Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF DRILLING AND ABRASIVE JET DRILLING ASSEMBLY
The invention is related to a method of drilling into
an object and to an abrasive jet drilling assembly. The
object can in particular be an earth formation.
An abrasive jet drill system and method of making a
hole in an object is disclosed in WO-A-2005/005767. Said
prior art 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. The conveyor means has a
magnet arrangement forming high-field bands and low-field
bands in a helical arrangement. Magnetic particles are
attracted by both bands onto a support surface, and are
rearranged to the high-field bands. The distance between
magnet arrangement and support surface decreases in the
direction of an inlet to a mixing chamber, therefore the
magnetic field at the support surface due to both high-
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field and low-field bands increases. During rotation of
the conveyor means the particles are transported along
the support surface to an inlet to a mixing chamber,
where they are entrained by drilling fluid streaming
along. A modulation means in form of a controllable drive
means for the conveyor 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 varied, and directional
control is achieved. Reference is also made in this
regard to WO 2005/05766.
In another abrasive jet drill system and method,
described in WO-A-2008/119821, a recirculation system
device is arranged, which can operate without a moving
action of the magnet. As much as it is advantageous not
to have moving parts or at least not continuously moving
parts operating downhole, such static magnet cannot
modulate the recycle rate of abrasive particles.
There is a need for an improved abrasive jet drilling
assembly and method of drilling, wherein modulation of
the abrasive particle concentration in the abrasive jet
can be provided with a minimum of moving parts or without
moving parts.
In accordance with the invention there is provided a
method of drilling into an object, the method comprising
- providing a drill string in a borehole in the object,
the drill string comprising an abrasive jet drill head at
its lower end, the drill head including a jet nozzle, the
drill string providing a passageway for fluid from the
object's surface to the jet nozzle;
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- supplying a fluid mixture comprising magnetic particles
via the drill string towards the abrasive jet drill head
and blasting from the jet nozzle an abrasive jet with an
erosive power into impingement with the object;
- modulating a jetting concentration of magnetic
particles in the abrasive jet;
wherein modulating the jetting concentration of magnetic
particles in the abrasive jet comprises modulating a
magnetic field at a collection surface arranged along a
flow path of fluid towards the jet nozzle between a first
value of the magnetic field, at which magnetic particles
are collected from the fluid mixture at the collection
surface, and a second value, at which magnetic particles
are released into the fluid mixture from the collection
surface.
Modulating magnetic field strength between a high
value at which particles are attracted to a collection
surface that is arranged along the passage of fluid with
particles towards jet nozzle, and a low value at which
particles are released from the collection surface,
provides an efficient and elegant way to avoid moving
parts in the modulation system.
The magnetic field can be modulated by moving a
magnet relative to the collection surface, and/or by
modulating a drive current of an electromagnet.
Modulating the magnetic field can comprise moving a
magnetic connector for magnetically connecting a magnet
and the collection surface between a connecting position
and a non-connecting position.
Preferably for directional drilling operation, the
particle concentration is controlled so as to be
modulated in relation with the position of the
impingement area on the selected trajectory. The jet
nozzle can in particular be rotated, and the modulation
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of the magnetic field can be controlled in dependence of
the rotation of the jet nozzle.
In one embodiment, a supply concentration of abrasive
particles in the fluid mixture received at an upstream
side of the abrasive jet drill head is modulated in order
to modulate a jetting concentration in the abrasive jet.
The jetting concentration can be different, in particular
on average higher, but still dependent on the supply
concentration, if a recirculation system
is provided, to recirculate abrasive particles after
their discharge from the jet nozzle to a mixing location
with the supply fluid mixture along the passageway, and
wherein the supply concentration is modulated upstream of
the mixing location. It is in principle also possible to
use the modulation of the present invention inside a
recirculation system, alternatively or in addition to
modulating the supply concentration. Preferably, when a
downhole recirculation system is provided, it is not used
for modulating the recycle rate.
