WO2011076847A1

WO2011076847A1 - Drilling a borehole and hybrid drill string

Info

Publication number: WO2011076847A1
Application number: PCT/EP2010/070492
Authority: WO
Inventors: Jan-Jette BLANGÉ
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2009-12-23
Filing date: 2010-12-22
Publication date: 2011-06-30
Also published as: BR112012015442A2; AU2010334863A1; EP2516789A1; US20120261194A1; CN102667048B; CA2785141A1; AU2010334863B2; CN102667048A

Abstract

Method of drilling a borehole into an object, the method comprising providing a first borehole section (5) extending into the object from its surface; running a first drill string part (3a) including a fluid jet drill head (10) in the first borehole section, wherein the first drill string part comprises a length of flexible tubing; generating fluid jet so as to blast with an erosive power on an impingement area of the borehole, thereby deepening the borehole to provide a second borehole section (5a), and assembling a plurality of jointed pipe elements (11) forming a second drill string part (11a) to the top of the length of flexible tubing in the course of providing the second borehole section; and hybrid drill string (16), comprising a length of flexible (3) tubing as well as a plurality of jointed pipe elements (11), and further comprising a bottomhole assembly comprising a fluid jet drill head (10).

Description

DRILLING A BOREHOLE AND HYBRID DRILL STRING

The invention is related to a method of drilling a borehole into an object, and to a hybrid drill string. The object can in particular be a subsurface earth formation .

A particular application is for drilling a borehole with a relatively small diameter at least in a lower section into a subsurface earth formation, for example, a borehole with a diameter of between 3 and 20 cm, e.g. 5-

12 cm, such as 7,4 cm (2,9 inch) or 10.5 cm (4,125 inch) .

Conventionally, a borehole can be drilled by a mechanical drilling method with a conventional mechanical cutting drill head, wherein a drill string formed of jointed pipe is used. For small diameter boreholes also so-called flexible tubing can be used, in particular coiled tubing that can be unrolled from a reel, also referred to as coiled tubing. Drilling with the help of a flexible tubing has the advantage that the drilling operation has to be interrupted less frequently in comparison with drilling operations wherein pipes are used, which have to be coupled to each other

subsequently. When connected to a reel, such tubing cannot be rotated, and typically a downhole drilling motor is required, driven by drilling fluid ("mud motor") in the bottom hole assembly. Directional control is often desired to provide a borehole along a desired trajectory. For small-diameter flexible tubing such directional control is conventionally obtained by means of a mud motor in combination with and a bent sub. Such mechanical drilling method for obtaining small diameter holes has however several disadvantages. One of these disadvantages is that, consequently, the mud motor has relatively small dimensions as well, resulting in a low efficiency. During directional drilling operation the drill string part above the mud motor is not rotated, which is not optimal for hole cleaning and penetration. Moreover, small diameter drill strings are often not sufficiently robust and are frequently damaged. As a result of the low mass of the drill string, furthermore weight on bit is low which gives rise to a poor directional control rate and reduced rate of penetration. For transmission of torque, the diameter of the drill string is of key importance. Consequently, in the rotary drilling method using a combination of jointed drill pipe and coiled tubing, that is known from EP 2 108 780, it is seen as an advantage of coiled tubing that a larger diameter coiled tubing can be used than jointed pipe. This is due to the lack of tool joints in coiled tubing which are wider than the actual drill pipe.

Alternatively to a conventional mechanical cutting drill bit a fluid jet drill head can employed, which directs a fluid jet with erosive power into impingement with the borehole wall. Preferably a fluid mixture including a quantity of abrasive particles is employed. Such jet drilling is particularly well suited for making boreholes with a small diameter. In contrast to

mechanical cutting drilling methods, no or minimum weight on bit is necessary for drilling.

