U.S. patent application number 13/517329 was filed with the patent office on 2012-10-18 for drilling a borehole and hybrid drill string.
Invention is credited to Jan-Jette Blange.
Application Number | 20120261194 13/517329 |
Document ID | / |
Family ID | 42153662 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120261194 |
Kind Code |
A1 |
Blange; Jan-Jette |
October 18, 2012 |
DRILLING A BOREHOLE AND HYBRID DRILL STRING
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).
Inventors: |
Blange; Jan-Jette;
(Rijswijk, NL) |
Family ID: |
42153662 |
Appl. No.: |
13/517329 |
Filed: |
December 22, 2010 |
PCT Filed: |
December 22, 2010 |
PCT NO: |
PCT/EP2010/070492 |
371 Date: |
June 20, 2012 |
Current U.S.
Class: |
175/67 ;
175/424 |
Current CPC
Class: |
E21B 19/22 20130101;
E21B 7/18 20130101; E21B 17/04 20130101; E21B 17/20 20130101 |
Class at
Publication: |
175/67 ;
175/424 |
International
Class: |
E21B 7/18 20060101
E21B007/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2009 |
EP |
09180572.1 |
Claims
1. 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.
2. The method according to claim 1, wherein the fluid jet is an
abrasive fluid jet and the fluid jet drill head is an abrasive
fluid jet drill head.
3. The method according to claim 1, wherein the drill string is
rotated while deepening the borehole.
4. The method according to claim 1, 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. The method according to claim 1, 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. The method according to claim 1, wherein the length of flexible
tubing comprises a coupling at its upper end.
7. The 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. The method according to claim 1, 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. The method according to claim 1, 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. The 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. The hybrid drill string according to claim 10, wherein the
length of flexible tubing comprises a single piece of tubing having
a length of at least 30 m.
12. The hybrid drill string according to claim 10, wherein the
length of flexible tubing comprises a single piece of tubing having
a length of at least 50 m.
13. The hybrid drill string according to claim 10, wherein the
outer diameter of the flexible tubing is at least 12 mm smaller
than the outer diameter of the jointed pipe elements as measured at
their joints.
14. The hybrid drill string according to claim 10, wherein the
outer diameter of the flexible tubing is at least 3 mm smaller than
the outer diameter of the jointed pipe elements as measured outside
their joints.
15. The hybrid drill string according to claim 10, wherein the
length of flexible tubing has an outer diameter in the range of
from 4.5 cm to 12.7 cm.
Description
[0001] 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.
[0002] 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).
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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 23/8 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] In accordance with the invention there is provided a method
of drilling a borehole into an object, the method comprising
[0011] providing a first borehole section extending into the object
from its surface;
[0012] 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;
[0013] 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
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] In one embodiment the method can further comprise
[0019] raising the assembled first and second drill string parts in
the borehole,
[0020] disassembling the jointed pipe elements from the length of
flexible tubing,
[0021] 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.
[0022] 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, 100 m or more, 200 m or more,
500 m or more.
[0023] 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.
[0024] 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:
[0025] providing a length of flexible tubing coiled on a reel,
[0026] obtaining a free end of the tubing,
[0027] connecting an abrasive jet drill head to the free end of the
tubing,
[0028] running said length of tubing in a borehole while unrolling
the tubing from the reel,
[0029] assembling a length of pipe, such as a steel pipe, to the
top of the length of tubing run in the borehole,
[0030] rotating the assembly comprising the length of pipe, the
length of tubing and the drill head,
[0031] providing a flow of a mixture of drilling fluid and abrasive
particles through the assembly,
[0032] eroding the borehole bottom by means of a jet of a mixture
of drilling fluid and abrasive particles discharged from the
rotating drill head.
[0033] 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.
[0034] Preferably, the method according to the invention comprises
the steps of:
[0035] 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,
[0036] unrolling the length of tubing as well as part of the
auxiliary flexible member while running the length of tubing into
the borehole,
[0037] disconnecting the length of tubing from the auxiliary
flexible member.
[0038] 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.
[0039] Furthermore, the method according to the invention comprises
the steps:
[0040] raising the assembly in the borehole,
[0041] disconnecting the length(s) of pipe from the tubing,
[0042] coiling the length of tubing onto the reel.
[0043] 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.
[0044] 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.
[0045] The invention will in the following be described further by
way of examples and with reference to the drawings, wherein FIGS.
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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] In FIG. 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.
[0050] As shown in FIG. 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.
[0051] 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
FIG. 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 FIG. 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.
[0052] 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 FIG. 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.
[0053] 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 FIG. 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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
[0065] 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 (27/8 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.
* * * * *