The invention also provides an abrasive jet drilling
assembly connectable to a tubular drill string part, and
comprising an abrasive jet drill head with a jet nozzle,
a passageway for fluid comprising magnetic abrasive
towards the jet nozzle,
and a modulation means for modulating the concentration
of the magnetic abrasive particles in fluid flowing,
during operation, through the jet nozzle;
the modulation means comprising
a collection surface,
a magnetic holdup device for exerting a magnetic field at
the collection surface, comprising a magnet arrangement
and a modulation control means for selectively changing
the magnetic field at the collection surface between a
first value, at which magnetic particles are collected at
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the collection surface, and a second value, at which
magnetic particles are released from the collection
surface.
The modulation means can be integrated with the
5 abrasive jet drill head, or can be separate, such as
connectable as part of a bottomhole assembly into the
drill string.
In one embodiment the abrasive jet drilling assembly
further comprises a control unit for controlling the
particle concentration of the abrasive jet in relation
with a position of an impingement area of the abrasive
jet drill head during operation. Preferably, when a
downhole recirculation system is provided, it is not
adapted for modulating the recycle rate, and/or the
control means is not adapted to control the recycle rate
of the recirculation system.
The magnet arrangement can comprise at least one
electromagnet, and then preferably modulation control
means is arranged to modulate a drive current of the
electromagnet. Alternatively or in addition, the magnet
arrangement can comprises at least one permanent magnet,
preferably then the at least one permanent magnet is
movable with respect to the collection surface. It can
e.g. be translationally and/or rotationally movable.
The magnetic holdup device can comprises an actuator
for changing the relative position of the magnet
arrangement and the collection surface.
Alternatively or in addition, the magnet arrangement
can further comprise a selectively movable magnetic
connector.
The passageway can be at least partially an annular
passageway having inner and outer annular walls, and the
collection surface can be at least partially arranged on
the inner annular wall. E.g. a magnet can be enclosed by
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the inner wall, or the magnet arrangement can be
surrounded by the inner annular wall.
The magnet arrangement can comprises at least two
magnets which are positioned at different angular
positions with respect to flow direction along the
passageway.
The magnet arrangement can be positioned outside the
passageway.
The abrasive jet drilling assembly can further
comprise a downhole recirculation system arranged to
recirculate abrasive particles during operation after
their discharge from the jet nozzle to a mixing location
along the passageway, and wherein the collection surface
is arranged along the passageway upstream of the mixing
location. In a particular embodiment the modulation means
can form part of the recirculation system.
However, by arranging the modulation means along the
fluid passageway from surface to the drill nozzle,
upstream of a mixing location with a return stream from a
recirculation system, i.e. so that it has effect in this
passageway and not in a return path of the recirculation
system, the modulation becomes independent of the
presence of and precise function of the recirculation
system.
The expressions upper, above, upstream, uphole,
lower, below, downstream, downhole, and the like, are
used with reference to a drill string with abrasive 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 abrasive jet drilling assembly can be part or a
drilling system including part or all of a drill stream,
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e.g. an entire drill string to surface or a conventional
component of a drill string such a s drill string
element, jointed pipe, bottom hole assembly, special
function sub such as for measurement while drilling
(MWD), stabilizer, etc.
The modulation means can be integrated with the
abrasive jet drill head, such as at an upper or upstream
end thereof, in particular upstream of the mixing
location with abrasive particles from a recirculation
system. It is however also possible to arrange the
modulation means separate from the abrasive jet drill
head, e.g. as part of the drill string above the abrasive
jet drill head, such as in a separate drill string
element or sub. Suitably the distance along the
drillstring between the modulation means and the jet
nozzle is not too large, otherwise a modulation of
particle concentration in the supply fluid can be
partially or fully smeared out when arriving at the
abrasive jet drill head. It will be understood that a
certain time lag between the modulation at an upstream
position in the passageway and the nozzle can occur.
Suitably the distance is 100 m or less, preferably 50 m
or less, more preferably 20 m or less, so that the supply
concentration received at the abrasive jet drill head is
well-defined. The modulation means can be provided just
upstream of the abrasive jet drill head or as part of a
bottomhole assembly.
The abrasive jet drilling assembly can further
comprise a measurement device for detecting the angular
orientation of the jet nozzle in the borehole, and the
modulation control means can be arranged to control the
magnetic holdup device in dependence on the detected
angular orientation.
