U.S. patent application number 13/517996 was filed with the patent office on 2012-11-01 for method of drilling and jet drillilng system.
This patent application is currently assigned to SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.. Invention is credited to Jan-Jette Blange, Pieter Van Nieuwkoop.
Application Number | 20120273277 13/517996 |
Document ID | / |
Family ID | 42196424 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120273277 |
Kind Code |
A1 |
Blange; Jan-Jette ; et
al. |
November 1, 2012 |
METHOD OF DRILLING AND JET DRILLILNG SYSTEM
Abstract
A method of drilling into an object comprises providing a drill
string in a borehole in the object, the borehole having at its
bottom a borehole axis, the drill string comprising a
borehole-centralized jet drill head at its lower end, the jet drill
head having a drill axis, and comprising a jet nozzle; providing a
stabilizer means for the drill string, the stabilizer means
determining an inclination angle between the borehole axis and the
drill axis; and generating a fluid jet at the jet nozzle, so as to
deepen the borehole, wherein the drill string is rotated while
deepening the borehole. Another system comprises a drill string
with an jet drill head at its lower end, the drill head being
provided with a jet nozzle, a stabilizer positioned above the jet
drill head, the stabilizer contacting the borehole wall, and means
for rotating the drill string.
Inventors: |
Blange; Jan-Jette;
(Rijswijk, NL) ; Van Nieuwkoop; Pieter; (Rijswijk,
NL) |
Assignee: |
SHELL INTERNATIONALE RESEARCH
MAATSCHAPPIJ B.V.
The Hague
NL
|
Family ID: |
42196424 |
Appl. No.: |
13/517996 |
Filed: |
December 22, 2010 |
PCT Filed: |
December 22, 2010 |
PCT NO: |
PCT/EP2010/070491 |
371 Date: |
June 21, 2012 |
Current U.S.
Class: |
175/67 ;
175/424 |
Current CPC
Class: |
B24C 1/045 20130101;
E21B 21/002 20130101; E21B 7/18 20130101; E21B 7/04 20130101; B24C
5/02 20130101; E21B 17/1014 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 |
09180563.0 |
Claims
1. A method of drilling into an object, the method comprising
providing a drill string in a borehole in the object, the borehole
having at its bottom a borehole axis, the drill string comprising a
borehole-centralized jet drill head at its lower end, the jet drill
head having a drill axis, and comprising a jet nozzle; providing a
stabilizer means for the drill string, the stabilizer means
determining an inclination angle between the borehole axis and the
drill axis; and generating a fluid jet at the jet nozzle, so as to
blast with an erosive power on an impingement area of the borehole,
thereby deepening the borehole, and wherein the drill string is
rotated while deepening the borehole.
2. A method according to claim 1, wherein the fluid jet is an
abrasive fluid jet and the jet drill head is an abrasive jet drill
head.
3. The method according to claim 1 wherein the jet drill head is a
self-centralizing jet drill head.
4. The method according to claim 1 wherein the stabilizer means is
fixed with respect to the drill string while deepening the
borehole.
5. The method according to claim 1 wherein the stabilizer means is
an eccentric stabilizer means.
6. The method according to claim 1 wherein the stabilizer means has
a largest diameter that is smaller than the borehole diameter.
7. The method according to claim 1 wherein at least one radial
dimension of the stabilizer means is adjusted in the course of a
drilling operation, so as to deepen the borehole along a
predetermined trajectory.
8. The method according to claim 7, wherein the stabilizer means
has a cross-section that is adjusted while rotating the drill
string.
9. A jet drilling system for drilling a borehole into an object,
comprising a drill string with an jet drill head at its lower end,
the drill head being provided with a jet nozzle, a stabilizer means
for the drill string at a position above the jet drill head, the
stabilizer means protruding radially with respect to adjoining
drill string parts so as to provide a point of contact with the
borehole wall, and a rotation means for rotating the drill
string.
10. The jet drilling system according to claim 9, further
comprising centralizing means for the jet drill head downhole of
the stabilizer means.
11. The jet drilling system according to claim 9 wherein the jet
drill head has a front end for engaging the borehole bottom and an
axis, and wherein the front end has a recess at least in an axial
region.
12. The jet drilling system according to claim 9 wherein the
stabilizer means is adjustable in at least one radial
dimension.
