METHOD AND SYSTEM FOR DIRECTIONAL DRILLING

Abstract

The invention relates to variation of concentrations of abrasive particles in a stream of drilling fluid mixed with abrasive particles, passed as an abrasive jet through abrasive nozzle(s) of a drill bit along rotations thereof, to vary the erosive power of the stream along angular sections of the borehole for directional drilling. During subsequent time periods majorities of abrasive particles are alternately deflected into two parallel channels with a different flow resistance. A resulting velocity difference between said majorities makes that the subsequently deflected majorities recombine downstream of the channels, so that high concentration stream portions of the combined majorities are formed which alternate low concentration stream portions. Synchronising the frequency of the stream portions with the rotational velocity of the drill bit results in a consistently higher erosive power of the abrasive jet within a selected angular section of the borehole than outside thereof.

Claims

1. A method for directional drilling of a borehole with a borehole bottom in an object, e.g. an earth formation, e.g. a subterranean earth formation, the method comprising: providing a drill bit, said drill bit being connected to a lower end of a drill string and comprising: a bit face, which during use faces the borehole bottom, one or more abrasive jet nozzles configured for directing a stream of drilling fluid mixed with abrasive particles into impingement with the borehole bottom in the form of an abrasive jet, which one or more abrasive jet nozzles, if in plural, are arranged at different adjacent azimuthal positions, an intermediate space between a bit fluid inlet port of the drill bit and said one or more abrasive jet nozzles, each of the one or more abrasive jet nozzles having a nozzle inlet for fluid communication with the intermediate space, from which each of the nozzle inlets extends; upstream of said bit fluid inlet port, passing the stream of drilling fluid mixed with abrasive particles through a supply channel having an supply channel outlet at a substantially constant supply velocity, simultaneously, rotating the drill bit, and thereby the one or more abrasive jet nozzles, at a rotational velocity and passing said stream of drilling fluid mixed with abrasive particles via the supply channel outlet and the bit fluid inlet port consecutively through the intermediate space, the one or more nozzle inlets, and the one or more abrasive jet nozzles into impingement with the borehole bottom, so as to deepen the borehole; and during said rotating of the drill bit while passing of the stream of drilling fluid mixed with abrasive particles, varying concentrations of said abrasive particles along subsequent stream portions of said stream flowing through the abrasive jet nozzles of the drill bit, such that alternatingly the concentration of abrasive particles is high in a first stream portion and low in a subsequent second stream portion, characterized in that said varying of the concentrations of the abrasive particles in the stream of drilling fluid mixed with abrasive particles comprises: upstream of said bit fluid inlet port, passing said stream from the supply channel outlet subsequently, in parallel through a first channel and a second channel to first and second outlets thereof, respectively, and from the first and second outlets into the bit fluid inlet port, while, alternatingly, during a first time period, deflecting into the first channel a majority of all abrasive particles in the stream that pass through the supply channel outlet, and during a second time period, following the first time period, not deflecting into the first channel a majority of all abrasive particles in the stream that pass through said supply channel outlet, and subsequently passing said stream from the first and second outlets into said one or more abrasive jet nozzles, wherein the difference between a flow resistance to said drilling fluid mixed with abrasive particles in said first channel and a flow resistance to said drilling fluid mixed with abrasive particles in said second channel results in a difference between a first velocity at which said drilling fluid mixed with abrasive particles flows through said first channel and a second velocity at which said drilling fluid mixed with abrasive particles flows through said second channel, wherein said difference between said first and second velocities is such that, downstream of the first and second outlets, the majority deflected into the first channel during the first time period and the abrasive particles passed into the second channel during the second time period, together with drilling fluid passed into the first and second channel during the first and second time period, respectively, are combined to form the first stream portion, and the abrasive particles passed into the first channel during the second time period and the abrasive particles passed into the second channel during a first time period following the second time period together with drilling fluid passed into the first and second channel during the second time period and a first time period following the second time period, respectively, are combined to form the second stream portion.

2. Method according to claim 1, wherein said varying of the concentrations of the abrasive particles in the stream of drilling fluid mixed with abrasive particles comprises: during the first time period, deflecting into the first channel a first majority of all abrasive particles in the stream that pass through the supply channel outlet, and during the second time period following the first time period, deflecting into the second channel a second majority of all abrasive particles in the stream that pass through said supply channel outlet, and subsequently passing said stream from the first and second outlets into said one or more abrasive jet nozzles, wherein the difference between a flow resistance to said drilling fluid mixed with abrasive particles in said first channel and a flow resistance to said drilling fluid mixed with abrasive particles in said second channel results in a difference between a first velocity at which said first majority flows through said first channel and a second velocity at which said second majority flows through said second channel, wherein said difference between said first and second velocities is such that, downstream of the first and second outlets, the first and second majority deflected into the first channel during the first time period and deflected into the second channel during the second time period, together with drilling fluid passed into the first and second channel during the first and second time period, respectively, are combined to form the first stream portion, and minorities of abrasive particles not being deflected together with drilling fluid passed into the first and second channel during the second and first time period, respectively, are combined to form the second stream portion.

3. Method according to claim 1, wherein the first and second stream portions pass through the one or more abrasive jet nozzles at a frequency synchronized with a rotational velocity of the drill bit, and the first and second time periods are timed such that the first stream portion passes said abrasive jet nozzles while the abrasive jet nozzles are directed towards a selected angular sector of the borehole bottom, and the second stream portion passes said abrasive jet nozzles while the abrasive jet nozzles are not directed towards the selected angular sector of the borehole bottom.

4. Method according to claim 1, wherein the abrasive particles are magnetic abrasive particles, and the deflection into the first and second channel is achieved by alternatingly directing in the first and second time periods a magnetic field over a cross-section of the stream directly upstream of the first and second channel towards the first channel and towards the second channel, respectively, wherein the density of the magnetic field is higher in a part of the cross-section covering the first channel than in a part covering the second channel during the first time period, and higher in the part of the cross-section covering the second channel than in the part thereof covering the first channel during the second time period.