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In one embodiment the modulation means is positioned
in the drill string above or at a lower outlet connection
towards the abrasive jet drill head. When connected, a
flow of drilling fluid with modulated abrasive particles
supply concentration enters an upper inlet connection of
the drill head. In one embodiment, the modulation means
is contained in a collar positioned in the drill string,
said collar having a through going channel forming part
of the passageway of the drill string. In one embodiment
the through going channel comprises an annulus and a
magnetic holdup device is positioned within the area
surrounded by the innermost wall of the annulus. In one
embodiment the modulation means comprises at least two
magnets which are positioned on opposite sides of the
through going channel. In one embodiment the magnet(s)
comprise at least one permanent magnet and the activating
means is provided comprising a mover, said mover being
carried out for displacing the permanent magnet with
respect to the through going channel. In one embodiment
the magnet(s) are mounted in a rotatable fashion between
a position parallel to the through going channel and an
oblique position with respect to the through going
channel. In one embodiment the modulation means is
provided with a paramagnetic collector and the mover is
arranged so as to establish respectively break a magnetic
contact between the magnet(s) and the collector.
In one embodiment of the invention, a method for
operating the abrasive jet drilling assembly comprises
the steps of obtaining a flow mixture comprising a
drilling fluid and abrasive particles, and varying the
operation of the activation device so as to obtain a
controlled attraction and/or release of the paramagnetic
abrasive particles. In particular the modulation means is
operated so as to hold an amount of particles within the
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through going channel, and subsequently making the
modulation means operate in a pulsating manner so as to
release a part and only a part of the abrasive particles
held within the through going channel during each
pulsation. In one embodiment the method comprises
providing at least two modulation devices in series,
activating said modulation means so as to hold a batch of
abrasive particles each, de-activating said modulation
devices such that a time difference is obtained in the
arrival time at the jet nozzle of said batches of
abrasive particles.
The invention will now be described by way of example
with reference to the drawings, wherein
Figure 1 shows schematically an abrasive jet drilling
system with abrasive jet drill assembly according to the
invention;
Figures 2 shows a first embodiment of a modulation
means;
Figure 3 A, B show a second embodiment of a
modulation means;
Figure 4 shows a third embodiment of a modulation
means;
Figures 5 A, B show a fourth embodiment of a
modulation means; and
Figures 6 A, B show a fifth embodiment of a
modulation means.
In the Figures, like reference numerals are used to
designate the same or similar objects.
As shown in Figure 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
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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 a collar 13 comprising a modulation means
14 in accordance with the invention. The collar 13 can
also be provided with a controller unit, such that the
controller unit is located inside the hole.
Alternatively, the controller unit can be positioned at a
different position in the drill string with abrasive jet
drill head, or at the surface 8.
At the lower end of the collar 13 an abrasive jet
drill head 16 with jet nozzle 18 is connected to or
integrated with collar 13. The drill string 1 has a
passageway 20 for fluid, which is in fluid communication
with the jet nozzle, via passages 22,24 of the collar 13
and abrasive jet drill head 16. The nozzle 18 is
obliquely oriented in a central area so that the
impingement area is located eccentric with respect to the
rotary axis of the drill head 16, 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.
The jet nozzle 18 is arranged above an optional foot
part 29, and is inclined relative to the longitudinal
direction of the system at an inclination angle of 15-30
relative to the rotary axis, but other angles can be
used. Preferably the inclination angle is about 21 which
is optimal for abrasively eroding the bottom of the bore
hole by axially rotating the complete tool inside the
bore hole.
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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.
The optional foot part provides 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. W02008/113843, WO
2008/113844.
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.
Simultaneously, the abrasive jet is rotated about the
rotary axis. Thus, the impingement area is moved along a
circular trajectory in the hole so that the formation can
be eroded at all azimuths. By modulating the erosive
power of the abrasive jet a high degree of directional
control can be achieved.
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.
When the erosive power is modulated, in particular by
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modulating the concentration of abrasive particles, a
modulation that is not synchronized with the moving of
the impingement area of the jet such as by rotation, is
not going to give rise to a directional effect. Thus,
straight borehole sections can in principle be drilled by
modulation that is asynchronous with respect to the
rotation. 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. The
erosive power of the abrasive jet can also be modulated
in order to deliberately excavate a curved hole.