13. The jet drilling system according to claim 12, further
comprising a control means for controlling the at least one radial
stabilizer dimension.
14. The jet drilling system according to claim 9 wherein the jet
drill head is an abrasive jet drill head.
Description
[0001] The invention relates to a method of drilling into an
object, in particular by jet drilling, and to a jet drilling
system. The object can in particular be a subsurface earth
formation.
[0002] In the course of making a borehole, it is often desirable to
control the drilling direction so as to provide a borehole along a
predetermined trajectory. In conventional mechanical drilling, a
common technology is to use equipment like bent subs, mud motors
and rotating seals, to set only the lower part of the drill string
with the drill bit to drill in a particular direction. Conventional
mechanical drilling uses drilling bits with mechanical cutters such
as roller-cones or polycrystalline diamonds, that produce cuttings
by scraping at the borehole bottom and at the sides. More recently,
rotary steerable systems (RSS) have been developed, which can
operate with the entire drill string rotating. Known RSS methods
point the mechanical drill bit into a desired direction using a
complex bending mechanism, or push the drill bit to a particular
side using expandable thrust pads. The side-cutting ability of the
mechanical drill bits used for directional drilling then allows to
deviate the borehole in the desired direction. For example, PDC
bits have cutters not only on the front end but also at the sides.
Known mechanical directional drilling systems are for example
described in U.S. Pat. No. 5,168,941, U.S. Pat. No. 5,520,255, U.S.
Pat. No. 5,857,531, U.S. Pat. No. 5,875,859, U.S. Pat. No.
6,092,610 and US2007/0163810.
[0003] Another type of drilling methods uses fluid jetting, instead
of mechanical interaction between the drill bit or drill head and
the object. However, directional drilling methods known from
mechanical drilling cannot generally be used with jet drill
systems. For example, equipment like bent subs, mod motors and
rotary seals cannot be used at the high fluid pressures applied in
jet drilling, which can be 100 bar or more, in particular 250 bar
or more, more in particular 350 bar or more. Moreover known jet
drilling heads bit do not have the side-cutting action that is
needed for known mechanical directional drilling to work.
Furthermore, abrasive jet drilling requires only minimal or even no
weight on bit, which makes such drilling system react totally
differently than conventional mechanical directional drilling
methods and devices.
[0004] A jet drill system and method of making a hole in an object
is 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 WO
2005/05766.
[0005] The control of directional effect via the recirculation
system is relatively complex, and in fact requires the presence of
abrasive particles in the fluid jet as well as a recirculation
system.
[0006] There is a need for a new method of directional drilling for
fluid jet drilling, which can be used independent of the presence
and/or operation of a recirculation system for abrasive particles
in the fluid jet.
[0007] In accordance with the present invention there is provided a
method of drilling into an object, the method comprising [0008]
providing a drill string in a borehole in the object, the borehole
having at its bottom a borehole axis, the drill string comprising a
borehole-centralized jet drill head at its lower end, the jet drill
head having a drill axis, and comprising a jet nozzle; [0009]
providing a stabilizer means for the drill string, the stabilizer
means determining an inclination angle between the borehole axis
and the drill axis; and [0010] generating a fluid jet at the jet
nozzle, so as to blast with an erosive power on an impingement area
of the borehole, thereby deepening the borehole, wherein the drill
string is rotated while deepening the borehole.
[0011] The invention is based on the insight that directional
tendency in a low weight-on-bit (WOB) jet drill system without
side-cutting action is mainly determined by the angle between the
drill axis and the borehole axis. Setting this angle to a non-zero
value by a suitable stabilizer means is relatively easy in view of
the low or virtually zero weight in bit. A stabilizer arranged on
the drill string above the jet drill head can deviate the drill
string slightly and sufficiently to form an inclination angle with
the borehole, so that the fluid jetting builds a curved borehole.
The rate at which the inclination angle of the drill bit increases
depends on said angle. The better the drill string is centralized,
the smaller this tendency is. The expression `stabilizer means` is
used to refer to a structure or device that protrudes radially with
respect to adjoining drill string parts so as to provide at least
one point of contact with the borehole wall during drilling. The
point of contact together with the borehole-centralizing property
of the jet drill bit mainly define a direction of progression of
drilling. The object into which is being drilled can in particular
be a subsurface earth formation containing a rock layer, such as a
layer of sandstone, limestone, granit, basalt, or combinations of
such layers. In such a formation, drilling system as for example
known from BE 1001450A3 is not usable, as it is designed for
providing a tunnel in the soil, wherein soil is compacted during
progression. This is not possible in a rock formation. The known
drilling system is moreover inadequate in that is does not rotate,
and in that there is no centralizing means for a jet drill
head.