5. Method according to claim 1, wherein said difference between the flow resistance to said drilling fluid mixed with abrasive particles in said first channel and said second channel is established by a difference between said first and second channel in respective lengths thereof in the longitudinal direction, in respective cross sections thereof, in respective surface roughnesses of inner wall surfaces thereof, and/or in variations of the respective cross-sections and/or surface roughness of the inner wall surfaces thereof along said respective lengths thereof.

6. Method according to claim 5, wherein said difference between the flow resistance to said drilling fluid mixed with abrasive particles in said first channel and said second channel is established by a difference between said first and second channel in respective cross sections thereof, and wherein said respective lengths and surface roughness of inner wall surfaces and said variations therein along said respective lengths are equal to each other.

7. Method according to claim 1, wherein the drill bit is a mechanical drill bit further comprising one or more wash nozzles on the bit face, and said rotating of said drill bit involves mechanical cutting of the borehole bottom by said mechanical drill bit to deepen the borehole, the method further comprising, simultaneously with said impingement with the borehole bottom by said stream in the form of said abrasive jet, straining the abrasive particles in said first and second stream portions upstream of said abrasive jet nozzles and said wash nozzles, e.g. within the intermediate space of the drill bit, and deflecting the strained abrasive particles into the abrasive jet nozzles, while passing the drilling fluid in said first and second stream portions of the stream into both the abrasive jet nozzles and the wash nozzles.

8. Method according to claim 1, wherein the method further comprises a downhole recirculation of the abrasive particles passed through the abrasive jet nozzles into impingement with the borehole bottom, said downhole recirculation comprising: capturing at least a part of the abrasive particles present in the stream downstream of said impingement thereof with the borehole bottom, e.g. at a substantially constant flow rate, and passing said captured abrasive particles into said stream upstream of said abrasive jet nozzles, e.g. upstream of said drill bit, e.g. downstream of said first and second channel, e.g. wherein said abrasive particles are magnetic abrasive particles, e.g. a steel shot, and said passing of said abrasive particles into said stream includes the employment of a magnetic field to convey said abrasive particles to the stream.

9. Method according to claim 1, wherein durations and/or timings of said first and second time period are set and/or adjusted, e.g. during said rotating of the drill bit, based on downhole measurements, including one or more of: detection of abrasive particles downstream of said deflection thereof, e.g. directly downstream of the first and second channel and/or at the first and second outlets and/or close to the drill bit, detection of a position, e.g. an azimuthal position, at which said impingement with the borehole bottom takes place, detection of a geometrical direction of the deepening of the borehole.

10. Method according to claim 1, wherein the abrasive particles are magnetic abrasive particles, e.g. a steel shot, wherein the method further comprises: activating a magnetic field in the first and/or second channel during a time interval such as to cause a local accumulation of magnetic abrasive particles in said first and/or second channel that results in a pressure pulse within that channel, and subsequently, deactivating said magnetic field, wherein said activation and deactivation are repeated to create a series of pressure pulses over time, of which the amplitudes and timings are determined such that said series of pressure pulses represents one of said downhole measurements for use in a mud pulse telemetry system, e.g. wherein the method further comprises decoding said series of pressure pulses into values of measured quantities, comparing said values with a predetermined reference values of said quantities, wherein said setting and/or adjusting of durations of said first and second time period are based a result of said comparing.

11. A directional drilling system for directional drilling of a borehole with a borehole bottom in an object, e.g. an earth formation, e.g. a subterranean earth formation, preferably for implementing a method according to claim 1, wherein the drilling system is connectable to a tubular drill string, the directional drilling system comprising: a drill bit, comprising: a bit face, which during use faces the borehole bottom, a bit fluid inlet port, one or more abrasive jet nozzles configured for ejecting a stream of drilling fluid mixed with abrasive particles into impingement with the borehole bottom in the form of an abrasive jet, which one or more abrasive jet nozzles, if in plural, are arranged at different azimuthal positions, and an intermediate space between the bit fluid inlet port and said one or more abrasive jet nozzles, each of the one or more abrasive jet nozzles having a nozzle inlet for fluid communication with the intermediate space, from which each of the nozzle inlets extends; and a sub, connected or connectable at a downhole end thereof to the drill bit, e.g. so as to be rotatable along therewith and at another end thereof to the tubular drill string, the sub comprising: a sub fluid inlet port, fluidly connectable to a supply channel through the drill string to receive from said supply channel the stream of drilling fluid mixed with abrasive particles when the system is connected to the drill string, and a sub fluid outlet port, fluidly connected or connectable to the bit fluid inlet port, characterized in that the sub further comprises, fluidly connected to the sub fluid inlet port, downstream thereof, a modulation unit configured to cause a variation of a concentration of abrasive particles along stream portions of the stream received from the supply channel that are subsequently passed through the sub fluid outlet port into the bit fluid inlet port, the modulation unit comprising: a first channel having a first flow resistance to the drilling fluid mixed with abrasive particles, a first inlet, and a first outlet fluidly connected to the sub fluid outlet port, a second channel arranged in parallel to the first channel, having a second flow resistance to the drilling fluid mixed with abrasive particles, a second inlet, and a second outlet fluidly connected to the sub fluid outlet port (20o), a particle deflection device between the sub fluid inlet port and the first and second inlets, comprising one or more actuators, and being connected to a control unit of the system, wherein the particle deflection device is configured to periodically, preferably based on control signals received from the control unit, during a first time period, deflect into the first inlet a majority of all abrasive particles received from the supply channel through the sub bit fluid inlet port, and during a second time period following the first time period, not deflect into the first inlet a majority of all abrasive particles in the stream received from the supply channel through the sub bit fluid inlet port, wherein the first and second channel are embodied such that a difference between the first flow resistance and the second flow resistance results in a velocity difference between said drilling fluid mixed with abrasive particles passing through said first channel and said drilling fluid mixed with abrasive particles passing through said second channel, wherein said velocity difference is such that in a combination section downstream of the first and second outlets, the majority deflected into the first inlet during the first time period and the abrasive particles passed into the second inlet during the second time period, together with any of said drilling fluid passed into the first and second inlet during the first and second time period, respectively, are combined into one of said stream portions, and abrasive particles passed into the first inlet during the second time period and abrasive particles passed into the second inlet during a first time period following the second time period, together with any drilling fluid passed into the first inlet during the second time period and into the second inlet during a first time period following the second time period, respectively, into a subsequent one of said stream portions.