When the abrasive jet is oriented to impinge the
formation in an area that requires more erosion in order
to establish the directional correction, the erosive
power of the abrasive jet can be periodically increased
resulting in a higher erosion rate in that area.
Alternatively, or in combination, the erosive power of
the abrasive jet can be reduced when the abrasive jet is
oriented to impinge the formation in an area that
requires less erosion.
A directional effect can be obtained by making the
abrasive particles emanate from the jet nozzle 10 at a
higher concentration at the same specific spot at each
rotation of the drill string 2 and the drill head 16.
Thereby, the borehole bottom 39 is eroded unevenly, which
makes that the further progression of the borehole 9 will
continue deviated with respect to the longitudinal
direction of the borehole 1.
The flow of drilling fluid with abrasive particles
can be modulated so as to obtain a pulsating effect.
That is to say, at each full rotation of the drill string
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1 and the drill bit 4 the jet can contain one phase with
a relatively high concentration of abrasive particles and
at least one phase with a relatively low concentration of
abrasive particles. It would also be possible to provide
a higher concentration only once every selected integer
number of rotations. Other schemes of preferential
erosion can be used in case more than one jet nozzle is
arranged. When more than one jet nozzle is arranged they
suitably provide an asymmetric distribution of impact on
the borehole.
Modulation of a concentration c means that the
concentration depends on time, c=c(t). The modulation can
have a period of repetition after 1, 2,3, up to 10, or
20, or more rotations. The time scale considered for a
particular modulation can be the duration of one or
several, say 10, 100 or more, 1000 or more, rotations of
the drill string. Over longer times the modulation can be
modified, or even stopped during certain periods, so as
to drill along a desired trajectory which may include
straight parts.
It is thus preferred that the modulation means
comprises modulation control means arranged to control
the modulation means such that the erosive power of the
abrasive jet is modulated in relation with the position
of the impingement area on the selected trajectory.
In order to establish the position of the impingement
area, the system can be provided with a positional
sensor, for instance a measurement while drilling sensor,
for providing a signal indicative of the position of the
abrasive jet. 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
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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.
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.
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.
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
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triggering a series of control actions required for the
desired direction drilling correction.
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.
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.
Modulating the erosive power of the abrasive jet can
be achieved by modulating the mass flow rate of the
abrasive particles in the abrasive jet. This can most
advantageously be achieved by modulating the
concentration of abrasive particles in the mixture. When
the quantity of similar particles impinging on an area
per unit of time is higher, the total erosive power of
the abrasive jet increases in that more of the formation
will be eroded. Modulation of the concentration of
abrasive particles in the mixture does not influence the
mechanical contact forces between the drilling system and
the formation.
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A relatively small variation in the concentration of
abrasive particles can be sufficient to achieve a
directional effect, such as 20 wt% of the particles or
less, 10 wt% or less, 5wt% or less between maximum and
minimum concentration of particles during modulation.
When the particles have substantially the same
size/weight and/or density, these figures can likewise be
expressed in vol% of abrasive particles.
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 (2,13,16).
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 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" or "magnetic particles", although it
does not need to have a permanent magnetization. The
recirculation system can comprise a magnet attracting
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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, W02008/119821, WO
2008/113844.
In accordance with the invention the supply
concentration of abrasive particles is modulated,
upstream of the mixing location 37, i.e. in the
passageway 20,22,24a above the mixing chamber 36. Thereby
the jetting concentration of abrasive particles in the
jet 19, which depends on the supply concentration, is
modulated.
The modulation means 14 is therefore arranged along
the passageway 20,22,24 of the drill string, in the
embodiment of Figure 1 at the upper end or just upstream
of the abrasive jet drill head 16.
In the embodiment of Figure 1 the modulation is
obtained in the collar 13, several embodiments of which
are shown in Figures 2-6.
In the embodiment of Figure 2, the collar 13, which
has a non-magnetic housing 111, has an upper inlet 112,
for fluid communication with the passage 20 of the upper
part of the drill string and a lower outlet 113 for fluid
communication with the passage 24 of the abrasive jet
drill bit. Between this inlet and outlet, the through
going channel 22 extends. Alongside this through going
channel 22, two pairs of electric holdup devices,
electromagnets 115,116, are positioned. Each pair of
electromagnets 115,116 is situated along a somewhat
obliquely oriented part of the through going channel 22.