A jet drill system and method of making a hole in an object is for example disclosed in WO-A-2005/005767. The known system comprises an excavating tool, herein also referred to as abrasive jet drill head, mounted on a lower end of a drill string that is inserted from the surface into a hole in a subterranean earth formation. The drill string is provided with a longitudinal passage for transporting a drilling fluid mixture comprising abrasive particles to the drill head. The drill head comprises jet means arranged to generate an abrasive jet in a jetting direction into impingement with the earth formation in an impingement area. The abrasive jet contains magnetic abrasive particles (steel shot) . A recirculation system is provided, which captures abrasive particles from the return stream to surface, after erosive impingement, by means of a magnet, and re-mixes the abrasive particles at a mixing location with the mixture received via the drill string. The magnet is arranged as a rotatable conveyor, attracting particles to be recycled and conveying them towards a mixing location with fresh fluid from surface. In the known system directional drilling is achieved by a modulation means in form of a controllable drive means for the conveyor, which is arranged so as to modulate the recirculation rate, and in this way the quantity of particles in the abrasive jet at the jet means is modulated. When the abrasive jet is moved along a trajectory in the hole, in particular in a rotating motion, the amount of erosion in each impingement area along the trajectory can be

selectively varied, and directional control is achieved. Reference is also made in this regard to other known abrasive jet drill systems and methods as described for example in WO 00/66872, WO 2002/034653, WO 2005/005766, WO 2008/119821, WO 2008/113843, WO 2008/113844.

For optimal results in jet drilling, the drill string outer diameter in abrasive jet drilling should have an outer diameter which is preferably in the range of 0,65 to 0,70 times the borehole diameter. In the case of drilling a 7,4 cm (2,9 inch) diameter hole, this would lead to a drill string of about 5 cm (2 inch) . However, jointed drill pipe of such dimensions has relatively large diameter connections. E.g., joints of 2 3/8 inch drill pipe (6 cm) can have a joint diameter of about 3,5 inch (8,9 cm) . Although this could be circumvented by applying jointed tubing pipe, which has smaller diameter connections, tubing pipe however suffers from frequent damage. In particular, tubing pipe connections are more prone to damages as a result of frequent making and breaking of said connections.

The second borehole section can in particular be a borehole section with a relatively small diameter, for example, with a diameter of between 3 and 20 cm, in particular 5-16 cm, e.g. 5-12 cm, such as 7,4 cm (2,9 inch), 10,5 cm (4,125 inch), or 15,2 cm (6 inch), which diameters correspond to respective sizes of fluid jet drill heads.

The use of flexible tubing, in particular coiled tubing, in jet drilling can be considered, however has the disadvantage that deformation and bending due

unrolling from a reel into the borehole at high internal fluid pressures may lead to a reduced lifetime. This is aggravated by the fact that the pressures which are applied in the process of jet drilling (100 bar or higher, in particular 200 bar or higher, such as 300 bar or higher) are appreciably higher than in conventional drilling rotating cutting drill bits which are driven by a mud motor (typically well below 100 bar) . A further disadvantage is that coiled tubing is used for non- rotating drill strings, in combination with a mud motor in such a way that the drill bit is rotating while the drill string is not rotating. For jet drilling, in particular abrasive jet drilling, a rotating drill string is preferred. The use of mud motors is even less

desirable than in mechanical cutting drilling, since the high pressures make it cumbersome if not practically impossible to operate mud motors and required rotating seal. Moreover string rotation is desirable in that it stimulates borehole cleaning.

There is a need for an improved method for drilling a borehole by means of jet drilling, which is faster and more economic, in particular for drilling relatively small diameter boreholes in a subsurface formation.

In accordance with the invention there is provided a method of drilling a borehole into an object, the method comprising

- providing a first borehole section extending into the object from its surface;

- running a first drill string part including a fluid jet drill head in the first borehole section, wherein the first drill string part comprises a length of flexible tubing;

- generating fluid jet so as to blast with an erosive power on an impingement area of the borehole, thereby deepening the borehole to provide a second borehole section, and

- assembling a plurality of jointed pipe elements forming a second drill string part to the top of the length of flexible tubing in the course of providing the second borehole section.

The method of the invention allows to profit from advantageous features of flexible tubing in abrasive jetting drilling operations while avoiding drawbacks. From within the first borehole section that can be conventionally drilled, a second borehole section is provided that can be drilled fast and economically using a fluid jet drill head together with a length of flexible tubing, which is extended by jointed pipe elements on top in the course of deepening the second borehole section. The second borehole section can be limited to just the diameter required since no joints have to be run into this section. In a preferred embodiment the fluid jet is an abrasive fluid jet and the jet drill head is an abrasive jet drill head. An abrasive fluid jet is a jet of a fluid mixture comprising a concentration of abrasive particles, e.g. steel shot in an drilling liquid, in particular aqueous drilling liquid such as water. With the method of the invention the entire drill string can be rotated, which is advantageous as pointed out above. The use of a length of flexible tubing avoids making the handling of the connections that are required for

conventional jointed drill pipe or tubing, elements typically not being longer than 15 m. The length of flexible tubing can comprise a single piece of tubing of 30 m length or more, preferably 50 m or more, and can be 100 m or more, 200 m or more, or even 500 m or more.