[0012] 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 aqueous
liquid such as water.
[0013] The borehole centralized jet drill head can have a
centralization means such as an annular centralizer, suitably at or
around the drill head, and in any event downhole from the
stabilizer means. In one preferred embodiment, the jet drill head
is a self-centralizing jet drill head, which by its design remains
centralized in the borehole during operation and does not require
additional centralizing means such as an annular centralizer. The
jet drill head can be self-centralizing as well as suitable for
jetting abrasive particles.
[0014] In a preferred embodiment the stabilizer means is fixed with
respect to the drill string while deepening the borehole, i.e. when
the drill string is rotated during operation for deepening the
borehole, the stabilizer means rotates together with the drill
string and does not need to be fixed against the borehole wall, and
released as the drill string progresses.
[0015] In a particular embodiment the stabilizer means is an
eccentric stabilizer means, preferably the eccentric stabilizer
means is rotated along with the drill string while deepening the
borehole, more preferably the outer dimensions of the eccentric
stabilizer means substantially conform to the borehole.
[0016] An eccentric stabilizer means has a non-uniform radial
extension around the drill string. When the drill string rotates
with the eccentric stabilizer means, and the eccentricity is fixed,
the inclination angle between drill axis and borehole axis changes
periodically. When the eccentric stabilizer conforms to the
borehole wall, the inclination angle circles about the borehole
axis. Depending on the relative direction of the jetting nozzle and
the eccentricity, this way a straight borehole of increased
("overgauged borehole") or in fact smaller diameter can be drilled
when compared with a centralized drill string.
[0017] In certain embodiments the stabilizer means has a diameter
smaller than the borehole diameter, preferably the largest diameter
of the stabilizer means is smaller than the borehole diameter. When
a borehole is somewhat deviated and no stabilizer is provided, an
abrasive jet drill string has a natural tendency to build
inclination angle, because the drill string due to the low weight
on bit will be pulled to the lower side of the borehole under the
influence of gravity. A stabilizer of smaller size than the
borehole makes use of this effect, but defines the point of contact
and therefore the inclination angle better.
[0018] In certain embodiments at least one radial dimension of the
stabilizer means is adjusted in the course of a drilling operation,
so as to deepen the borehole along a predetermined trajectory. A
radial dimension is a dimension or direction perpendicular to the
drill axis of the drill string. By changing a radial dimension the
inclination angle can be influenced. When the stabilizer means is
for example concentric with the drill string and smaller than the
borehole, the inclination angle is set by setting the diameter of
the stabilizer means arranged at a certain distance above the
centralized jet drill head. When the stabilizer means is an
eccentric stabilizer, the eccentricity can be changed, for example
changing the borehole diameter that is being drilled.
[0019] In certain embodiments the stabilizer means has a
cross-section that is adjusted while rotating the drill string,
preferably wherein the stabilizer cross section is adjusted to
maintain the drill axis geostationary during at least one rotation,
more in particular wherein the geostationary drill string axis has
a smaller angle with the vertical than the borehole axis. Such a
stabilizer means can for example be an actively controlled means
that maintains an eccentricity that is geostationary during
rotation. The inclination angle can then be fixed, and in
particular a negative inclination, i.e. an arrangement wherein the
geostationary drill string axis has a smaller angle with the
vertical than the borehole axis can be provided.
[0020] The invention moreover provides a jet drilling system for
drilling a borehole into an object, comprising a drill string with
an jet drill head at its lower end, the drill head being provided
with a jet nozzle, a stabilizer means for the drill string at a
position above the jet drill head, the stabilizer means protruding
radially with respect to adjoining drill string parts so as to
provide a point of contact with the borehole wall, and a rotation
means for rotating the drill string.
The jet drilling system can further comprise centralizing means for
the jet drill head downhole of the stabilizer means.