The modulation of the concentration of abrasive particles
which pass through this through going channel 22 is
obtained by controlled activation and de-activation of
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these electromagnets. This is obtained by starting or
stopping the current which is fed to the electromagnets.
The current can also be varied more smoothly and/or
between various non-zero values, but the latter is less
preferred as a constant energizing from a power source
would be required.
Thus, at collection surfaces at the wall of the
passage 22 the magnetic field is modulated between a
first value, at which at least some magnetic particles
are collected from the fluid mixture at the collection
surface, and a second value, at which magnetic particles
are released into the fluid mixture from the collection
surface. The first value is a higher value and the second
value is a lower value of the magnetic field. It will be
understood that particles on the collection surface can
move along the surface under the influence of the
streaming fluid, but in periods of higher magnetic field
strength more particles are present on the collection
surface that in periods of lower or zero magnetic field
strength. Only one collection surface is indicated at
114. Preferably, magnetic particles are collected by a
substantial part of the support surface, preferably at
least 25% of the area or the support surface, more
preferably at least 50% of the area, even more
preferably at least 75% of the area, such as
substantially all area, when the magnetic field is at its
first value. Preferably, magnetic particles are released
from substantial part of the support surface, preferably
at least 25% of the area or the support surface, more
preferably at least 50% of the area, even more
preferably at least 75% of the area, such as
substantially all area, when the magnetic field is at its
second value. Preferably, when the magnetic field is at
the first value, no or only an insignificant part of
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magnetic particles is re-entrained by fluid streaming
along the support surface, such as less than 25% of the
particles collected on the support surface are re-
entrained then during a period at which the magnetic
field is at its first value.
In the embodiment shown in Figures 3A, B, permanent
magnets 117, 118 have been applied as magnetic holdup
devices alongside the through going channel 22. These
magnets can be swung around the swing axis 123, so as to
obtain the different positions shown in Figure 3A and in
Figure 3B. In the position shown in Figure 3A, the
magnets 117, 118 are swung away from the through going
channel 22, which means that a relatively large part of
the paramagnetic abrasive particles will pass along the
passage 22. Conversely, in the position shown in Figure
3B, the magnets 117, 118 are swung towards and against
the collection surfaces 114 at the wall of the through
going channel 22, in such a way that the magnetic field
at the collection surfaces is higher and a portion of the
magnetic abrasive particles are collected at said
surfaces. By changing between the position is shown in
Figures 3A, B, such as by means of a mechanic actuator
(not shown), modulating effects can be obtained in that
the drilling fluid flow will show different
concentrations of abrasive particles.
The embodiment of Figure 4 contains an electromagnet
119, which is connected to a so-called paramagnetic
collector 120, together forming a magnetic holdup device.
Furthermore, the box 121 contains a battery or connection
to external power source and an electronic control unit.
Under the influence of the control unit, the
electromagnet is energized by the battery/power source in
such a way that the magnetic field at the paramagnetic
collector becomes strong enough so that magnetic abrasive
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particles will collect on the paramagnetic collector 20.
By varying the activation of the electromagnet, the
concentration of the magnetic particles in the through
going channel 14 can be varied so as to obtain the
modulation required. The paramagnetic collector can
comprise a permanent magnet providing a relatively low
magnetic field strength, which is just insufficient to
collect particles at the collection surface 114, in this
case surrounding the collector 120. So a moderate
increase of field strength at the collection surface
provided by electromagnet 119 is sufficient for
collection.
The embodiment shown in Figures 5A, B comprises a
permanent magnet 119a, which by means of the magnetic
connector 122 can be disconnected (Figure 2A) from or
connected (Figure 2B) two the paramagnetic collector 120,
together forming a magnetic holdup device. By changing
between the disconnected and the connected state of the
connector 22, the concentration of the abrasive particles
in the through going channel 14 can be varied.
