AU2010334861A1

AU2010334861A1 - Method of drilling and jet drilling system

Info

Publication number: AU2010334861A1
Application number: AU2010334861A
Authority: AU
Inventors: Jan-Jette Blange; Pieter Van Nieuwkoop
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2009-12-23
Filing date: 2010-12-22
Publication date: 2012-07-05
Anticipated expiration: 2030-12-22
Also published as: CN102667047B; CN102667047A; CA2784545A1; BR112012015440A2; AU2010334861B2; EP2516788A1; US20120255792A1; WO2011076845A1

Abstract

Method of drilling, comprising providing a drill string (1) in a borehole (2), the drill string comprising an abrasive jet drill head (16) including a jet nozzle (18), the drill string providing a passageway (20) for fluid to the jet nozzle; supplying a supply fluid mixture via the drill string to the abrasive jet drill head comprising a supply concentration of abrasive particles; generating an abrasive jet (19) of a jetting fluid mixture comprising a jetting concentration of abrasive particles at the jet nozzle, so as to blast with an erosive power on an impingement area of the borehole, wherein the jetting concentration depends on the supply concentration; and modulating (14) the jetting concentration of the abrasive particles; wherein the supply concentration of abrasive particles in the supply fluid mixture is modulated in order to modulate the jetting concentration in the abrasive jet.

Description

WO 2011/076845 PCT/EP2010/070490 METHOD OF DRILLING AND JET DRILLING SYSTEM 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. 5 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 10 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 15 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 20 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 25 with fresh fluid from surface. 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 30 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 WO 2011/076845 PCT/EP2010/070490 -2 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 5 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. 10 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 is obtained independent of whether a recirculation system with moving parts is provided. 15 To this end the present invention provides 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 20 drill string providing a passageway for fluid to the jet nozzle; - supplying a supply fluid mixture via the drill string to the abrasive jet drill head, wherein the supply fluid mixture received at the abrasive jet drill head comprises 25 a supply concentration of abrasive particles; - generating an abrasive jet of a jetting fluid mixture comprising a jetting concentration of abrasive particles at the jet nozzle, so as to blast with an erosive power on an impingement area of the borehole, wherein the 30 jetting concentration depends on the supply concentration; - moving the impingement area of the abrasive jet along a selected trajectory in the borehole; and WO 2011/076845 PCT/EP2010/070490 -3 - modulating the jetting concentration of the abrasive particles while the impingement area is being moved; wherein the supply concentration of abrasive particles in the supply fluid mixture is modulated in order to 5 modulate the jetting concentration in the abrasive jet. By modulating the supply concentration of abrasive particles in the supply fluid mixture that is received via the drill string, rather than modulating a recycle rate of a recirculation system, the modulation becomes 10 independent of the presence and precise function of a recirculation system. When a downhole recirculation system is provided, to recirculate abrasive particles after their discharge from the jet nozzle to a mixing location with the supply fluid 15 mixture along the passageway, the supply concentration suitably is modulated upstream of the mixing location. It has been found that a modulation applied to the supply concentration can be passed on to the abrasive jet at the jet nozzle after mixing with recirculated abrasive 20 particles, even though the jetting concentration of the fluid mixture blasted by the jet nozzle is, averaged over time, higher than the supply concentration. Preferably, when a downhole recirculation system is provided, it is not used for modulating the recycle rate. 25 Suitably the jetting concentration is controlled so as to be modulated in relation with the position of the impingement area on the selected trajectory. Suitable modulation correlated with the moving of the impingement area, e.g. by rotation, allows preferential erosion in 30 selected parts of the borehole and therefore directional drilling action. In particular, the jetting concentration is controlled by controlling the supply concentration. If a recirculation system is present, its recycle rate is WO 2011/076845 PCT/EP2010/070490 -4 preferably not controlled or controllable by a control system during normal operation. In one embodiment, the position of the jet nozzle along the trajectory is determined, and used in order to 5 control the supply concentration. This allows to trigger the blasting of jetting fluid with a selectively increased, or decreased, jetting concentration at one or more selected positions along the trajectory. In one preferred embodiment the moving of the 10 impingement area comprises rotating the jet nozzle, preferably together with the abrasive jet drill head, more preferably together with the drill string. An eccentric jet nozzle blasting obliquely into an off-axis direction with respect to rotary axis then conducts a 15 circular motion of the impingement area. The jet nozzle can be rotated at a selected rotational frequency, and the supply concentration be modulated at a modulation frequency that is the same or an integer fraction of the rotational frequency. In this way a single jet nozzle can 20 blast an increased (or decreased) jetting concentration of abrasive particles at a certain angular position in the borehole. In a preferred embodiment the abrasive particles comprise magnetic abrasive particles, and the supply 25 concentration is modulated by modulating a magnetic field in the passageway of the drill string. Preferably modulating the supply concentration of magnetic particles in the supply fluid mixture comprises modulating a magnetic field at a collection surface arranged along the 30 passageway between a first value, 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 WO 2011/076845 PCT/EP2010/070490 -5 collection surface. Such modulation can be implemented with no or a minimum of moving parts. In accordance with the invention there is further provided an abrasive jet drilling assembly connectable to 5 a tubular drill string part, and comprising an abrasive jet drill head with a jet nozzle, a passageway for fluid from a connected tubular drill string part to the jet nozzle, and a modulation means for modulating the concentration of abrasive particles in fluid flowing, 10 during operation, from the tubular drill string part through the fluid passageway towards the jet nozzle of the abrasive jet drilling bit, wherein the modulation means is arranged along the fluid passageway. Such abrasive jet drilling assembly can be equipped without a 15 downhole recirculation system. If the abrasive jet drilling assembly further does 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, 20 wherein the modulation means is arranged along the passageway upstream of the mixing location. By arranging the modulation means along the fluid passageway, i.e. so that it has effect there and not in a return path of a recirculation system, the modulation 25 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 30 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.