[0021] The jet drill had can have a front end for engaging the
borehole bottom and an axis, and wherein the front end has a recess
at least in an axial region. During operation of such a jet drill
head, the undrilled material is present in the recess in the axial
region, e.g. substantially in the form of a cone, and centralizes
the jet drill head during rotation. Such a jet drill head is
therefore self-centralizing. The front end engaging the borehole
bottom can be a substantially circumferential front end. In the
recess the jet nozzle can be arranged, preferably directed
obliquely with respect to the drill axis, such as to form a cone in
the recess during rotation.
[0022] The stabilizer means can be adjustable in at least one
radial dimension, and to this end preferably further comprises an
actuator for manipulating the at least one radial dimension. Such
stabilizer means can comprise a plurality of pads the radial
extension of which can be set by the actuator, e.g. hydraulically
or with an electrical motor such as a stepper motor. An adjustable
stabilizer is for example known from U.S. Pat. No. 4,572,305.
[0023] An adjustable jet drilling system preferably further
comprises a control means for controlling the at least one radial
stabilizer dimension, in particular downhole control means.
Preferably the control means comprise a control signal receiver,
such as a weight-on-bit sensor, a fluid pressure sensor, a
rotational speed sensor.
[0024] In one preferred embodiment the jet drill head is an
abrasive jet drill head.
[0025] In the following the invention will be described in more
detail and by way of example also with reference to the
accompanying drawings, wherein
[0026] FIG. 1 shows schematically a first embodiment of the
invention; and
[0027] FIGS. 2-5 show schematically several further embodiments of
the invention.
[0028] In the Figures, like reference numerals are used to refer to
the same or similar objects.
[0029] In one aspect the invention is related to a jet drill method
and system, in particular an abrasive jet drill system, comprising
a rotatable drill string, a rotatable abrasive jet drill head
connected to the lower end of the drillstring, the drill head being
provided with at least one jet nozzle for discharging a mixture of
drilling fluid and abrasive particles, drive means for rotating the
drill bit and drill string and pump means for generating a flow of
said mixture comprising drilling fluid and abrasive particles.
Abrasive jet drill systems are known for example from WO 00/66872,
WO 2002/034653, WO 2005/005766, W02008/119821, WO 2008/113843, WO
2008/113844.
[0030] By the combined action of the high pressure mixture jet
which contains hard particles, and the rotational movement of the
drill string and the drill bit, a borehole is formed in a rock
formation as a result of the gradual erosion of the rock material.
This erosion is solely obtained by the abrasive jet, without any
mechanical cutting action of a cutter or drill bit, due to the high
pressure drop over the jet nozzle of e.g. 250 bar or more,
preferably 350 bar or more of. The abrasive particle concentration
in the drilling fluid may be in the range of 2-10 vol %, such as 3
vol % or more. However, in case use is made of a downhole abrasive
particle recycling device, the abrasive particle concentration
passed from surface through the drill string may be as lower, such
as 1.5 vol % or less while still giving a good penetration
rate.
[0031] An aim of the present invention is to provide an abrasive
jet drilling system wherein use is made of more simple devices and
method steps for obtaining directional control. This aim can be
achieved by means of at least one stabilizer at a position above
the drill head, said stabilizer protruding radially with respect to
the adjoining drill string parts. The influence of such stabilizer
on the direction of the drilling process relies on the angle
between the drill bit axis and the borehole axis. The stabilizer
may rotate together with the drill string, in which case the
stabilizer may extend around the drill string. For a fixed
directional tendency, a stabilizer with a specific fixed radial
dimension may be used. When a stronger tendency is required, a
stabilizer with a reduced radial dimension or diameter should be
used. As an example, a fixed spiral melon shaped drill collar
stabilizer can be used. Such stabilizer can be positioned directly
behind the abrasive jet drill head, possibly with a downhole
abrasive particle recirculating device in between or integrated in
the drill head.
[0032] Alternatively, the stabilizer may have an adjustable radial
dimension. In that case, the directional tendency of the drilling
system may be influenced by changing the radial dimension of the
stabilizer. Such adjustable stabilizers can be embodied in several
ways. For instance, in case the stabilizer comprises segments which
are arranged next to each other in circumferential direction, these
segments can be adjustable in radial direction. Preferably, said
segments are adjustable independently from each other, e.g. by
steering means such as stepper motors or hydraulic piston/cylinder
devices.