The embodiment shown in Figures 6A, B contains a
permanent magnet 119a which by means of the connectors
122 can be connected to the double paramagnetic
collectors 120, together forming a magnetic holdup
device. The area or annulus 124 on the outside of the
collectors 120 should be large enough to collect a
sufficiently large volume of abrasive particles, so as to
significantly vary the concentration of the abrasive
particles which is fed to the bit within a single
rotation of the string. Instad of permanent magnet 119a a
selectively energizable electromagnet can be used (not
shown).
Instead of moving paramagnetic connectors in Figures
5 and 6, the permanent magnets themselves could be moved
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up and down to connect/disconnect with the paramagnetic
collector. Alternatively or in addition, only one of the
two paramagnetic collectors 120 shows can be arranged. It
might also be advantageous to mount an additional
magnetic guide (not drawn) parallel to the magnet in
order to form a magnetic shortcut with the open
connectors. This is to avoid that when the connectors are
open the magnetic field coming from the open end of the
connectors disturb the particle flow in the surrounding
annulus.
So, modulation can be e.g. obtained by selectively
energizing electromagnets, by physical movement of the
permanent magnets to or from the collection surface, by
repositioning a magnetic conductor between the magnet and
the wall of the fluid channel, and/or by creating (or
removing) a magnetic shortcut at the magnet(s). If
possible, the magnets are preferably permanent magnets,
e.g. rare earth permanent magnets like NdFeB, or SmCo
magnets in order to avoid the continuous supply of
current to the magnets when they have to be activated.
Advantages of the embodiments of Figures 5 and 6 are
that only one chamber with electronics is required; the
fluid pass through area at the height of the paramagnetic
collector is large which reduces the average fluid and
particle velocity around the device; only the switching
between activation or deactivation of the collector
requires power and not the (de-)activation itself. In the
embodiment of Figure 5 the magnetic holdup device is
coaxial with the housing. This can be advantageous for
manufacturing and for connecting to other (electronic)
devices like surveying sensors and electronic control
units.
The design and material of the magnet, the magnetic
connector and the collector should match each other so
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that magnetic flux is not wasted and the strength of the
magnetic field on the outside of the collector is big
enough to collect temporarily the ferromagnetic particles
in the drilling fluid stream along the collector. The
collection surface should be large enough to collect a
sufficiently large volume of abrasive particles to be
able to significantly vary the concentration of the
particles to the bit within one rotation of the string.
In an illustrative example, the rotation of the
string can typically take 1 sec. In the case a downhole
recirculation device is used, the 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.
Each 100 litres of drilling fluid pumped to the bit per
minute then contains up to 0.017 litres/s of abrasives. A
collector for a particle flow rate of 1 vol% at 200 L/min
fluid flow rate and a 1 sec modulation typically needs to
be able to collect for 0.017 L during 0.5 sec and release
it during the other half of the second.
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%. The 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
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the abrasive jet drill head excavating tool, is typically
between 0.3 and 3 Hz.
The intended bending radius of the drilled trajectory
can be increased by modulating not continuously, but, for
instance, only two or every three subsequent rotations.
In order to obtain directional control, the
activation and deactivation of the collector are suitably
triggered by a measurement of the angular orientation of
the jet nozzle of the drilling bit with respect to the
desired drilling direction. The power for the
(de-)activation could e.g. come from a down hole battery
pack or a turbine generator or a combination thereof. In
an example bottom hole configuration, a battery pack or
turbine, control unit and memory, and a sensor package,
are arranged, e.g. in this order, between the modulation
means (cf. 14 in Figure 1), and the mixing location 37,
either all integrated in an abrasive jet drill head or in
several connected components. It is however also possible
that no recirculation system is arranged.
It can be desired to avoid release of all collected
particles at the same time, i.e. to avoid a spike. To
this end it can be considered to use:
-a modulation means comprising more than one
modulation device in series with each other that have
deactivation times that are chosen such that there is a
time difference between the arrival time of the released
particle batches at the jet nozzle; for example, the
collection surface closest to the bit could be
deactivated first and possibly also be activated first;
- a by-pass channel that ensures that a constant
fraction of the particles in the flow reaches the
abrasive jet drilling device;
-a pulsed release of particles from the collector; by
deactivating the collector only very briefly the
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particles that were released at the top of the collector
will be recollected, and this way a few pulses with
varying number of particles can be released within a
short time frame;
-design of the collector such that it saturates after
a fraction such as a quarter of the duration of the
modulation cycle and releases particles half a cycle
later duration of the modulation cycle later;
-a combination of the options mentioned above.