12. Directional drilling system according to claim 11, wherein the control unit and the particle deflection device are configured to during a first time period, deflect into the first channel a first majority of all abrasive particles in the stream that passes through the supply channel outlet, and during a second time period following the first time period, deflect into the second channel a second majority of all abrasive particles in the stream that passes through said supply channel outlet, and subsequently passing said stream from the first and second outlets into said one or more abrasive jet nozzles, wherein said velocity difference is such that in a combination section downstream of the first and second outlets, the majority deflected into the first channel during the first time period and the second majority passed into the second inlet during the second time period, together with any of said drilling fluid passed into the first and second inlets during the first and second time period, respectively, are combined into one of said stream portions, and abrasive particles passed into the first inlet during the second time period and the abrasive particles passed into the second inlet during a first time period following the second time period, together with any drilling fluid passed into the first inlet during the second time period and into the second inlet during the first time period following the second time period, respectively, into a subsequent one of said stream portions.

13. Directional drilling system according to claim 12, wherein the control unit is configured such that the signals thereof received by the particle deflection device cause the time periods in which the actuators of the particle deflection device deflect said first and second majority of the abrasive particles into the first and second channel to be synchronized with the rotational velocity of the drill bit, and to be timed such, that said one of said stream portions passes through the one or more abrasive jet nozzles while the abrasive jet nozzles are directed towards a selected angular sector of the borehole bottom, together with any of said drilling fluid passed into the first and second channel during the first and second time period, respectively, and said subsequent one of said stream portions passes through the one or more abrasive jet nozzles while the abrasive jet nozzles are not directed towards said selected angular sector of the borehole bottom.

14. Directional drilling system according to claim 11, wherein the abrasive particles are magnetic abrasive particles, e.g. a steel shot, and the actuators of the particle deflection device comprise a magnetic switch, which is configured to during the first time period establish an inhomogeneous magnetic field over a cross-section directly upstream of the first and second inlets that in the plane of said cross-section directs the abrasive particles towards the first inlet and to during the second period establish an inhomogeneous magnetic field over a cross-section directly upstream of the first and second inlets that in the plane of said cross-section directs the abrasive particles towards the second inlet, wherein the density of the magnetic field produced in the first time period is higher in a part of the cross-section covering the first channel than in a part thereof covering the second channel, and the density of the magnetic field produced in the second time period is higher in the part of the cross-section covering the second channel than in the part thereof covering the first channel, e.g. wherein the magnetic switch comprises multiple magnets arranged at different azimuthal positions along an outer circumference of the stream directly upstream of the first and second inlets, e.g. along a circumference of a channel accommodating said stream at that location, the multiple magnets together producing said inhomogeneous magnetic fields.

15. Directional drilling system according to claim 14, wherein the magnets are movable permanent magnets, and the actuators further comprise drive means, connected to the magnets and configured to move, based on the signals received from the control unit, the magnets as a unity along the circumference upon a switch between the respective time periods to establish that said magnetic fields direct the abrasive particles towards the respective channels during the respective time periods.

16. Directional drilling system according to claim 11, wherein said difference between the flow resistance to said drilling fluid mixed with abrasive particles in said first channel and said second channel is established by a difference between said first and second channel in respective lengths thereof in the longitudinal direction, in respective cross-sections thereof, in respective surface roughnesses of inner wall surfaces thereof, and/or in variations of the respective cross sections and/or surface roughness of the inner wall surfaces thereof along said respective lengths thereof.

17. Directional drilling system according to claim 15, wherein said difference between the flow resistance to said drilling fluid mixed with abrasive particles in said first channel and said second channel is established by a difference between said first and second channel in respective cross-sections thereof, said respective lengths and surface roughnesses of inner wall surfaces and said variations therein along said respective lengths being equal to each other.

18. Directional drilling system according to claim 11, wherein the drill bit is an abrasive jet drill bit, wherein the bit face is devoid of any wash nozzles and mechanical cutters, e.g. wherein the abrasive jet nozzles of the drill bit consist of one single abrasive jet nozzle.

19. Directional drilling system according to claim 11, wherein the drill bit is a mechanical drill bit, e.g. a PDC drill bit or tricone drill bit, further comprising: one or more mechanical cutters, arranged on the bit face, one or more wash nozzles arranged at respective adjacent azimuthal positions different from those of the one or more abrasive jet nozzles, a strainer, arranged inside the intermediate space of the drill bit and rotating along with the drill bit, configured to direct the abrasive particles in said stream as received through the bit fluid inlet port into the abrasive jet nozzles, while passing drilling fluid within said stream into both the abrasive jet nozzles and the wash nozzles.

20. Directional drilling system according to claim 11, wherein the abrasive particles are magnetic abrasive particles, the system further comprising a downhole recirculation unit for recirculation of the abrasive particles passed through the abrasive jet nozzles into impingement with the borehole bottom, said downhole recirculation unit comprising one or more magnets arranged such that one or more magnetic fields produced thereby attract abrasive particles from the stream downstream of said impingement thereof with the borehole bottom, and convey the attracted particles in a recirculation stream at a substantially constant flow rate to a mixing section through which said stream passes upstream of said abrasive jet nozzles.