WO 2011/076845 PCT/EP2010/070490 -6 The tubular drill string part can e.g. be 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 5 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 10 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 drill 15 string 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 20 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. 25 The modulation means can be provided just upstream of the abrasive jet drill head or as part of a bottomhole assembly. In one embodiment the abrasive jet drilling assembly further comprises a control unit for controlling the 30 jetting concentration 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 WO 2011/076845 PCT/EP2010/070490 -7 recycle rate, and/or the control means is not adapted to control the recycle rate of the recirculation system. In one embodiment the modulation means comprise a collection surface, a magnetic holdup device for 5 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 the collection 10 surface, and a second value, at which magnetic particles are released from the collection surface. 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 15 modulation control means can be arranged to control the magnetic holdup device in dependence on the detected angular orientation. In one embodiment, the magnet arrangement comprises at least one electromagnet, and preferably the modulation 20 control means is arranged to modulate a drive current of the electromagnet. In one embodiment the magnet arrangement comprises at least one permanent magnet, preferably wherein the at least one permanent magnet is movable with respect to the 25 collection surface. Movable means that the permanent magnet can perform a translational and/or rotational motion with respect to the collection surface, so as to change the magnetic field at that surface. In particular the the magnetic holdup device can comprises an actuator 30 for changing the relative position of the magnet arrangement and the collection surface. In one embodiment the magnet arrangement further comprises a selectively movable magnetic connector. A magnetic connector is a material that guides field lines WO 2011/076845 PCT/EP2010/070490 -8 from a magnet preferentially to an area at a distance, and can in particular provide a shortcut to a distant paramagnetic or ferromagnetic object such as a collection surface. The magnetic connector is of a paramagnetic or 5 ferromagnetic material. By using a magnetic connector the effect of a magnet, in particular of a permanent magnet can be guided some distance to another area, and this connection can easily be changed or modulated by moving the magnetic connector, not requiring to move the entire 10 magnet. 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 15 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 20 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 25 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 30 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 WO 2011/076845 PCT/EP2010/070490 -9 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 5 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 10 operated so as to hold an amount of particles within the 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 15 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 20 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 25 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 30 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 WO 2011/076845 PCT/EP2010/070490 - 10 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. 5 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 10 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 a collar 13 comprising a modulation means 15 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 20 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 25 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 30 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 WO 2011/076845 PCT/EP2010/070490 - 11 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 5 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 210 which is optimal for abrasively eroding the bottom of the bore hole by axially rotating the complete tool inside the 10 bore hole. 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 15 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 20 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 25 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 30 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.

WO 2011/076845 PCT/EP2010/070490 - 12 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 5 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. 10 When the erosive power is modulated, in particular by modulating the supply concentration of abrasive particles, a modulation that is not synchronized with the moving of the impingement agrea of the jet such as by rotation, is not going to give rise to a directional 15 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 20 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 25 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 30 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.