The length of flexible tubing can be unrolled from a reel, allowing efficient transport to the rig site. The length of flexible tubing can be connected to an

auxiliary flexible member, e.g. comprising an auxiliary part of flexible tubing or a cable, when the first drill string part is run into the first borehole section. In this way, an assembly the flexible tubing and the

auxiliary flexible member can be rolled on a reel, and unrolled again, so the flexible tubing can easily be used and re-used for similar drilling operations. Suitably, a coupling is provided at the upper end of the length of flexible tubing. This coupling can be used for coupling to the auxiliary flexible member and/or for coupling the jointed pipe (second drill string part) to the flexible tubing (first drill string part) . The coupling can comprise a cross-over to jointed pipe.

Preferably flexible tubing guidance equipment is provided at surface, and the coupling and the flexible tubing guidance equipment are dimensioned such that the coupling can pass through the flexible tubing guidance equipment .

In one embodiment the method can further comprise -raising the assembled first and second drill string parts in the borehole,

-disassembling the jointed pipe elements from the length of flexible tubing,

-coiling the length of flexible tubing onto the reel. The invention also provides a hybrid drill string, comprising a length of flexible tubing as well as a plurality of jointed pipe elements, and further comprises a bottomhole assembly comprising a jet drill head. The jointed pipe elements are preferably connected to a first end of the length of flexible tubing.

Preferably also, bottomhole assembly with jet drill head is provided at a second end of the length of flexible tubing. The length of flexible tubing can comprise a single piece of tubing of 30 m length or more, 50 m or more, 100m or more, 200m or more, 500 m or more.

Flexible tubing used in the present invention is preferably flexible enough to follow the drilled

trajectory and stiff enough to be pushed through the bore hole while being rotated. Furthermore, it preferably should be of such torsional strength that the required torque for drill string and drill head rotation can be transmitted .

The invention moreover provides a method for drilling a hole in a formation by means of an abrasive jet of a mixture of drilling fluid and abrasive particles, comprising the steps of:

-providing a length of flexible tubing coiled on a reel ,

-obtaining a free end of the tubing, -connecting an abrasive jet drill head to the free end of the tubing,

-running said length of tubing in a borehole while unrolling the tubing from the reel,

-assembling a length of pipe, such as a steel pipe, to the top of the length of tubing run in the borehole,

-rotating the assembly comprising the length of pipe, the length of tubing and the drill head,

-providing a flow of a mixture of drilling fluid and abrasive particles through the assembly,

-eroding the borehole bottom by means of a jet of a mixture of drilling fluid and abrasive particles

discharged from the rotating drill head.

The assembly consisting of the flexible tubing, the drill pipe(s) and the drill head can be rotated as a unity. The length of tube can be applied without making up or breaking of a high number of connections, as would have been the case in a traditional drill string which completely consists of drill pipes, thus speeding up the process of drilling. Flexible tubing is well fit for handling the relatively low torque which is exerted on the drill string for rotating the drill head.

Furthermore, high pressures can be applied in the

flexible tubing.

Preferably, the method according to the invention comprises the steps of:

-providing a reel with a length of an auxiliary flexible member, such as an auxiliary part of tubing or a cable, coiled on said reel as well as a length of

flexible tubing connected to said auxiliary flexible member and coiled on said flexible member,

-unrolling the length of tubing as well as part of the auxiliary flexible member while running the length of tubing into the borehole, -disconnecting the length of tubing from the

auxiliary flexible member.

The auxiliary flexible member is permanently

connected to the reel and simplifies the process of reeling and unreeling the tubing. This process can be further improved by providing a coupling between the length of tubing and the auxiliary flexible element.

Furthermore, the method according to the invention comprises the steps:

-raising the assembly in the borehole,

-disconnecting the length (s) of pipe from the tubing,

-coiling the length of tubing onto the reel.

The invention is also related to a hybrid drill string, comprising a length of flexible tubing as well as at least one length of pipe connected to the upper end of the length of tubing.