[0033] In the latter embodiment, in case one of the segments is
adjusted to a relatively large radial dimension and the opposite
segment to a relatively small radial dimension, the axis of the
drill head will show a non-zero angle with respect to the borehole
axis. This adjustment may for instance be carried out at the earth
surface and be kept during the ensuing drilling cycle.
Alternatively however, downhole control means may be provided for
controlling the stabilizer dimension. In the latter case, the
direction of the borehole may be changed during the course of the
drilling process, whereby a very flexible control of the drilling
trajectory is obtained.
[0034] The direction of the drilling trajectory may be controlled
from the earth surface. In that case, the drilling system may
comprise downhole control means with at least one sensor, such as a
weight-on-bit sensor, a fluid pressure sensor, a rotational speed
sensor and the like, as well as a control unit for controlling the
steering means on the basis of data detected by said sensor.
Preferably, the downhole control means may comprise a memory
containing stored trajectory data as well as control means
containing preprogrammed control data for controlling the
stabilizer radial dimension dependant on the trajectory.
[0035] In one aspect the invention relates to a method for
operating an abrasive jet drilling system as described before,
comprising the steps of: [0036] making the drill string and
abrasive jet drill head rotate, [0037] applying a specific data
sequence, such as a weight-on-bit sequence, as a code, [0038]
detecting said code by the sensor, [0039] controlling the radial
dimension of the stabilizer on the basis of said detected code.
[0040] Alternatively, the method may comprise the steps of: [0041]
making the drill string and abrasive jet drill head rotate, [0042]
applying a specific data sequence, such as a weight-on-bit
sequence, as a code, [0043] detecting said code by the sensor,
[0044] controlling the directional action of the stabilizer on the
basis of said detected code.
[0045] According to yet another possibility, the method may
comprise the steps of: [0046] making the drill string and abrasive
jet drill head rotate, [0047] applying a specific data sequence,
such as a weight-on-bit sequence, as a code, [0048] detecting said
code by the sensor, [0049] controlling the eccentricity of the
stabilizer on the basis of said detected code.
[0050] The radial dimension of the stabilizer may be adapted while
carrying out a drilling operation. According to another
possibility, the stabilizer settings may be controlled as a
function of a pre-programmed borehole trajectory in combination
with downhole measurements while drilling. Such method may thus
comprise the steps of: [0051] making the drill string and abrasive
jet drill head rotate, [0052] detecting a downhole condition by the
sensor, [0053] adjusting the stabilizer size on the basis of the
stored trajectory data and the detected downhole condition.
[0054] The invention will now be described further by way of
example with reference to the embodiments shown in the
drawings.
[0055] As shown in FIG. 1, an abrasive jet drilling system
including an abrasive jet drilling assembly according to the
invention comprises a drill string 1 in a borehole 2 in an object.
This object is here a subterranean earth formation 5, in particular
to provide a borehole for the manufacture of a well for production
of mineral hydrocarbons. The drill string 1 is which at its upper
end at surface 8 connected to a rotational drive device (not shown,
but indicated by arrow 10) and at the other, lower, end to an
abrasive jet drill head 16 with jet nozzle 18. The drill string 1
has a passageway 20 for fluid, which is in fluid communication with
the jet nozzle, via a passage through the stabilizer means and
passage 24 of abrasive jet drill head 16. Furthermore, pump means
(not shown) are provided at surface for circulating the drilling
fluid from the surface through the drill string 2 to the drill head
16.
[0056] The nozzle 18 is obliquely oriented in a recess 17 in an
axial area so that the impingement area is located eccentric with
respect to the drill axis or rotary axis 21, and in this case
rotating the abrasive jet in the hole results in the jet 19 and the
impingement area moving along an essentially circular trajectory in
the hole. Preferably, the eccentric impingement area overlaps with
the centre of rotation, so that also the middle of the bore hole is
subject to the erosive power of the abrasive jet.
[0057] The jet nozzle 18 is arranged above an optional foot part
29, and the nozzle is inclined relative to the longitudinal
direction of the system (drill axis 20) at a nozzle angle of
15-30.degree. relative to the drill axis, but other angles can be
used. Preferably the nozzle angle is about 21.degree. which is
optimal for abrasively eroding the bottom of the bore hole by
axially rotating the complete tool inside the bore hole.
