Active magnetic ranging

Abstract

A drill string capable of determining distance to remote conductive objects. An active magnetic ranging tool arranged on the drill string, where the active magnetic ranging tool at least comprises one first current transmitter configured to transmit a current vector in a controlled direction and phase in a direction away from the drill string; and an electromagnetic field sensitive receiver arranged on the drill string in an axial distance, S3, from the current transmitter. The active magnetic ranging tool further comprises two or more isolators for suppression of noise induced from the first current transmitter to the electromagnetic field sensitive receiver.

Description

Active magnetic rangingTechnical field

[0001] The present invention relates to a drill string capable of determining distance to remote conductive objects.Background

[0002] Active Magnetic Ranging (AMR) is an advanced technique used in the oil and gas industry to accurately determine the position of a well using a known magnetic source. Method is particularly useful for achieving precise positioning of two wells relative to each other, which is critical in situations such as relief drilling, collision avoidance, improved recovery methods, identification and avoidance of abandoned wells, and complex plugging and abandonment operations.

[0003] Active Magnetic Ranging systems use precise sensors to collect data necessary for several drilling applications. In a typical operation, current is injected into the formation near the casing of a target well from drill well which creates a short circuit and causes the current to flow along the casing of the target well. This current produces a magnetic field which has a specific shape and strength based on the intensity and direction of the current. The sensors in the drill well measure the strength and direction of the magnetic field, which varies depending on the distance and orientation of the target well. By analysing these measurements, operators can calculate the exact position of the drill well in relation to the target well.

[0004] In the later lifetime of a petroleum field's production, it is common to drill wells in between old and often abandoned wells, so-called in-fill drilling to increase the drainage of the field. In these in-fill drilling operations collision with existing wells or abandoned wells must be avoided. This infill well can pass remarkably close to several old wells. Some of these may be in production and some may be abandoned wells. To hinder potential collisions with the other wells, it is necessary to measure wherethe old well paths exist while drilling. Traditionally, the logging tools are placed on a cable or wireline. Therefore, to be able to measure the distance and direction towards the other wells, the drill pipe must be pulled out of the well which is an operation taking several hours. Thus, use of such wireline operations significantly increases the drilling operation time. The logging operations to locate old well trajectories during in-fill drilling are normally like those applied during well intercept drilling where the objective is to hit another well in relief well drilling.

[0005] A well intercept operation in a relief well drilling is a drilling operation where the scope is to drill into an existing well at large depth. In the, fortunately rare, case of a blowout, drilling a relief well can be part of the necessary last step to kill the well. In such blowout cases, there may be no access to the target well. Thus, there are no possibilities to directly pump heavy fluid, so-called kill mud, into the well to stop the blowout. The solution is thus to drill a relief well from a location at a safe distance from the blowout well, typically from a distance around 500m from the well head of the well with blowout. The plan is to drill into a position in the target well as deep as possible to be able to pump high density kill fluids into this well to stop the blowout. Normally, it is not possible to locate the open section of that well. Therefore, frequent practice is to drill the relief well to the assumed vicinity of a cased section of the blowing well and then navigate such that this well is intercepted in the bottom of this cased section. Typically, 10 - 25 wireline operations are needed to intercept the well with the blowout. Therefore, by use of an Active Magnetic Ranging on the drill pipe itself instead of using wireline operations may shorten the time needed to kill a blowout by for example 75%.

[0006] Most well intercept operations do not have the scope of relief well drilling. They need to be intercepted for instance to be connected to another well for production purposes. Another reason could be during plug and abandonment where different sones needs to be temporary or permanent plugged for several reasons. Also, the spacing and connection ofadvanced geothermal wells rely on such technology. These other applications are in volume far more important than the relief well drilling. Although these cases may include large environmental spills, this type of emergency actions is fortunately needed only a handful of times per year. Geothermal drilling has an immense potential for power generation in several locations globally. On some locations it is necessary to drill deep using two wells in a closed loop to adopt enough heat to be able to produce electricity. In such cases with closed loop systems, one well is drilled to target depth where the well is split into several parallel lateral well sections. These laterals merge again at the bottom of a return well. "Cold" water is pumped down into one of these wells and hot water is returned in the other well. This hot water then energises a producing site for electricity before the fluid is returned into the "cold" water well and the circulation is repeated. Use of such radiator wells require good control on the distance between each lateral well and good control on hitting the correct junction point.