Also, the invention is related to an installation for carrying out the method as described before, comprising a drilling rig provided with guiding means for guiding the length of flexible tubing, fixing means for fixing the assembly consisting of length of tubing, the at least one length of pipe and the drill head as well as lifting means for lifting said assembly.

The invention will in the following be described further by way of examples and with reference to the drawings, wherein Figures 1-3 show the method and hybrid drill string according to the invention in various stages of operation. Like reference numerals are used in the drawings to refer to the same or similar objects.

In a first step of the method of the present

invention a first borehole section is provided extending into the object from its surface. This object is here a subterranean earth formation, in particular to provide a borehole for the manufacture of a well for production of mineral hydrocarbons. Such a first borehole extends downwardly from the earth's surface, and can be for example provided by conventional drilling methods, in particular mechanical drilling e.g. using roller-cone or PDC bits.

The expressions upper, above, upstream, uphole, lower, below, downstream, downhole, and the like, are used herein with reference to a drill string with jet drill head in a borehole, wherein upper or above is closer to surface than lower or below; and upstream and downstream are with respect to drilling fluid flowing generally downwards through the drill string, and upwards to surface though the annulus with the borehole wall.

The first borehole section can in particular be provided with well completion, in particular casing, such as a casing string comprising coaxial casing sections of narrower diameters in downhole direction.

In Figure 1, the first borehole section 5 is shown only schematically completed with a single casing

section. It does not need to be vertical as shown. Into this borehole section a first drill string part 3 is run, including a fluid jet drill head 10 at its lower end. The first drill string part comprises a length of

flexible tubing 3a.

As shown in figure 1, a reel 1 is positioned next to the drilling rig 4 which is used in the process of drilling a borehole 5. The drilling rig 4 is provided with a guide 7 over which a length of flexible tubing 3a, which is coiled tubing, and an auxiliary flexible member 8 are guided during the transfer of the length of

flexible tubing 3a from a coiled position on the reel 1 into the first borehole section 5. The length of flexible tubing 3a is connected to said auxiliary flexible member 8 by a coupling 9. Said auxiliary flexible member 8 may be carried out as a cable or as a further piece of tubing .

Before the flexible tubing 3 is introduced into the borehole 5, the jet drill head 10, preferably an abrasive jet drill head, has been mounted to the lower end

thereof. In the position shown in figure 1, the flexible tubing 3a has been unrolled fully from the reel 1, and is still connected to the auxiliary flexible member 8 through the coupling 9. In the position shown in figure 1, the flexible tubing 3 is subsequently held fixed with respect to the drilling rig 4 by the fixing means 17, and then the coupling 9 is released and the auxiliary

flexible member 8 is a reeled onto the reel 1.

The first drill string part 3 is run to a desired depth. In many cases this will be the bottom of the first borehole section 5 as shown in Figure 2. It is however also possible to drill the second borehole section as a deviated section from a more uphole position, e.g. as part of a multilateral well. Providing e.g. a laterally deviating second borehole above the bottom of the first is also regarded as deepening the borehole. At the desired depth, drilling the second borehole section 5a is to be started by operating the jet drilling bit,

generating a fluid jet blasting with an erosive power on an impingement area of the borehole, such as at the borehole bottom. The abrasive jet drill head suitably comprises one or more jet nozzles each blasting against a certain area in the borehole. When the drill string is rotated, impingent areas circle and even erosion in different is obtained.

Preferably but not necessarily, first a first pipe element, such as a piece of drill pipe, is connected to the upper end of the first drill string 3. To this end the flexible tubing 3 is still held fixed with respect to the drilling rig 4, and a piece of drill pipe 11 (see figure 2), which is for instance a stiff steel pipe, is connected by a coupling 12 which can be the same, a similar or a coupling co-operating with coupling 9, to the flexible tubing 3.

As the second borehole 5a section is deepened, more pipe elements are jointed on top, to form a second drill string part 11a. During the drilling process, a mixture consisting of a drilling fluid and abrasive particles is pumped through line 13 into the bore which runs through the hybrid drill string 16 comprising drill pipes 11 and the flexible tubing 3. Thus, the drill pipes 11 and the flexible tubing 3 behave as a single unity; because of the fact that a great length of flexible tubing 3 can be introduced into the borehole without making up

connections, the process of drilling is greatly

accelerated. The second borehole section 5a can have a smaller diameter than the first borehole section. If the connections for not fit through the second borehole section, the length of the coiled tubing effectively determines the maximum depth of the second borehole section. Both the first and the second borehole sections can be several hundred meters long. The first and/or the second borehole sections can be directionally drilled. Directional drilling using abrasive jetting drilling can for example be done as known from WO 2005/005767.