A variation in the amount of magnetic flux that goes
from a selected magnet, e.g. 119, 119a, to the
paramagnetic collector. By varying the contact area
between the magnetic connector and the magnet or the
connector and the collector the flux to the collector can
be regulated and thereby the number of particles that can
be collected by the collector.
Preferably, the toolface and direction of the drill
head are measured close to the bit and may require the
measurement of the earth magnetic field. To avoid
influence of the magnet(s) in the modulator on this
measurement it is preferred to have the modulator at a
distance of typically at least 1 meters away from the
magnetic sensors. To match the modulation of the abrasive
concentration with the orientation of the abrasive jet
nozzle, i.e. the toolface of the abrasive jet dril head,
the timing of the modulation has to compensate for the
travel time of the particles from the modulator to the
bit. In the case of stationary permanent magnets in both
the modulator and the abrasives recycling device the
relative position of the magnets with respect to the
magnetic sensors only changes by the bending of the
assembly and by its rotation. These effects can largely
be eliminated by a calibration.
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If it is found that the passage of the particles
along the magnetic field sensors disturbs their
measurement, it can be considered to do a magnetic field
measurement after activating de collector and before the
collector is saturated with abrasive particles. Again, a
time delay (in this case for the particles to travel from
the collector to the magnetic sensors) should be taken
into account.
The correlation between the particle concentration
and drill string rotation can be arranged by taking into
account several parameters. First of all, the rotational
position of the drill bit is of importance. Furthermore,
the rotational speed of the drill bit plays a role. Also,
by measuring the flow rate or by calibrating the system
for a specific flow rate the travel time of the particles
between their time of release and their time of arrival
at the drill bit may be corrected for.
When the modulation means is accommodated in the
drill string above the abrasive jetting drill head, it is
possible to deliver a supply flow of drilling fluid with
modulated abrasive particle concentrations so as to
obtain a certain desired eroding effect at the downhole
bottom. Therefore, it is no longer necessary to rely on
the modulation of the downhole recirculation circuit.
In one embodiment the modulation means is contained
in a collar positioned in the drill string, said collar
having a through going channel along which the modulation
means is positioned. Said collar can be positioned at
specific desired positions along the drill string. Best
results of the modulation effect are however obtained in
case the collar is positioned close to the drill bit.
The modulation means may comprise at least one magnet
as well as activating means for influencing the magnetic
field of the magnet(s) at the location of and outside the
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through going channel. By properly varying the magnetic
field, the paramagnetic abrasive particles are influenced
in such a way that concentration differences can be
obtained. Thus, the drill bit is supplied with a drilling
fluid flow having varying concentrations of abrasive
particles over time, without the necessity to recirculate
the abrasive particles downhole.
The magnets and the through going channel may be
positioned according to several possibilities. For
instance, the through going channel may comprise an
annulus, in which case the magnet is positioned within
the area surrounded by the innermost wall of the annulus.
According to yet another possibility, the modulation
means may comprise at least two magnets which are
positioned on opposite sides of the through going
channel.
With the aim of obtaining the required modulation, it
is possible to apply a magnetic conductor which is
displaceable between the magnet and the through going
channel. The magnetic conductor influences the magnetic
field experienced by the abrasive particles, depending on
the position of said conductor. Other possibilities exist
as well. For instance, the magnet(s) may comprise at
least one permanent magnet and the activating means may
comprise a mover. Said mover is carried out for
displacing the permanent magnet with respect to the
through going channel. The physical movement of the
magnet with respect to the wall of the through going
channel makes that the magnetic field experienced by the
abrasive particles passing by in the through going
channel varies, such that some particles are held and
other particles are passed through. This different
behavior of the abrasive particles in the through going
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channel evokes a modulation of the particle concentration
in the drilling fluid.
The magnet(s) may be mounted in a rotatable fashion
between a position parallel to the through going channel
and an oblique position with respect to the through going
channel. Other displacement mechanisms are possible as
well, such as slides.