21. Directional drilling system according to claim 11, comprising one or more sensors, including one or more of: one or more positional sensors, configured to, and arranged on or directly above the drill bit such as to, provide a signal to the control unit indicative of the position, e.g. the azimuthal position, at which said impingement of the stream, e.g. of the first stream portion, with the borehole bottom takes place, one or more presence detection sensors, e.g. high frequency acoustic sensors or magnetic sensors, arranged at a location downstream of said deflection, e.g. at the first and second inlets and/or at the first and second outlets and/or close to the drill bit, configured to provide a signal to the control unit indicative of the presence of abrasive particles at said location, e.g. at least indicative of which of said first stream portion and said second stream portion passes the sensor, one or more navigational sensors, configured to, and arranged on or directly above the drill bit such as to provide a signal to the control unit indicative of a geometrical direction of said deepening of the borehole, the control unit being configured to, based on signals of the sensors, control the actuators of the particle deflection device e.g. wherein the control unit is connected to the one or more sensors such as to receive signals provided thereby, and is configured to compare the values of quantities measured thereby indicated by said signals with a predetermined reference value of quantities measured thereby, and provide in dependence of the result of said comparing, said control signals to the particle deflection device.

22. Directional drilling system according to claim 21, wherein the abrasive particles are magnetic abrasive particles, e.g. a steel shot, the system further comprising a mud pulse telemetry unit, comprising: a telemetric control unit, configured to receive one or more of said signals provided by one or more of said sensors and encode these into series of pulses with predetermined timings and amplitudes, a switchable magnet arranged such as to produce in the first and/or second channel a magnetic field, configured to during activation thereof, cause a local accumulation of the magnetic abrasive particles in said first and/or second channel that results in a pressure pulse within that channel, and upon deactivation thereof, stops said causing of said local accumulation, wherein said telemetric control unit is configured to control the activation and deactivation of the switchable magnet such that the switchable magnet repeatedly produces said pressure pulse to form a series of pressure pulses of which the timings and amplitudes correspond to said encoded series of pulses, e.g. wherein the mud pulse telemetry unit further comprises, downstream of the first and second outlets, e.g. externally from the borehole, e.g. in particular when the object is a subterranean earth formation, at the surface of said earth formation, a conversion device connected to the control unit, the conversion device being adapted to register the timings and amplitudes of said series of said pressure pulses and to provide corresponding signals to the control unit, e.g. a series of voltages with corresponding timings and amplitudes, wherein said control unit is configured to decode said corresponding signals produced by said conversion device into the values of the quantities measured by said one or more sensors, to compare said values with a predetermined reference value of said quantity, and to provide in dependence of the result of said comparing, said control signals to the particle deflection device.

23. A steerable sub for use in a directional drilling system, e.g. the directional drilling system according to claim 11, connectable at a downhole end thereof to a drill bit of the system, and at another end thereof to a tubular drill string of the system, comprising: a sub fluid inlet port, fluidly connectable to a supply channel through the tubular drill string, to receive from the supply channel a stream of drilling fluid mixed with abrasive particles from the supply channel when the sub is connected to the drill string, and a sub fluid outlet port, fluidly connectable to the bit fluid inlet port, to pass to the drill bit said stream when the sub is connected to the drill bit, a modulation unit configured to cause a variation of a concentration of abrasive particles along stream portions of the stream received from the supply channel, the modulation unit being fluidly connected to the sub fluid inlet port, downstream thereof, the modulation unit comprising: a first channel having a first flow resistance to the drilling fluid mixed with abrasive particles, a first inlet, and a first outlet fluidly connected to the sub fluid outlet port, a second channel arranged in parallel to the first channel, having a second flow resistance to the drilling fluid mixed with abrasive particles, a second inlet, and a second outlet fluidly connected to the sub fluid outlet port, a particle deflection device arranged at a location along the stream between the sub fluid inlet port and the first and second inlets, comprising one or more actuators connectable to a control unit, wherein the particle deflection device is configured to periodically, preferably based on control signals received from the control unit, during a first time period, deflect a first majority of all abrasive particles received from the supply channel through the sub fluid inlet port into the first inlet, and during a second time period following the first time period, deflect a second majority of all abrasive particles in the stream received from the supply channel through the sub fluid inlet port into the second inlet, wherein the first and second channel are embodied such that a difference between the first flow resistance and the second flow resistance results in a velocity difference between said first majority of abrasive particles passing through said first channel and said second majority of abrasive particles passing through said second channel, wherein said velocity difference is such that in a combination section downstream of the first and second outlets, the first and second majority, together with any of said drilling fluid passed into the first and second channel during the first and second time period, respectively, are combined into one of said stream portions, and minorities of abrasive particles not being deflected, together with any drilling fluid passed into the first and second channel during the second and first time period, respectively, into a subsequent one of said stream portions.

24. An abrasive particle pulse generator for use in a directional drilling system, e.g. in the directional drilling system (1) according to claim 11, configured to cause a variation of a concentration of abrasive particles along stream portions of a stream of abrasive particles mixed with drilling fluid to be passed through one or more abrasive nozzles of the system, the pulse generator comprising: a first channel having a first flow resistance to the drilling fluid mixed with abrasive particles, a first inlet, and a first outlet, a second channel arranged in parallel to the first channel, having a second flow resistance to the drilling fluid mixed with abrasive particles, a second inlet, and a second outlet, a particle deflection device, arranged at a location along the stream directly upstream of the first and second inlets, comprising one or more actuators connected to or connectable to a control unit, wherein the particle deflection device is configured to periodically, preferably based on control signals received from the control unit, during a first time period, deflect a first majority of all abrasive particles of the stream passing said location into the first inlet, and during a second time period following the first time period, deflect a second majority of all abrasive particles in the stream passing said location into the second inlet, wherein the first and second channel are embodied such that a difference between the first flow resistance and the second flow resistance results in a velocity difference between said first majority of abrasive particles passing through said first channel and said second majority of abrasive particles passing through said second channel, wherein the velocity difference is such that in a combination section downstream of the first and second outlets, the first and second majority, together with any of said drilling fluid passed into the first and second inlet during the first and second time period, respectively, are combined into one of said stream portions, and minorities of abrasive particles not being deflected, together with any drilling fluid passed into the first and second channel during the second and first time period, respectively, are combined into a subsequent one of said stream portions.