The second borehole section 5a can be an open hole section, i.e. not provided with casing. In one example it can have a diameter of 7.4 cm. The (minimum) internal diameter of the casing in the first borehole section back to surface can e.g. be 8.9-10.2 cm, in this case even drill pipe can be used for the top part of the drill string. If the casing is narrow, jointed tubing can be used for the second drill string part. The mixture of drilling fluid and adhesive particles is pumped by a pump 14; by means of the mixer 15 adhesive particles are added to the flow of drilling fluid which is discharged by the pump 14.

After the target depth of the second borehole section

5a has been reached, the hybrid drill string 16

comprising the flexible tubing 3 and the drill pipes 11 interconnected through couplings 12, 12a, is lifted by the lifting means 18 of the drilling rig 4, and

subsequently the drill pipes 11 are disconnected. After the last drill pipe 11 has been removed, the coupling 9 of the flexible tubing 3 is connected to the

corresponding coupling of the auxiliary flexible member 8. Finally, said auxiliary flexible member 8 and the flexible tubing 3 are coiled onto the reel 1. The entire process can then be repeated at another location, or using the same first borehole section a further borehole section such as a lateral well section can be drilled. This method is beneficial in particular when a sequence of similar wells have to drilled, e.g. batch drilling, for which a fixed length of coiled tubing and

standardized reel can be used.

Operating the jet drill head can be generally done as knows in the art, with the main difference the use of and the operation via the hybrid drill string of the

invention. Suitable abrasive jet drill heads, systems and methods of operation are e.g. disclosed in WO 00/66872, WO 2002/034653, WO 2005/005766, WO 2005/005767, WO

2008/119821, WO 2008/113843, WO 2008/113844, incorporated herein by reference. A recirculation system as for example described in WO 2002/034653, WO 2005/005766, WO2008/119821, WO 2008/113844 can be used, but this is not required. In one embodiment, the jointed pipe elements have a maximum diameter larger than the diameter of the second borehole section. The maximum diameter is typically the outer diameter at the joints. The second drill string part in the first borehole section has a maximum diameter larger than the diameter of the second borehole section. The connector between first and second drill string parts is regarded as part of the second drill string part.

In one embodiment, the second borehole section is provided up to a depth at which the second drill string is near the uphole end of the second drill string part, e.g. within 100 m of the uphole end, or within 50 m, or within 10 m.

Then, the second drill string part is pulled up to surface and disconnected from the first drill string part, and the first drill string part is extended by connecting a further length of flexible tubing or

replaced by a longer length of flexible tubing,

preferably having the same diameter. Then, the second borehole section can be extended to greater depth at the same diameter by running back into the hole, operating the jetting head and re-connecting a second drill string part of jointed pipe elements. This process can be repeated to reach greater depths.

In one embodiment, the diameter of the flexible tubing is smaller than the diameter of the jointed pipe elements as measured at the joints. In particular, the diameter of the flexible pipe can be at least 12 mm, more in particular at least 25 mm, even more in particular at least 37 mm, such as even at least 50 mm, smaller than the joint diameter of the jointed pipe.

In one embodiment, the diameter of the flexible tubing is smaller than the diameter of the jointed pipe elements as measured outside the joints. In particular, the diameter of the flexible pipe can be at least 3 mm, more in particular at least 6 mm, even more in particular at least 12 mm, such as even at least 22 mm or at least 25 mm, smaller than the joint diameter of the jointed pipe .

It is an advantage of the present invention that relatively small diameter flexible tubing can be used for drilling even extended lengths of e.g. more than 100 m, or more than 500 m, even more than 1000 m. Since no or minimum torque needs to be transmitted to the jetting head, even very small diameters are not a problem, e.g. between 4,5 and 7,5 cm o.d., such as 5 cm, 6 cm or 7,3 cm o.d. tubing. The diameter of flexible tubing is

preferably in an optimum range for hydraulic performance of the jetting system in the borehole section created by fluid jetting (second borehole section). In particular the ratio of the diameter of the flexible tubing to the diameter of this borehole section is preferably in the range of 0.6 - 0.75, more preferably 0.62-0.72, such as 0.65 - 0.7. An optimum value of this ratio provides optimum hydraulic transmission through the tubing to the jetting head, as well as return of the flow via the annulus to surface.