[0058] The abrasive jetting drill head in this embodiment moreover
comprises a recirculation system for abrasive particles, which is
generally indicated as 30, with an inlet 32 in fluid communication
with the annulus 33 between abrasive jet drill head 16 and the
borehole 2, and an outlet 34 to a mixing chamber 36 arranged at a
mixing location 37 of the passageway 24.
[0059] The optional foot part 29 forms a front end of the drill
head and engages the borehole bottom providing for a distance from
the borehole bottom and suitably contains slots for drilling fluid
and cuttings to flow via the annulus 33 upwardly. The abrasive jet
drill head 18 can for example be a head as described in e.g.
WO2008/113843, WO 2008/113844.
[0060] In operation, the system works as follows. A stream of
drilling fluid including abrasive particles such as steel shot, is
pumped from the object's surface (e.g. earth's surface) by a
suitable pump (not shown) through the longitudinal passage 20 of
the drill string 2. Part or all of the drilling fluid is led to the
jet nozzle 18 where an abrasive jet 19 is generated. The abrasive
jet is blasted into impingement with the formation. The formation
is eroded in the impingement area as a result of the abrasive jet
19 impinging the formation 5, thereby deepening the borehole 2. The
positioning and oblique orientation of the nozzle in the axial area
provides that a substantially cone-shaped central area of the
borehole bottom is created, which centralizes the jet drill head
16, i.e. the head is self-centralizing.
[0061] Simultaneously, the abrasive jet is rotated about the rotary
axis 20. Thus, the impingement area is moved along a circular
trajectory in the hole so that the formation can be eroded at all
azimuths at the borehole bottom. By keeping the erosive power of
the abrasive jet constant, the formation is eroded evenly on all
sides of the hole and consequently the hole is excavated straight.
Nevertheless, distortions in the rotating of the excavation tool,
or variations in rock formation properties in the hole region, or
other causes may result in uneven erosion in the hole. A
directional correction may be required by modulating the erosive
power to compensating for the unintentional uneven erosion.
[0062] As shown in FIG. 1, the trajectory of the borehole 2 is
curved. This curvature is obtained by the action of the stabilizer
means 40 which is connected to or provided around the drill string
1 immediately above or at a small distance above the drill head 2.
Said stabilizer 40 is in contact with the wall of the borehole at
42 under the influence of gravity. The distance between the contact
point 42 and the point where the drill head is centralized (in this
example at the front end of the foot part 29) can be between 0.1 m
and 50 m, such as between 0.5 m and 10 m.
[0063] The stabilizer 40 of this embodiment is coaxial with the
drill string 1, defining a circular cross-section at the widest
radial dimension. It is fixedly arranged around the drill string so
that it rotates together with the drill string 1, and it has a
diameter smaller than the borehole diameter. The diameter together
with the distance from the centralized drill head determines the
inclination angle 45 between the drill axis 20 and the borehole
axis 48. The better the jetting assembly is centralized at the
first point of contact with the borehole above the drill head, the
smaller the directional tendency is. Thus, if the first point of
contact is at a stabilizer, the outer diameter of this stabilizer
determines the building tendency. If the outer diameter of the
near-bit stabilizer can be changed while drilling, the building
tendency can be controlled while drilling.
[0064] For directional drilling operation the inclination angle is
normally non-zero, and can for example be between 0.01 and 20
degrees, such as between 0.1 and 5 degrees. The inclination angle
in the Figure is positive, i.e. the borehole is deviated upwardly.
The invention can however also provide negative inclination angles
(deviation towards the vertical), and lateral inclinations, so that
directional drilling into any desired spatial direction can be
provided.
[0065] It shall be clear that the schematic drawing illustrates the
various components but does not necessarily represent their
relative size correctly. For relatively large inclination angles
the diameter of the drill pipe or collar can be limiting, so that
the drill pipe or collar 3 between drill head 16 and stabilizer 40
and/or the drill string or tubular above stabilizer 40 has to be
sufficiently thin for a desired inclination angle, to ensure that
the stabiliser above the bit is touching the borehole wall.
[0066] 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.
[0067] The mechanical forces on the drilling system that is based
on abrasive jetting are much smaller than is the case for systems
based on mechanical rock removal. This has the advantage that the
sensors can be located very close to the excavating tool, making
early and accurate signal communication possible to the modulation
control means. The sensors can for instance be provided in the same
chamber as the modulation control means. The control means can
comprise a memory for storing trajectory data.