WO 2011/076845 PCT/EP2010/070490 - 13 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. 5 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 10 can be modulated so as to obtain a pulsating effect. That is to say, at each full rotation of the drill string 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 15 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 20 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 25 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 30 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 WO 2011/076845 PCT/EP2010/070490 - 14 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 5 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 10 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 15 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 20 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 25 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. 30 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 WO 2011/076845 PCT/EP2010/070490 - 15 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 5 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 10 signal can be sent to the down hole control means 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 15 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 20 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. 25 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 30 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 WO 2011/076845 PCT/EP2010/070490 - 16 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 5 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 10 the formation. 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 15 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 20 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 25 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, schematically shown at 30, and arranged to recirculate at least a part of the abrasive particles 30 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 WO 2011/076845 PCT/EP2010/070490 - 17 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. 5 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 10 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 15 recirculation means are for example described in WO 2002/034653, WO 2005/005766, W02008/119821, WO 2008/113844. A supply fluid mixture comprising a supply concentration of abrasive particles is supplied via the 20 drill string, in particular via passage 22 communicating with the passage 24 to the abrasive jet drilling head 16. In accordance with the invention the supply concentration of abrasive particles is modulated, upstream of the mixing location 37, i.e. in the passageway 20,22,24,24a 25 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 30 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.

WO 2011/076845 PCT/EP2010/070490 - 18 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 5 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 10 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 15 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 20 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 25 surface, and a second value, at which magnetic particles are released into the fluid mixture from the collection surface. 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 30 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.

WO 2011/076845 PCT/EP2010/070490 - 19 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 5 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 or all of the magnetic abrasive particles will pass along 10 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 15 or all 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 20 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 25 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 30 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 22 can be varied so as to obtain the modulation required. The paramagnetic collector can WO 2011/076845 PCT/EP2010/070490 - 20 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 5 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 10 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 15 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 20 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 25 rotation of the string. Instead 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 30 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 WO 2011/076845 PCT/EP2010/070490 - 21 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 5 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 10 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, SmCo, AlNiCo-5, or a combination thereof, in order to avoid the 15 continuous supply of current to the magnets when they have to be activated. Preferably the magnet also has a magnetic energy content of at least 140 kJ/m3 at room temperature, preferably more than 300 kJ/m3 at room temperature such as is the case with NdFeB-based magnets. 20 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 25 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) 30 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 that magnetic flux is not wasted and the strength of the WO 2011/076845 PCT/EP2010/070490 - 22 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 5 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 10 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 15 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. 20 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 25 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 can for 30 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.

WO 2011/076845 PCT/EP2010/070490 - 23 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 5 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 10 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, 15 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 20 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 25 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 30 abrasive jet drilling device; -a pulsed release of particles from the collector; by deactivating the collector only very briefly the particles that were released at the top of the collector will be recollected, and this way a few pulses with WO 2011/076845 PCT/EP2010/070490 - 24 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 5 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 10 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. 15 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 20 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 25 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 30 assembly and by its rotation. These effects can largely be eliminated by a calibration. 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 WO 2011/076845 PCT/EP2010/070490 - 25 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 5 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, 10 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. 15 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 20 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 25 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 30 as well as activating means for influencing the magnetic field of the magnet(s) at the location of and outside the through going channel. By properly varying the magnetic field, the paramagnetic abrasive particles are influenced in such a way that concentration differences can be WO 2011/076845 PCT/EP2010/070490 - 26 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. 5 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. 10 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 15 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 20 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 25 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 30 behavior of the abrasive particles in the through going 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 WO 2011/076845 PCT/EP2010/070490 - 27 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 5 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, 10 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 15 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 20 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 25 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 30 amount of abrasive particles is freed each time, resulting in batches the size of which can be determined accurately. Alternatively, the method according to the invention may comprise the step of: WO 2011/076845 PCT/EP2010/070490 - 28 -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 5 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. 10 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 15 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 20 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 25 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 30 for instance by means of a turbine or a PDM section. The electric power generated can be supplied to an electric motor. The electric motor 23 may be controlled by an electronic control system.