[0007] The document AU2015200086B2 pertains to the technology of active magnetic ranging. One of the critical aspects highlighted in the document is the use of insulators to prevent short circuiting. By incorporating insulators, the system can effectively avoid electrical short circuits.

[0008] The document D2 US 2015369036A1 discusses a method and system for Active Magnetic Ranging (AMR). This technology is used to determine the relative position of a target wellbore to a reference wellbore. The system employs magnetic field sensors to detect magnetic fields generated by an induced current in the casing / lines in the target wellbore. The data collected by these sensors is then processed to calculate the distance and direction between the wellbores.

[0009] It is an object of the present invention to simplify the AMR process where wireline must be pulled in and out of a borehole for measuring the distance to neighbouring wells.Summary of the invention

[0010] The present invention provides a drill string capable of determining distance to remote conductive objects. The drill string at least comprises: a) an active magnetic ranging tool arranged on the drill string.The active magnetic ranging tool at least comprises: i. one first current transmitter configured to transmit a current vector in a controlled direction and phase in a direction away from the drill string; ii. an electromagnetic field sensitive receiver arranged on the drill string in an axial distance, S3, from the current transmitter, where the active magnetic ranging tool further comprises two or more isolators for suppression of noise induced from the first current transmitter to the electromagnetic field sensitive receiver.

[0011] A first isolator may be arranged in an axial distance S3 between the first current transmitter and the electromagnetic sensitive receiver. The axial distance may be between 3 and 30 meters.

[0012] A second isolator may be at least one gap sub arranged between the first current transmitter and the electromagnetic sensitive receiver.

[0013] A third isolator may be an outer non-conductive layer applied on the drill string at least in a portion between the first current transmitter and the electromagnetic sensitive receiver. The outer non-conductive layer applied on the drill string may be provided between the first current transmitter and at least below a part of the electromagnetic sensitive receiver.

[0014] One or more wear rings may be mounted on the drill string in the portion of the non-conductive layer to protect the non-conductive layer from wear.

[0015] A fourth isolator may be a noise cancelling second current transmitter configured to cancel noise at the electromagnetic field sensitive receiver induced from the first current transmitter by transmitting current which is opposite in phase relative to the first current transmitter.

[0016] The drill string may further include a pickup devise detecting signals from the first current transmitter which is used as a reference signal for the noise cancelling second current transmitter.

[0017] The reference signal from the pickup device may be used to make a coherent and low noise detector like a lock-in receiver.

[0018] The reference signal from the pickup device may be configured to make an impedance measurement device to detect conductivity of a formation.

[0019] A fifth isolator may be an axially oriented magnetic isolator arranged close to the electromagnetic sensitive receiver the magnetic isolator enclosing the drill string attenuating unwanted currents flowing on the drill string.

[0020] The electromagnetic sensitive receiver may be arranged downstream and closer to the BHA than the first current transmitter is.

[0021] The first current transmitter may be arranged downstream and closer to the BHA than the electromagnetic sensitive receiver.

[0022] The invention provides in a further aspect, use of isolators on drill string in active magnetic ranging, where the isolators are: a) a first isolator is an axial distance, S3 from a first current transmitter on the drill string, b) a second isolator is a gap sub arranged between the first current transmitter and an electromagnetic sensitive receiver on the drill string, and c) one third isolator is a radial isolator encapsulating the drill string.

[0023] It is disclosed an electric device (electrode(s) connected to a transmitter or receiver) and at least one magnetic device (coil(s) or fluxgate magnetometers connected to a receiver or transmitter) that enable electric and magnetic fields to flow between the AMR. tool and the formation surrounding the AMR. tool.