[0068] Alternatively, the position and and/or the direction of
progress through the formation of the abrasive jet can be
determined on the basis of parameters available on the surface 8,
including torque on the drill string 2 and azimuthal position of
the drill string 2, and axial position and velocity of the drill
string 2.
[0069] A decision to change or correct drilling direction may also
be taken via the operator of the directional system at surface. In
case of the signal originating from a down-hole measurement while
drilling sensor, a mud-pulse telemetry system or any other suitable
data transfer system can be employed to transfer the data to the
surface. Via similar means of data transfer a control signal can be
sent to the down hole control means triggering a series of control
actions required for the desired direction drilling correction.
[0070] A thruster (not shown) is advantageously provided for
pressing the abrasive jetting system upon the bottom 39 of the hole
2. Best results are obtained when the pressing force is not much
higher than what is required to keep the abrasive jet drill head 16
at the bottom, in order to avoid unnecessary wear on the abrasive
jet drill head 6, bending of the system, and loss of directional
control. Thus, the pressing force is preferably just sufficient to
counteract the axial recoil force of the abrasive jet and the
friction forces in the thruster and between the abrasive jet system
and the hole wall. Typically, the pressing force is well below 10
kN.
[0071] A suitable abrasive jet comprises a mixture containing a
fluid, such as the drilling fluid, and a certain controlled
concentration of abrasive particles. The erosive power of the jet
correlates with the total power vested in the abrasive particles
entrained in the mixture. This depends on the mass flow rate of
abrasive particles and on the square of the velocity of the
abrasive particles.
[0072] Still referring to FIG. 1, the abrasive particles will be
entrained in a return stream of drilling fluid through the
excavated hole, running for instance through an annular space 33
between the hole 1 and the drilling system (2,16, 40).
[0073] In order to reduce the concentration of abrasive particles
to be transported all the way back to the surface, the drilling
system, in particular the abrasive jet drill head 16, can be
provided with recirculation means 30 arranged to recirculate at
least a part of the abrasive particles from the return stream
downstream from impingement with the formation, back into the
abrasive jet 10 again. The abrasive particles to be recirculated
can be mixed with the fresh stream of drilling fluid containing a
supply concentration of abrasive particles, for instance in a
mixing chamber to which both the fresh stream of drilling fluid and
the recirculated abrasive particles are admitted, to obtain a
jetting fluid mixture comprising a jetting concentration of
abrasive particles. The abrasive particles referred to herein
preferably comprise or consist of magnetisable material, i.e.
paramagnetic or ferromagnetic material, such as for instance steel
shot or steel grit. This will herein also be referred to as
"magnetic material" although it does not need to have a permanent
magnetization. The recirculation system can comprise a magnet
attracting magnetic particles from the drilling fluid flowing
upwardly in annulus 33, and conveying the particles via outlet 34
to the mixing chamber 36. Generally suitable recirculation systems
are for example described in WO 2002/034653, WO 2005/005766,
WO2008/119821, WO 2008/113844. A recirculation system is however
optional and not required for the present invention to
function.
[0074] In an illustrative example, the rotation of the string can
typically take 1 sec. In the case a downhole recirculation device
is used, the supply concentration of particles pumped through the
drill string is typically in the range of 0.1 to 4% by volume, such
as 0.4 to 2 vol %, considering steel shot in an aqueous fluid, e.g.
water. When a recirculation system is used, the drilling fluid in
the abrasive jet may contain a jetting concentration of up to 10%
by volume, typically up to 5 vol % of magnetic abrasive particles,
and is on average higher than the supply concentration. When there
is no recirculation system, the supply concentration via the drill
string is typically the same as the jetting concentration, apart
from a possible time lag of changes, and can e.g. be in the range
of 0.5 to 10 vol %, such as 2-5 vol %, e.g. 3 vol%. Recycle
frequency preferably exceeds the rotational frequency of the drill
string. The recycle frequency can for example be between between 10
and 40 Hz. The rotation of the drill string, or at least the
abrasive jet drill head excavating tool, is typically between 0.3
and 3 Hz.