WO 2011/076845 PCT/EP2010/070490 - 29 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 5 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, 10 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. 15 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. 20 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 25 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 30 jet drill head. The positional sensor can suitable be housed together with the electronic control system. The electronic control system may include an electronic memory module that stores data including one or more of motor voltage, current, rotational frequency, temperature WO 2011/076845 PCT/EP2010/070490 - 30 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 5 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.

Claims

1. A method of drilling into an object, the method comprising - providing a drill string in a borehole in the object, 5 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 to the jet nozzle; - supplying a supply fluid mixture via the drill string 10 to the abrasive jet drill head, wherein the supply fluid mixture received at the abrasive jet drill head comprises a supply concentration of abrasive particles; - generating an abrasive jet of a jetting fluid mixture comprising a jetting concentration of abrasive particles 15 at the jet nozzle, so as to blast with an erosive power on an impingement area of the borehole, wherein the jetting concentration depends on the supply concentration; - moving the impingement area of the abrasive jet along a 20 selected trajectory in the borehole; and - modulating the jetting concentration of the abrasive particles while the impingement area is being moved; wherein the supply concentration of abrasive particles in the supply fluid mixture is modulated in order to 25 modulate the jetting concentration in the abrasive jet.

2. Method according to claim 1, wherein a downhole 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 30 passageway, and wherein the supply concentration is modulated upstream of the mixing location. WO 2011/076845 PCT/EP2010/070490 - 32

3. Method according to claim 1 or 2, wherein the jetting concentration is controlled so as to be modulated in relation with the position of the impingement area on the selected trajectory. 5

4. Method according to claim 3, wherein the position of the jet nozzle along the trajectory is determined, and used in order to control the supply concentration.

5. Method according to any one of claims 1-4, wherein moving the impingement area comprises rotating the jet 10 nozzle.

6. Method according to claim 5, wherein the jet nozzle is rotated at a selected rotational frequency, and wherein the supply concentration is modulated at a modulation frequency that is the same or an integer fraction of the 15 rotational frequency.

7. Method according to any one of claims 1-6, wherein the abrasive particles comprise magnetic abrasive particles, and wherein the supply concentration is modulated by modulating a magnetic field in the passageway. wherein 20 modulating the supply concentration of magnetic particles in the supply fluid mixture comprises modulating a magnetic field at a collection surface arranged along the passageway between a first value, at which magnetic particles are collected from the fluid mixture at the 25 collection surface, and a second value, at which magnetic particles are released into the fluid mixture from the collection surface.

8. Abrasive jet drilling assembly connectable to a tubular drill string part, and comprising an abrasive jet 30 drill head with a jet nozzle, a passageway for fluid from a connected tubular drill string part to the jet nozzle, and a modulation means for modulating the concentration of abrasive particles in fluid flowing, during operation, from the tubular drill string part through the fluid WO 2011/076845 PCT/EP2010/070490 - 33 passageway towards the jet nozzle of the abrasive jet drilling bit, wherein the modulation means is arranged along the fluid passageway, and, if the abrasive jet drilling assembly further 5 comprises 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, wherein the modulation means is arranged along the passageway upstream of the mixing 10 location.

9. Abrasive jet drilling assembly according to claim 8, wherein the modulation means is integrated with the abrasive jet drill head.

10. Abrasive jet drilling assembly according to claim 8, 15 wherein the modulation means is arranged to form part of a drill string above the abrasive jet drill head.

11. Abrasive jet drilling assembly according to any one of claims 8-10, wherein the modulation means comprise a collection surface, 20 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 25 the collection surface, and a second value, at which magnetic particles are released from the collection surface.

12. Abrasive jet drilling assembly according to claim 10, further comprising a measurement device for detecting the 30 angular orientation of the jet nozzle in the borehole, and wherein the modulation control means is arranged to control the magnetic holdup device in dependence on the detected angular orientation. WO 2011/076845 PCT/EP2010/070490 - 34

13. Abrasive jet drilling assembly according to claim 11 or 12, wherein the magnet arrangement comprises at least one electromagnet, preferably wherein the modulation control means is arranged to modulate a drive current of 5 the electromagnet.

14. Abrasive jet drilling assembly according to claim 11 or 12, wherein the magnet arrangement comprises at least one permanent magnet, preferably wherein the at least one permanent magnet is movable with respect to the 10 collection surface.

15. Abrasive jet drilling assembly according to any one of claims 8-14, further comprising a control unit for controlling the jetting concentration in relation with a position of an impingement area of the abrasive jet drill 15 head during operation.