Description

[0234] The invention will now be described with reference to the appended drawings. In the drawings:

[0235] FIG. 1 schematically shows a system according to the invention being used for directional drilling of a curved borehole in a subterranean earth formation,

[0236] FIG. 2 schematically shows embodiments of a system according to the invention, along with magnifications of a mechanical drill bit thereof and of the interior of a steerable sub thereof,

[0237] FIG. 3 schematically shows a system according to the invention during use, during a first time period,

[0238] FIG. 4a schematically shows a top view of cross-section A-A indicated in FIG. 3 of the system of FIG. 3, during a first time period,

[0239] FIG. 4b schematically shows a top view of cross-section A-A indicated in FIG. 3 of the system of FIG. 3, during a second time period,

[0240] FIG. 5a schematically shows a system according to the invention during use, during a first time period,

[0241] FIG. 5b illustrates schematically the action of the deflection device in the system of FIG. 5a,

[0242] FIG. 6 schematically shows a system according to the invention during use, during a first time period along with the action of the deflection device,

[0243] FIG. 7 schematically shows a recirculation sub and a drill bit being used within a system according to the invention.

[0244] The figures illustrate embodiments of a directional drilling system 1 according to the invention.

[0245] FIG. 1 illustrates, highly schematically, an embodiment of the system 1 while directional drilling of a curved borehole 4a in a subterranean earth formation 2. The drilling has progressed through limestone layer 2a and sandstone layer 2b into a rock layer 2c of the subterranean earth formation. As best seen in the magnification of the system 1, the system 1 is connected to a drill string 40, which is rotated by top drive 3b of drilling tower 3a at the surface 2d. Within the cement casing of a main, vertical borehole 4, an anchor 3c is arranged and a whipstock 3d, which guides the drill string 40 through the casing to deviate into borehole 4a. Borehole 4a is the last of four curved boreholes 4a, 4b, 4c, 4d deviating from the main borehole 4 being drilled. All deviating curved boreholes 4a, 4b, 4c, 4d comprise a curved section and a subsequent straight section. System 1 is currently deepening the straight section of borehole 4a. Borehole 4a has a borehole bottom 4a′.

[0246] At the surface 2d, besides the tower 3a and top drive 3b, a pump 98 is provided which pumps drilling fluid 91 through a particle injection device 99. In particle injection device 99, magnetic abrasive particles 92 from an abrasive particles supply 95 are combined with the drilling fluid 91 to form a stream 90 of drilling fluid 91 mixed with abrasive particles 92. The stream 90 has a substantially constant flow rate and concentration of abrasive particles 92. The stream 90 is passed through a supply channel that runs through the drill string 40 into the system 1, inside which it runs subsequently through a steerable sub 20 and a recirculation sub 50 and drill bit 10. The drill bit 10 is in this case an abrasive jet drill bit. After passing the drill bit 10, the stream 90 impinges the borehole bottom 4a′ in the form of an abrasive jet of said stream 90, so as to erode the borehole bottom 4a′. After this impingement, the stream 90 progresses upwardly again towards the surface 2d, moving in between the annular space in between the cylindrical borehole wall and the system 1. While passing the recirculation sub 50, a portion of the abrasive particles 92 inside the stream is captured by the recirculation sub 50, and recirculated within the recirculation sub as a recirculation stream 93 to the stream 90. After the capture of the abrasive particles 92 by the recirculation sub from the stream 90, it progresses further towards the surface as return stream 94. The particles 92 still left in the recirculation stream 94 are filtered at the surface 2d to join the supply 95 of abrasive particles.

[0247] FIG. 2 shows, schematically, two possible embodiments of a system 1 according to the invention. Both have an identical steerable sub 20, the interior of which is shown schematically to the right of both embodiments in a magnification. In the leftmost system 1, the drill bit 10 is a mechanical drill bit. In the rightmost system 1, the drill bit 10 is an abrasive jet drill bit, and the system comprises a recirculation unit 50. The recirculation unit 50 and the AJD bit of this system is shown in more detail in FIG. 7.

[0248] As indicated for the mechanical drill bit 10 in a magnification thereof, the drill bit 10 comprises a bit face, which during use faces the borehole bottom 4a′, a bit fluid inlet port 10i, one or more abrasive jet nozzles 17a and an intermediate space between the bit fluid inlet port 10i and one or more abrasive jet nozzles 17a. These parts are also comprised by the abrasive jet drill bit of the rightmost embodiment, as shown in FIG. 7.

[0249] The abrasive jet nozzles 17a are configured for ejecting stream 90 of drilling fluid 91 mixed with abrasive particles 92 into impingement with the borehole bottom 4a′ in the form of an abrasive jet 90. The mechanical drill bit comprises multiple abrasive jet nozzles 17a arranged at different azimuthal positions. The AJD drill bit has only one single abrasive jet nozzle 17a, as shown in FIG. 7.

[0250] Each of the abrasive jet nozzles 17a have a nozzle inlet for fluid communication with the intermediate space, from which each of the nozzle inlets extends at least during rotation of the drill bit 10.

