along the annulus between the temporary tubing and the edge of the wellbore.

29. The method of any one of claims 26 to 28, wherein the fluid is pumped into the wellbore along the annulus between the temporary tubing and the edge of the wellbore and out of the wellbore along the bore of the temporary tubing.

30. The method of any one of claims 26 to 29, wherein the acquiring of the downhole DAS measurements means detecting Rayleigh backscatter.

31. The method of any one of claims 26 to 30, wherein the acquiring of the downhole DAS measurements means detecting Brillouin backscatter.

32. The method of any one of claims 26 to 31, wherein the acquiring of the downhole DAS measurements means detecting Raman backscatter.

33. The method of any one of claims 26 to 32, wherein the characteristic is selected from the group consisting of: density, turbulence, viscosity, amount of suspended particulate matter, attenuation of acoustic energy, generation of acoustic energy, and resonance frequency.

34. The method of any one of claims 26 to 33, wherein the adjusting of the wellbore cleanout operation means adjusting one or more of the RIH speed, the POOH speed, the pump rate or the fluid composition.

35. The method of any one of claims 26 to 34 wherein the at least one optical waveguide is a single mode optical waveguide.