[0024] Other advantageous features will be apparent from the accompanying claims.Brief description of the drawings

[0025] To make the invention more readily understandable, the discussion that follows will refer to the accompanying drawings, in which:

[0026] Figure 1 shows a prior art example of active magnetic ranging 10 on wireline 11. A first current transmitter TX 12 is attached to the wireline 11. A current vector radiating in different directions from the first current transmitter 12, the phase, the direction and the absolute value may be controlled by the first current transmitter 12. An electromagnetic field sensitive receiver R.X 14 is arranged at a distance Si from the first current transmitter TX 12. The wireline with first current transmitter and the electromagnetic field sensitive receiver 14 is arranged in a new wellbore 13;

[0027] Figure 2 shows a prior art example of active magnetic ranging 20 on coiled tubing 21. A first current transmitter TX 12 is attached to the coiled tubing 21. A current vector radiating in different directions from the first current transmitter 12, the phase, the direction and the absolute value may be controlled by the first current transmitter 12. An electromagnetic field sensitive receiver R.X 14 is arranged at a distance S2 from the first current transmitter TX 12. A gap sub 22 is arranged under the first current transmitter 12. The gap sub stops currents from the first current transmitter 12 to reach the electromagnetic field sensitive receiver 14. The coiled tubing with first current transmitter and the electromagnetic field sensitive receiver 14 is arranged in a new wellbore 13;

[0028] Figure 3 shows an example of active magnetic ranging on a drill string including several isolators;

[0029] Figure 4 shows an example of active magnetic ranging on a drill string including several isolators and wear rings;

[0030] Figure 5 shows an example of active magnetic ranging on a drill string including several isolators where the position of a first current transmitter and an electromagnetic field sensitive receiver is shifted in relation to what is shown in figure 3;

[0031] Figure 6 shows an example of active magnetic ranging on a drill string including several isolators and wear rings where the position of a first current transmitter and an electromagnetic field sensitive receiver is reversed compared to what is shown in figure 3; and

[0032] Figure 7 shows an example of active magnetic ranging on a drill string with bottom hole assembly, EM-fields at receiver, RX, position is suppressed because there are no non-isolated drill strings close to the receiver, RX, position and below the transmitter, TX, position.Detailed description of the Invention

[0033] In the following, general embodiments as well as exemplary embodiments of the invention will be described. Reference will be made to the accompanying figures and to reference numerals in the accompanying figures. It shall be noted, however, that the drawings are exemplary embodiments only, and that other features and embodiments may well be within the scope of the invention as described. The exemplary embodiments are not limiting for the scope of the invention.

[0034] In the following it is described how to measure range and orientation towards a target well while drilling a new well, while achieving the object of the present invention, with different purposes such as: a) Avoiding drilling into a nearby well, also called collision avoidance. b) Drill into the nearby well also called well intercept. c) Position the new well at a given position relative to the nearby well.

[0035] The phrase "new well" shall be interpreted as a new well that is drilled with one of the mentioned purposes a - c above, whilst the nearby well or target well is an existing well which, dependent on the purpose, shall be intercepted, avoided to collide with or be positioned in a defined distance from the new well.

[0036] AMR relies on a first current transmitter in the new well transmitting an electric current vector of a known direction and size. An electromagnetic field sensitive receiver in the new well detects a magnetic field vector induced in a target well for example from the casing of the target well. The first current transmitter and the electromagnetic field sensitivereceiver is arranged on a drill string in the new well. Electromagnetic fields from the first current transmitter can disturb the electromagnetic field sensitive receiver. These electromagnetic fields are unwanted and must be supressed.

[0037] The unwanted electromagnetic fields from the first current transmitter can reach the electromagnetic field sensitive receiver and the area close to the electromagnetic field sensitive receiver either wirelessly through weak conductors such as the formation or mud in the new wellbore, but also guided as a current on good conductors such as a solid drill string.

[0038] These unwanted electromagnetic fields can also reach the electromagnetic field sensitive receiver by partly flowing both on the solid drill string and outside the drill string in the new well.