Example

A borehole from surface of 15,2 cm diameter is provided to a first depth, e.g. with a conventional drilling system, and a casing of 12,7 cm o.d. is provided therein. A conventional rotary drilling system with a bit diameter of 10,5 cm can just be operated through such a casing, to provide an extension of the borehole at diameter 10,5 cm to a second depth. The 10,5 cm bit can be operated by a drill string of 7,3 cm (2 7/8 inch, outside the joints), and a joint diameter of typically about 9,8-11,1 cm, such as 10,5 cm. In the extension to the second depth a liner of 8,9 cm o.d. is installed, liner joints may be a bit wider. The borehole until the second depth represents the first borehole section. The liner has an inner diameter through which a 7,4 cm (2.9 inch) jet drilling head fits. This head is mounted at the lower end of a flexible tubing of 5,1 cm (2 inch) o.d., which is operated to drill the second borehole section. It would be very difficult and inefficient to operate a rotary drilling bit through a liner of 8.9 cm o.d., in particular for extended lengths because transmission of torque and weight-on-bit is very difficult. This is not an issue for a jetting system, in particular an abrasive jetting system. The second borehole section can be extended to a third depth which can be approximately twice the second depth. The entire drill string can be pulled up, and flexible tubing with the jetting head with the same sizes as before can be re-inserted up to the third depth, and operated to extend the second borehole section to greater depth while connecting jointed pipe elements as discussed before.

Claims

C L A I M S

1. Method of drilling a borehole into an object, the method comprising

2. Method according to claim 1, wherein the fluid jet is and abrasive fluid jet and the fluid het drill head is an abrasive fluid jet drill head.

3. Method according to claim 1 or 2, wherein the drill string is rotated while deepening the borehole.

4. Method according to any one of claims 1-3, wherein the second drill string part in the first borehole section has a maximum diameter larger than the diameter of the second borehole section.

5. Method according to any one of claims 1-4, wherein the length of flexible tubing is connected to an auxiliary flexible member when the first drill string part is run into the first borehole section.

6. Method according to any one of claims 1-5, wherein the length of flexible tubing comprises a coupling at its upper end.

7. Method according to claim 6, wherein the coupling is used for connecting one of the second drill string part and the auxiliary flexible member according to claim 6 to the first drill string part.

8. Method according to any one of claims 1-7, wherein flexible tubing guidance equipment is provided at

surface, and wherein the coupling and the flexible tubing guidance equipment are dimensioned such that the coupling can pass through the flexible tubing guidance equipment.

9. Method according to any one of claims 1-8, wherein the method further comprises

-raising the assembled first and second drill string parts in the borehole,

-disassembling the jointed pipe elements from the length of flexible tubing,

- providing a longer length of flexible tubing as first drill string part and repeating the steps of the method of claim 1.

10. Hybrid drill string, comprising a length of flexible tubing as well as a plurality of jointed pipe elements, and further comprising a bottomhole assembly comprising a fluid jet drill head.

11. Hybrid drill string according to claim 10, wherein the length of flexible tubing comprises a single piece of tubing of 30 m length or more, preferably 50 m or more.

12. Hybrid drill string according to claim 10 or 11, wherein the length of flexible tubing comprises a single piece of tubing of 30 m length or more, preferably 50 m or more .

13. Hybrid drill string according to any one of claims 10-12, wherein the outer diameter of the flexible tubing is smaller than the outer diameter of the jointed pipe elements as measured at their joints, in particular at least 12 mm, more in particular at least 25 mm, even more in particular at least 37 mm smaller.

14. Hybrid drill string according to any one of claims

10-13, wherein the outer diameter of the flexible tubing is smaller than the outer diameter of the jointed pipe elements as measured outside their joints, in particular at least 3 mm, more in particular at least 6 mm, even more in particular at least 12 mm smaller.

15. Hybrid drill string according to any one of claims 10-14, wherein the length of flexible tubing has an outer diameter in the range of from 4,5 cm to 12,7 cm, in particular 4,8 to 10,5, more in particular 5,0 to 8,9 cm.