[0075] Referring to FIG. 2, there is shown a drill string 1,3 with
jet drill bit at its lower end, and a stabilizer 40 that is
concentric and fixed with the drill string, and circumferentially
engaging the borehole wall. The stabilizer acts as a centralizer,
the drill axis 20 is collinear with the borehole axis 48, i.e.
there is no inclination angle. If the stabilizer is of a type that
can change its diameter, by lowering the diameter a configuration
similar to FIG. 1 in accordance with the invention would be
obtained.
[0076] FIG. 3 shows an embodiment of the invention similar to FIG.
1, but in which the drill string above the stabilizer is of a
narrower diameter than the drill string part 3, so that the
stabilizer 40 provides the first point of contact 42. The upper
drill string part can also be of a flexible tube. The drill string
part 3 can be thicker or also thin drill pipe, or could be a collar
containing e.g. a control means and/or a sensor means and/or a
communication means.
[0077] FIG. 4 shows an embodiment with an eccentric stabilizer 50.
It has a non-uniform radial extension around the drill string. The
extension in the radial dimension 52 is smaller than in radial
dimension 54. The stabilizer conforms to the borehole wall, so that
the contact 42 is around the circumference. Here it rotates with
the drill string, with the long and short radial dimensions also
turning, and therefore the drill axis 48 turns around the borehole
axis at the inclination angle. When a nozzle is arranged like in
FIG. 1, obliquely in the axial area of the recess 17, and with the
eccentricity as drawn, the angle between nozzle and borehole axis
is increased by the inclination angle, and a straight, but wider
borehole is created. If the nozzle would be turned by 180 degrees
to jet against the opposite side of the borehole bottom, a narrower
straight borehole would be created. If the eccentric stabilizer
would not conform to the borehole, a wider or narrower curved
borehole would be drilled.
[0078] For larger overgauge hole sizes the bottom profile and the
internal conical profile of the distance holder (foot part 29)
should be modified to allow for the angle between the drilling
assembly and the borehole axis 48.
[0079] FIG. 5 is similar to FIG. 4, but the eccentric stabilizer 60
has a cross-section that is adjusted while rotating the drill
string to maintain the drill axis 20 geostationary during at
rotation. The drill axis 20 has a negative inclination with regard
to the borehole axis 48. The stabilizer means can actively
controlled. An adjustable eccentric stabilizer can be made as
follows: If the stabilizer has four pads that can be moved radially
independent of each other, of which one pad is moved outward and
one pad moved inward, the axis of the connected drilling assembly
is not parallel to the bore hole axis anymore. The pads of an
adjustable eccentric stabilizer are adjustable down hole with, for
instance, controlled stepper motors or hydraulically operated
pistons that are controlled by a down hole control unit, e.g. in
collar 3, so that the radial dimensions 62 and 64 remain constant
as shown during rotation. Varying the pad positions synchronized
with the rotation of the drilling assembly provides directional
control in all drilling directions, i.e. both azimuth and
inclination control.
[0080] In order to establish the current drilling direction through
the formation, the system can be provided with a navigational
sensor, for instance a measurement while drilling sensor, for
providing a signal indicative of the direction under which the
making of the hole in the earth formation progresses.
Such a navigational sensor can be provided in the form of one of or
a combination of a directional sensor providing a signal indicative
of the direction of the device relative to a reference vector; a
positional sensor providing a signal indicative of one or more
positional coordinates relative to a reference point; a formation
density sensor providing information on a distance to a change of
formation type or formation content nearby; or any other suitable
sensor.
[0081] Over longer time scales than a rotation period, a downhole
control unit can moreover adjust the adjustable stabiliser settings
as a function of a preprogrammed trajectory and down hole
measurements while drilling. Also, the instructions can be sent
from surface to the control unit, e.g. by data sequences such as a
weight on bit sequence, a rotational speed (RPM) sequence, or a
hydraulic pressure sequence, e.g. to adjust the magnitude of the
inclination or the direction. Based on such transmission of data,
one or more of an outer diameter of the stabilizer, and
eccentricity, or a directional drilling mode can be set.
[0082] It is attractive to combine such options with jet drilling
because of the minimal mechanical loading of the drilling assembly
compared to mechanical drilling. The mechanical forces on the
drilling system that is based on fluid jetting are much smaller
than is the case for systems based on mechanical rock removal. This
has the advantage that the sensors can be located very close to the
jet drill head, making early and accurate signal communication
possible to the modulation control means. The sensors can for
instance be provided in the same chamber as the modulation control
means.
* * * * *