According to a further alternative, the collar may be
provided with a paramagnetic collector in which case the
mover is operated for establishing respectively breaking
a contact between the magnet(s) and the collector.
Further, the magnet may comprise an electromagnet. By
energizing respectively de-energizing such electromagnet,
the modulation effect is obtained.
The invention is also related to a method for
operating the abrasive jet drilling described before,
comprising the step of varying the operation of the
activation device so as to obtain a controlled attraction
and/or release of the paramagnetic abrasive particles. In
this way, batches of abrasive particles may be generated
in the drilling fluid flow which provide a pulsating
effect.
According to a further possibility, the method
according to the invention may comprise the step of:
-operating the modulation means so as to hold an
amount of particles within the through going channel,
-subsequently making the modulation means operate in
a pulsating manner so as to release a part and only a
part of the abrasive particles held within the through
going channel during each pulsation.
By means of a quick pulsation, some of the particles
will be released and also be attracted again, depending
on the duration of the pulses. In this way, a limited
amount of abrasive particles is freed each time,
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resulting in batches the size of which can be determined
accurately.
Alternatively, the method according to the invention
may comprise the step of:
-providing at least two modulation means in series,
-activating said modulation means so as to hold a
batch of abrasive particles each,
-de-activating said modulation means such that a time
difference is obtained in the arrival time at the drill
bit of said batches of abrasive particles.
As an example, the modulation means which is closest
to the drill bit can be de-activated before the other
modulation means is de-activated.
Furthermore, the method according to the invention
may comprise the steps of:
-measuring the flow rate of the flow mixture or
calibrating the drilling system for a specific flow rate,
-applying a correction for the time of travel of the
particles from the time of release thereof by the
modulation means and the time of arrival thereof at the
drill bit.
Down hole power systems used in conjunction with the
present invention can extract power from the pressurised
drilling fluid stream. Only a small fraction of the
hydraulic energy present in the fluid circulating through
the hole, typically less than 5 % needs to be extracted.
Thus, the generator can be made much smaller than, for
instance, a down hole turbine or positive displacement
motor (PDM) that aims at converting a large fraction of
the available energy for driving a conventional drill
bit.
A first type of down hole power system comprises an
electric generator drivable by the drilling fluid flow
for instance by means of a turbine or a PDM section. The
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electric power generated can be supplied to an electric
motor. The electric motor 23 may be controlled by an
electronic control system.
More than one turbine/generator module can be mounted
in series in order to convert the required power. This
can improve the directional flexibility of the down hole
power system, because such modular approach can be
constructed mechanically less stiff than a non-modular
turbine assembly with a similar power rating.
A second, alternative, type of down hole power system
comprises a passive hydraulic motor, such as for instance
a turbine or a positive displacement motor (PDM) section,
drivable by the drilling fluid flow. Means are provided
for controlling the power on the output shaft. Such means
can be provided in the form of flow control means
controlling the flow of drilling fluid through the
passive hydraulic motor, such as an adjustable valve.
Alternatively, a generator can be mounted around the
output shaft and act as a controlled brake that is
electronically adjustable by adjusting the load in the
generator circuit. The electronically adjustable valve or
load may be controlled by an electronic control system.
The erosive power of the abrasive jet with the
abrasive jet can be modulated via an electronic control
system. The electronic control system may be arranged to
receive a signal indicative of the position of the
impingement area of the abrasive jet along its trajectory
on the bottom of the borehole, which it can then use to
modulate the erosive power of the abrasive jet in
dependence on the position along the trajectory. The
signal can be received directly from a downhole
positional sensor located in the vicinity of the abrasive
jet drill head. The positional sensor can suitable be
housed together with the electronic control system.
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The electronic control system may include an
electronic memory module that stores data including one
or more of motor voltage, current, rotational frequency,
temperature and other data. A selection of this data may
be transmitted to the surface via a measurement while
drilling MWD system when provided. Such measurement while
drilling system can be electronically connected to the
electronic control system by means of a male stabber. The
electronic control system may be programmable, such that
selected conditions can be maintained or achieved. Any
electronic components can be placed in an atmospheric
chamber or a pressure-balanced chamber.