[0039] It is an aim to supress these unwanted electromagnetic fields by using multiple isolators. These isolators will hinder electromagnetic fields generated by the first current transmitter from reaching the electromagnetic field sensitive receiver.

[0040] Another purpose is to hinder the electromagnetic field generated by the first current transmitter to reach objects close to the electromagnetic field sensitive receiver. These objects can be mud inside the new wellbore and the formation just outside the new wellbore.

[0041] To summarise the electromagnetic fields generated by the first current transmitter can reach the electromagnetic field sensitive receiver by different coupling mechanisms, and it is an aim of the invention to eliminate all the essential coupling mechanisms utilising isolators. Isolator in this context includes all means that eliminate these coupling mechanisms.

[0042] One important isolator is to keep the distance between the first current transmitter and the electromagnetic field sensitive receiver large and typically in the order of 3 - 30 meters. In one example the distance between the first current transmitter and the electromagnetic field sensitive receiver is between 5 and 10. In cases where it is beneficial tokeep the receiver and transmitter in different resistivity layers distances up to thirty meters can occur.Another isolator in axial orientation such as a gap sub will stop currents from the first current transmitter to reach the electromagnetic field sensitive receiver. A gap sub can be described as an insulating piece of pipe providing axial isolation, but it is more sophisticated than that. It is designed to function as an electrical insulator between two sections of a drill string and must withstand the demanding conditions of drilling operations.

[0043] Yet an isolator can be a magnetic material enclosing the drill string of the new well. The magnetic material enclosing the drill string of the new well will attenuate unwanted currents flowing on the drill string of the new well. A benefit of this solution is that the physical strength of the drill string is not compromised as a gap sub can do if made by layers of metal and less conductive composite or thermoplastics.

[0044] Such a magnetic material enclosing the drill string used to attenuate unwanted current from flowing on the drill string of the new well can also be augmented with a second transmitter signal tailored in opposite phase of the unwanted current, thereby cancelling the effect of the unwanted current. In this case the isolation in axial direction can be controlled and even electronically removed when needed to change or increase the beam pattern of a transmitter antenna.

[0045] This isolator in axial orientation or an extra isolator of the same type will also stop currents and electromagnetic fields induced into the bottom hole assembly, BHA, from reaching back to the electromagnetic field sensitive receiver.

[0046] Another type of isolator is oriented in the radial direction, i.e. orthogonal to the axial direction. This isolator is placed on the outside of the drill string of the new well as a non-conductive layer with a given thickness. This isolator will hinder currents and electromagnetic fields close to the electromagnetic field sensitive receiver to enter the formation and mud located close to the electromagnetic field sensitive receiver. Outside thisnonconductive layer there can be rings of wear elements. The wear elements can be metallic, ceramic, or other high wear resistant materials.

[0047] An important aspect of the current invention is the combination of different type of isolators - the aggregated effect. All together they become more efficient. This efficiency improves the range potential of the AMR. system because unwanted EM fields i.e. noise are lowered.

[0048] It should also be understood that the active magnetic ranging system is reciprocal in the sense that the first current transmitter and electromagnetic field sensitive receiver can swap places. One solution is to connect the electrode to a first current transmitter and the coils to receivers. The other way around is also possible where the coils are magnetic first current transmitter or transmitters, and the electrode become an electric receiver picking up weak currents. Another solution is to use one coil that rotates to achieve at least two measurements in different directions.