[0251] The mechanical drill bit 10 further comprises wash nozzles 17w, mechanical cutters 18, and a strainer 19. The strainer is configured and arranged within the drill bit 10 such that the abrasive particles from the stream 90 are deflected into the abrasive jet nozzles 17a only, and the drilling fluid 91 from the stream 90 passes into both the abrasive jet nozzles and into the wash nozzles 17w.

[0252] Both embodiments of the system 1 further comprise the same sub 20, which is connected at a downhole end thereof to the drill bit 10 so as to be rotatable along therewith, and at another end thereof to the tubular drill string 40.

[0253] The sub 20 comprises a sub fluid inlet port 20i, fluidly connectable to the supply channel through the drill string 40 to receive from this supply channel the stream 90 of drilling fluid 91 mixed with abrasive particles 92 when the system 1 is connected to the drill string 40. It further comprises a sub fluid outlet port 20o, fluidly connected or connectable to the bit fluid inlet port 10i.

[0254] The sub 20 further comprises, fluidly connected to the sub bit inlet port 20i, downstream thereof, a modulation unit configured to cause a variation of a concentration of abrasive particles 92 along stream portions 90h, 90l of the stream 90 received from the supply channel that are subsequently passed through the sub bit fluid outlet port 20o into the bit fluid inlet port 10i.

[0255] The modulation unit comprises a first channel 21 and a second channel 22. The first channel 21 has a first flow resistance to the drilling fluid 91 mixed with abrasive particles 92, a first inlet 21i, and a first outlet 21o fluidly connected to the sub bit fluid outlet port 20o. The second channel 22 is arranged in parallel to the first channel 21, and has a second flow resistance to the drilling fluid 91 mixed with abrasive particles 92, a second inlet 22i, and a second outlet 22o fluidly connected to the sub bit fluid outlet port 20o.

[0256] The modulation unit further comprises a particle deflection device 23 between the sub bit fluid inlet port 20i and the first and second inlets 21i, 22i. The particle deflection device is indicated in FIGS. 2 and 3, and is shown in more detail in FIG. 4, in a top view of a cross-section A-A as indicated in FIG. 3. It comprises one or more actuators 23m, and is connected to a control unit (not shown) of the system 1.

[0257] The particle deflection device 23 is configured to periodically, based on control signals received from the control unit, during a first time period, deflect a first majority 92m1 of all abrasive particles 92 received from the supply channel through the sub bit fluid inlet port 20i into the first inlet 21i, and during a second time period following the first time period, deflect a second majority 92m2 of all abrasive particles 92 in the stream 90 received from the supply channel through the sub bit fluid inlet port 20i into the second inlet 22i.

[0258] FIG. 3 illustrates the particle deflection device 23 deflecting a first majority 92m1 into the first channel 21.

[0259] The first and second channel 21, 22 are straight channels with equal internal surface roughness and both have a constant cross-section along their lengths, namely the cross-section shown in FIGS. 4a and 4b. Together they form a cylinder. Because they are separated by one plate-shaped wall, they have equal lengths and start and end at the same location along the stream. Because the cross-sectional areas of the channels 21 and 22 are not equal to each other, there is a difference between the first flow resistance and the second flow resistance. This difference in flow resistances results in a velocity difference between said first majority 92m1 of abrasive particles 92 passing through said first channel 21 and said second majority 92m2 of abrasive particles 92 passing through said second channel 22. The flow resistance of the first channel 21 is larger than that of the second channel 22, because its cross-sectional area is smaller than that of the second channel 22. Therefore the first majority 92m1 travels slower through the first channel 21 than the second majority 92m2 travels through the second channel 22.

[0260] The velocity difference is such that in a combination section downstream of the first and second outlets 21o, 22o, the first and second majority 92m1, 92m2, together with any of said drilling fluid 91 passed into the first and second channel 21, 22 during the first and second time period, respectively, are combined into a first stream portion 90h. Minorities of abrasive particles 92 not being deflected, together with any drilling fluid 91 passed into the first and second channel 21, 22 during the second and first time period, respectively, are combined into a subsequent second stream portion 90l. In FIG. 3, a first and second majority are about to combine directly downstream of the outlets 21, 22.

[0261] In FIG. 3, the drill bit 10 is an abrasive jet drill bit (AJD bit). It has one single abrasive jet nozzle 17a. The bit face is devoid of any wash nozzles and mechanical cutters.

[0262] The control unit is configured such that the signals thereof received by the deflection device 23 cause the time periods in which the actuators 23m of the deflection device 23 deflect said first and second majority 92m1, 92m2 of the abrasive particles 92 into the first and second channel 21, 22 to be synchronized with the rotational velocity of the AJD bit 10 such, that said first stream portions 90h passes through the abrasive jet nozzle 17a while it is directed towards a selected angular sector 4a″ of the borehole bottom 4a′, together with any of said drilling fluid 91 passed into the first and second channels 21, 22 during the first and second time periods, respectively, and said subsequent second stream portion 90l passes through the abrasive jet nozzle 17a while it is not directed towards the selected angular sector 4a″. In FIG. 3, a first stream portion is being ejected from the abrasive jet nozzle 17a being directed towards the selected angular sector 4a″, impinging the selected angular sector 4a″. The selected angular sector 4a″ is to form the outer bend of the curved borehole section being drilled.

[0263] To illustrate the principle most clearly, the difference between the concentrations of the first stream portion 90h and the second stream portion is shown as 100%. That is, in the second stream portion 90l no abrasive particles 92 are present. In practice, this difference will be less than 100% when employing an AJD bit 10, to still accomplish some erosion of the borehole 4a′ outside of the selected section 4a″ as well, that is, at least deepening the inner bend of the curved borehole section to some extent. The concentration of abrasive particles determines the erosive power of the abrasive jet 90 being ejected, and therefore, the radius of the curved borehole section increases as the concentration difference between the stream portions 90h, 90l decreases.