[0049] An example embodiment 30 of active magnetic ranging on a drill string 31 including several isolators is illustrated in Figure 3. A non-conductive layer 32 is arranged on the outside of drill string 31. The non-conductive layer provides a radial isolation. The drill string 31 is provided with a first current transmitter TX 12. Current vector radiating in different directions from the first current transmitter, the phase, the direction, and the absolute value may be controlled by the first current transmitter. The drill string 31 is provided with an electromagnetic field sensitive receiver R.X 14. The electromagnetic field sensitive receiver R.X 14 is arranged a distance S3 from the first current transmitter TX 12. The receiver is arranged vertically below the first current transmitter TX 12 as illustrated in Figure 3. The receiver R.X 14 may be configured to perform ID, 2D, or 3D measurements by e.g. one or more coils. Wear rings 33 are arranged on the outside of the non-conductive layer 32. As illustrated in the example embodiment in Figure 3, a first wear ring 33 is arranged near the first current transmitter TX 12, a second wear ring 33 is arranged about in the middle between the first current transmitter TX 12 and the receiverRX 14, a third wear ring 33 is arranged near the receiver RX 14, a fourth wear ring 33 is arranged on the receiver RX 14, and a fifth wear ring 33 is arranged below the receiver RX 14 position outside the isolating non- conductive layer 32. Wear rings 33 may also be arranged in other positions on the outside of the non-conductive layer 32. The enlarged view in Figure 3 shows a portion of the wellbore 13 with drill string 31 with receiver RX 14, the third wear ring above and near the receiver RX 14, the fourth wear ring 33 is close to the receiver RX 14. Extra wear rings may also be stretched to a position below the receiver position. The non-conductive layer 32 is arranged on the outside of drill string 31. d is an outer diameter of drill string with the non-conductive layer 32 (d < D). D is an outer diameter of the wear rings 33 (D > d). A fifth wear ring 33 is arranged a distance below the receiver.

[0050] Figure 4 shows an example of active magnetic ranging on a drill string including several isolators and wear rings. Reference number 40 shows a drill string 31 with active magnetic ranging and multiple isolators, including a noise cancelling second transmitter TX2. A non-conductive layer 32 is arranged on the outside of drill string 31. The drill string 31 is provided with a first current transmitter TX 12. Current vector radiating in different directions from the first current transmitter, the phase, the direction, and the absolute value may be controlled by the first current transmitter. The drill string 31 is provided with a second transmitter TX2 41. The second transmitter TX2 41 is a noise cancelling second transmitter, out of phase with the first transmitter TX 12. The second transmitter TX2 41 is arranged a distance S3 from the first current transmitter TX 12. Current vector radiating in different directions from a second current transmitter, the phase, the direction, and the absolute value may be controlled by the second current transmitter. The drill string 31 may be provided with an electromagnetic field sensitive receiver RX 14 below the second transmitter TX2 41. The receiver RX 14 may be configured to perform ID, 2D, or 3D measurements by e.g. one or more coils. Three wear rings 33 are arranged between the first currenttransmitter TX 12 and the second transmitter TX2 41. As illustrated in the example embodiment in Figure 4, a first wear ring 33 is arranged near the first current transmitter TX 12, a second wear ring 33 is arranged about in the middle between the first current transmitter TX 12 and the second transmitter TX2 41, and a third wear ring 33 is arranged near the second transmitter TX2 41 and the receiver R.X 14. The wear rings 33 may also be arranged in other positions between the first current transmitter TX 12 and the second transmitter TX2 41. A fourth wear ring 33 is arranged on the receiver R.X 14.

[0051] Figure 5 shows an example embodiment of active magnetic ranging on a drill string 31 including several isolators where the position of a first current transmitter TX 12 and an electromagnetic field sensitive receiver R.X 14 is shifted in relation to what is shown in figure 3. 30 is drill string with active magnetic ranging and multiple isolators. Current vector radiating in different directions from the first current transmitter 12, the phase, the direction and the absolute value may be controlled by the first current transmitter. A non-conductive layer 32 is arranged on the outside of drill string 31. The first current transmitter TX 12 is arranged a distance S3 downhole from the electromagnetic field sensitive receiver R.X 14. The receiver R.X 14 may be configured to perform ID, 2D, or 3D measurements by e.g. one or more coils. A gap sub 22 is arranged above and in relation to the first current transmitter TX 12.