[0264] The abrasive particles 92 are magnetic abrasive particles 92, namely ferromagnetic abrasive particles, and the actuators 23m of the deflection device 23 comprise a magnetic switch. The magnetic switch is shown in the top views of cross-section A-A of FIG. 3 directly upstream of the first and second inlets 21i, 22i in FIGS. 4a and 4b. FIG. 4a shows the magnetic switch during the first time period, that is, in the situation of FIG. 3. FIG. 4b shows the magnetic switch during the second time period.

[0265] This magnetic switch is configured to during the first time period establish an inhomogeneous magnetic field 23B over the shown cross-section that directs the abrasive particles 92 towards the first inlet 21i and to during the second period establish an inhomogeneous magnetic field 23B over the shown cross-section that directs the abrasive particles 92 towards the second inlet 22i.

[0266] The magnetic switch comprises multiple magnets 23m arranged at different azimuthal positions along an outer circumference of the stream 90 in the shown cross-section, namely along a circumference of a channel accommodating said stream 90 at that location. The multiple magnets 23m together produce the inhomogeneous magnetic fields 23B.

[0267] There are seven magnets 23m along the circumference. The arrows inside the magnets 23m indicate the direction of the N-poles thereof. They are directed relative to each other such as to produce the oval-shaped magnetic field lines over the cross-section. The magnets 23m are unevenly distributed along the circumference: in the first time period, most of the magnets 23m are at the side of the circumference of the first channel 21, see FIG. 4a, and in the second time period, most of the magnets are at the side of the circumference of the second channel 22, see FIG. 4b. As a consequence the density of the magnetic field 23B produced in the first time period is higher in a part of the cross-section covering the first channel 21 than in a part covering the second channel 22. As shown in FIG. 4b the density of the magnetic field 23B produced in the second time period is higher in the part of the cross-section covering the second channel 22 than in the part covering the first channel 21.

[0268] To achieve the different positions of the magnets 23m along the circumference, the magnets 23m are movable permanent magnets 23m, and the actuators further comprise drive means (not shown), connected to the magnets 23m and configured to move, based on the signals received from the control unit, the magnets 23m as a unity along the circumference upon a switch between the respective time periods. The movement is shown by the curved arrows along the circumference: in FIG. 4a, illustrating the first time period, the magnets 23m have just been rotated clockwise to direct the particles into the first channel 21, and in FIG. 4b, illustrating the second time period, the magnets 23m have just been rotated counter clockwise to direct the particles into the second channel 22.

[0269] The rightmost embodiment of the system 1 with the AJD bit 10 shown in FIG. 2, the system 1 further comprises a recirculation unit 50 for recirculation of the abrasive particles 92 passed through the abrasive jet nozzles 17a into impingement with the borehole bottom 4a′. This recirculation unit 50 is shown in more detail in FIG. 7. The downhole recirculation unit 50 comprises a magnet 51, which is arranged such that one or more magnetic fields produced thereby attract abrasive particles 92 from the stream 90 downstream of said impingement thereof with the borehole bottom 4a′. Thereafter it conveys the attracted particles 92 in a recirculation stream 93 at a substantially constant flow rate to a mixing section 52c of channel 52 of the recirculation unit 50, through which said stream 90 passes towards the abrasive jet nozzle 17a after it has passed the steerable sub 20.

[0270] As shown in FIG. 3, the system 1 further comprises one or more sensors 81, 82. These sensors include positional sensors 82 which are configured to, and arranged directly above the drill bit 10 such as to, provide a signal to the control unit indicative of the position at which said impingement of the stream 90 with the borehole bottom 4a′ takes place. The sensors furthermore include navigational sensors 82, configured to, and arranged on or directly above the drill bit such as to provide a signal indicative of a geometrical direction of said deepening of the borehole 4a.

[0271] The sensors also include presence detection sensors 81, in the form of high frequency acoustic sensors or magnetic sensors, arranged at a location directly downstream of said first and second outlets 21o, 22o. These presence detection sensors 81 are configured to provide a signal to the control unit indicative of the presence of abrasive particles 92 at that location. These signals at least indicate of which of said first stream portion 90h or said second stream portion 90l passes the sensors 81.

[0272] The control unit is configured to, based on signals of the sensors 81, 82, control the actuators 23m of the particle deflection device 23.

[0273] The control unit is connected to the one or more sensors 81, 82 such as to receive signals provided thereby, and is configured to compare the values represented by said signals with a predetermined reference value of quantities measured thereby, and produce in dependence of the result of said comparing, the mentioned control signals to the particle deflection device 23.

[0274] FIGS. 5a-b and 6 illustrate an embodiment with a different particle deflection device 24 and a different arrangement of the first and second channel. Instead of being radially adjacent, the first channel 21 is, arranged inside the second channel 22 so that the first channel is radially enclosed by the second channel. Furthermore the deflection of the particles into the channels 21, 22 is established by mechanical, instead of magnetic, deflection means. A particle concentrating device 25 is arranged directly upstream of the first and second inlet 21i, 22i, between the supply channel outlet and the first and second inlets 21i, 22i. Through this concentrating device 25 the abrasive particles are supplied in a higher concentration in a first cross-sectional portion 25o1 of an outlet 25o thereof than in a second cross-sectional portion 25o2 of this outlet. A radial moving of the first inlet 21i relative to the first and second portion 25o1, 25o2 makes that a smaller or larger cross-sectional portion of the first inlet 21i is within the contours of the first and second portion 25o1, 25o2, so that the respective inlet receives a higher or lower concentration of abrasive particles 92. The relative radial movement is driven by an actuator mechanism of the deflection device 24.

[0275] The particle concentrating device 25 comprises an inlet fluidly connected to the supply channel outlet for receiving the stream 90 of drilling fluid 91 mixed with abrasive particles 92 from the supply channel, and the mentioned outlet 25o. The concentrating device 25 is configured to direct a majority of the abrasive particles 92 of the supply stream 90 into the first portion 25o1 of the outlet 25o, and a minority of the abrasive particles 92 into the second portion 25o2 of the outlet, so that the concentration, and thus the flow rate, of abrasive particles 92 is high in the first portion 25o1 and low in the second portion 25o2. In this example, the concentrating device 25 is a strainer. In effect, the concentrating device 25 forms an extension of the supply channel.

[0276] As illustrated in FIG. 5b, the first channel 21 is movable such that its inlet 21i is in a first position 21i′ in which it is radially coincident within the contour of, and axially in line with, the first portion 25o1 of the outlet 25o of the concentrating device 25 during the first time period, so that the first inlet 21i receives the abrasive particles 92 of the supply stream 90 at the high concentration—and thus, flow rate. This position is shown both in FIGS. 5a and 5b. The first channel 21 is furthermore movable such that during the second time period, its inlet 21i is in a second position 21i″ in which it is radially outside the contour of, and axially not in line with, the first portion 25o1 of the outlet 25 during the second time period, so that the first inlet does not receive, or receives less of, the abrasive particles 92 of the supply stream 90 at the high concentration—and thus, flow rate, so as to receive the abrasive particles at a lower concentration. This position is illustrated in FIG. 5b by the dashed lines of the outer contour of the first channel 21. The first inlet 21i is in the second position 21i″ during the second time period radially inside the contour of the second portion 25o2 of the outlet 25o of the concentrating device 25, so as to receive the abrasive particles 92 of the supply stream 90 at a lower concentration.

[0277] To move the first inlet 21 into the first and second position 21i′ and 21i″ thereof, the first channel 21 is pivotable about a radially extending pivot axis 24p remote from the inlet 21i and near the outlet 21o of the first channel. The actuator mechanism of the deflecting device 24 is arranged in the second channel 22, and is configured to move the first channel 21 between the first and second position 21i′,21i″ of its inlet, by pivoting the first channel 21 around the pivot axis 24p.

[0278] As can be verified from FIGS. 5a-b, by the movement of the first inlet 21i, the second inlet 22i is simultaneously brought into and out of the contour of the first portion 25o1 of the outlet 25. In the second position 21i″ of the first inlet 21i, the second inlet 22i is radially within the contour of, and axially in line with, the first portion 25o1 during the second time period, and radially within the contour of the second portion 25o2 during the first time period. The effect is that the first and second channel 21, 22 alternatingly—during the first and second time period [0279] receive the abrasive particles 92 of the supply stream 90 at a high concentration.

[0280] As can be verified from the detail of the cross-section B-B at the interface between the outlet 25o and the inlets 21i, 22i in FIG. 5b, the first portion 25o1 of the concentrating device outlet 25o is radially enclosed by the second portion 25o2. The first and second portion 25o1, 25o2 are concentric.

[0281] The first channel 21 is arranged inside the second channel 22 so that the inlet 21i of the first channel 21 is concentric with the second channel 22 in the first position 21i′ of the first inlet 21i. Thus, in the first position 21i′ of the first inlet 21i, the first channel is axially in line with the first portion of the outlet of the concentrating device. In the second position 21i″ of the inlet 21i of the first channel 21i, the first channel 21 is eccentrically arranged within the second channel 22 and out of axial alignment with the first portion 25o1.

[0282] The actuator mechanism of the deflection device 24 comprises a linear motor 24m, which is controllable by the control unit. The motor 24m is fixed to the second channel 22 and indirectly connected to the first channel via a cable 24c, so as to drive the movement of the first channel 21 inside the second channel 22. The actuator mechanism comprises a transfer mechanism to convert the output movement of the motor 24m to the relative movement of the first channel 21. In this embodiment the transfer mechanism comprises the cable 24c and a cable guide 24cg fixed to the second channel 22. The cable 24c runs from the linear motor 24m to the first channel 21 via the cable guide 24cg. The cable 24c engages on the first channel 21 near the first inlet 21i to effectuate the pivoting movement. The linear output movement of the motor 24m is in the axial direction, indicated by the double arrow in FIG. 5a, and the cable guide 24cg guides the cable 24c to engage the first channel 21 in a radial direction, so that the operation of the linear motor 24m makes the cable 24c pull the first channel 21 in a radial direction. This pulling movement is indicated in FIG. 5b by the single arrow. The transfer mechanism further comprises an elastic element in the form of a spring 24s, diametrically opposite the radial location at which the cable 24c engages the first channel 21. It is fixed to both the first and second channel, such as to counteract a pulling action of the cable 24c by extending upon a movement of the inlet 21i towards the second position 21i″, indicated by the double arrow in FIG. 5b, and contracting to effectuate the opposite movement. The actuator mechanism comprises two limiters 24l, which limit the movement range of the first channel 21 to the two positions shown in FIG. 5b.

[0283] In an alternative embodiment, shown in FIG. 6, the transfer mechanism comprises instead of the cable guide 24cg and the cable 24c, a channel wall guide 24wg. The motor 24m is not fixed to the cable 24c but to the channel wall guide 24wg. The channel wall guide 24wg cooperates with the outer wall of the first channel 21 for converting the output movement of the motor 24m to the pivoting movement of the first channel 21. The wall guide 24wg and the first channel wall engage at complementary slanted surfaces near the inlet 21i in the axial-radial direction, so that an axial movement by the motor 24c makes the slanted surfaces slide over one another, thereby inducing a radial movement component of the first channel 21, and therewith the pivoting movement around the pivot axis 24p. The spring 24s interconnects the first and second channel at the same radial location at which the channel wall guide 24wg engages the first channel 21, at some axial distance therefrom, to counteract the action of the motor 24c. The limiter 24l, limits the movement range of the first channel 21 in the radial direction, in addition to the wall guide 24wg.