[0052] Figure 6 shows an example embodiment of active magnetic ranging on a drill string including several isolators and wear rings where the position of a first current transmitter TX 12 and an electromagnetic field sensitive receiver R.X 14 is reversed compared to what is shown in figure 4. The first current transmitter TX 12 is arranged a distance S3 from a second transmitter TX2 41. The first transmitter TX 12 is below the second transmitter TX2 41. In the exemplary embodiments in Figure 6 the distance S3 is from lowest part of second transmitter TX2 41 and to upper part of first transmitter TX 12. The receiver R.X 14 may be configured to perform ID, 2D, or 3D measurements by e.g. one or more coils. A non-conductive layer 32 is arranged on the outside of drill string 31. The non- conductive layer 32 shall not encapsulate active transmitters such as TX 12. Current vector radiating in different directions from the first current transmitter TX 12, the phase, the direction, and the absolute value may be controlled by the first current transmitter. The electromagnetic field sensitive receiver R.X 14 has the second transmitter TX2 41 arranged below and close to the electromagnetic field sensitive receiver R.X 14 as illustrated in Fig.6. The noise cancelling second transmitter TX2 41 is out of phase with first transmitter TX 12. Current vectors are radiating in different directions from the second current transmitter TX2 41, the phase, the direction and the absolute value of these vectors may be controlled by the second current transmitter TX2 41. A gap sub 22 is arranged above and in relation to the first current transmitter TX 12. Wear rings 33 are arranged between the second current transmitter TX2 41 and the first current transmitter TX 12. As illustrated in the example embodiment in Figure 6, a first wear ring 33 is arranged just below the second current transmitter TX2 41, a second wear ring 33 is arranged approximately mid between the first wear ring 33 and the first current transmitter TX 12. A third wear ring 33 is arranged above and in relation to the first current transmitter TX 12. A fourth wear ring 33 is arranged around the first transmitter TX 12. The wear rings 33 may also be arranged in other positions between the second current transmitter TX2 41 and the first current transmitter TX 12. A fifth wear ring 33 is arranged a distance below the first current transmitter TX 12.

[0053] Figure 7 shows active magnetic ranging on a drill string where electromagnetic fields at R.X position is supressed. The configuration resembles that of Figure 3. The upper screen shot of Figure 7 shows a screen shot of how E-fields at an R.X position between the non isolated bottom hole assembly and electrode Tx position is about 39dB attenuated compared with E-field at the electrode position. The lower screen shot of Figure 7 shows B fields at isolated R.X position is about 75dB attenuated compared to the B fields at the electrode at TX position.

[0054] Figure 7 shows a screen shot of how electromagnetic fields are suppressed at receiver, R.X, position with an arrangement in accordance to Figure 3. The upper screen shot of Figure 7 shows the electric E-field close to the receiver, R.X, position is more than 39dB attenuated compared to the E-field close to the electrode of the transmitter, TX. The lower screen shot of Figure 7 shows the magnetic B-field close to the receiver, R.X, position is about 75dB attenuated compared to the B-field close to the electrode position of the transmitter, TX. Note that Figure 7 shows that AMR. is manageable with bottom hole assembly present according to a configuration such as shown in figure 3 - 6.

[0055] The disclosed active magnetic ranging discloses in exemplary embodiments: a) An insulation distance is maintained between the electric and magnetic device. b) The current flowing radially on the drill string close to the magnetic device may be suppressed by at least one radially isolator enclosing the magnetic device. c) The current flowing axially on the drill string close to the magnetic device may be supressed by at least one axially isolator placed inside or next to the radially isolator. d) The signal generated by the transmitter may be detected by a pickup device and used as a reference for the receiver section. A pick-up device may be a reference receiver and a reference coil, e.g. a Z- coil. e) The backscattered signal amplitude and phase from different orientations around the tool may be used to find distance and orientation to neighbour well(s) or other object(s). f) Signal from the pickup device may be used to make a coherent and low noise detector like a lock-in receiver. The signal maybe weaker than noise. The signal may be identified by knowing phase and frequency.g) Signal from the pickup device may be used to make an impedance measurement device to detect the conductivity of the formation.

[0056] Damping of unwanted currents in the drill string may be provided by use of coil solutions stopping passing currents. Composite may be used as a barrier as an alternative. Epoxy solutions in threads combined with coil solutions.

[0057] With respect to electric current, the receiver and transmitter are (partly) axially isolated from each other. The isolation can be placed as a separate pipe element or be included as a part of a pipe which includes both the transmitter and the receiver. o The isolation can be due to setting up an extra transmitter in opposite phase to the original transmitter. o The partly isolation can be due to inserting a pipe section with an alloy with a (significantly) lower conductivity than the remaining drill string. o The isolation is due to mounting in pipe elements with non- conductive couplings or bodies.

[0058] The isolation pipe or the isolation part of the continuous tool should be isolated electrically towards flow to the formation or well fluids. o This radial isolation can be conducted by using an isolating coating on the pipe body.

[0059] Ref table

Claims

Claims1. A drill string (30, 40) capable of determining distance to remote conductive objects, where the drill string at least comprises: a. an active magnetic ranging tool arranged on the drill string (30, 40), where the active magnetic ranging tool at least comprises: i. one first current transmitter (12) configured to transmit a current vector in a controlled direction and phase in a direction away from the drill string (30, 40); ii. an electromagnetic field sensitive receiver (14) arranged on the drill string (30, 40) in an axial distance, S3, from the current transmitter (12), where the active magnetic ranging tool further comprises two or more isolators for suppression of noise induced from the first current transmitter (12) to the electromagnetic field sensitive receiver (14).

2. A drill string according to claim 1, where a first isolator (32) is an axial distance S3 between the first current transmitter (12) and the electromagnetic sensitive receiver is between 3 and 30 meter (14).

3. A drill string according to claim 1 or 2, where a second isolator is at least one gap sub (22) arranged between the first current transmitter (12) and the electromagnetic sensitive receiver (14).

4. A drill string according to any of the previous claims, where a third isolator is an outer non-conductive layer applied on the drill string at least in a portion between the first current transmitter and the electromagnetic sensitive receiver.

5. A drill string according to claim 4, where one or more wear rings (33) are mounted on the drill string (30, 40) in the portion of the non-conductive layer (32) to protect the non-conductive layer (33) from wear.

6. A drill string according to any of the previous claims, where a fourth isolator is a noise cancelling second current transmitter (41) configured to cancel noise at the electromagnetic field sensitive receiver (14) induced from the first current transmitter (12) by transmitting current which is opposite in phase relative to the first current transmitter (12).

7. A drill string according to any of the previous claims where the drill string (30, 40) further comprises a pickup devise detecting signals from the first current transmitter which is used as a reference signal for the noise cancelling second current transmitter (41).

8. A drill string according to claim 7 where the reference signal from the pickup device is used to make a coherent and low noise detector like a lock-in receiver.

9. A drill string according to claim 7 or 8 where the reference signal from the pickup device is configured to make an impedance measurement device to detect conductivity of a formation.

10. A drill string according to any of the previous claims, where a fifth isolator is an axially oriented magnetic isolator arranged close to the electromagnetic sensitive receiver (14) the magnetic isolator enclosing the drill string (30, 40) will attenuate unwanted currents flowing on the drill string (30, 40).

11. A drill string according to any of the previous claims, where the electromagnetic sensitive receiver (14) is arranged downstream and closer to the BHA than the first current transmitter is (12).

12. A drill string according to any of claims 1-10, where the first current transmitter (12) is arranged downstream and closer to the BHA than the electromagnetic sensitive receiver is (14).

13. A drill string according to claim 4, wherein the outer non-conductive layer applied on the drill string may be provided between the first current transmitter (12) and at least below a part of the electromagnetic sensitive receiver (14).

14. Use of isolators on drill string in active magnetic ranging where the isolators are: a. a first isolator is an axial distance, S3 from a first current transmitter on the drill string, b. a second isolator is a gap sub arranged between the first current transmitter and an electromagnetic sensitive receiver on the drill string, and one third isolator is a radial isolator encapsulating the drill string.

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