Method and system for directional drilling

1. A system for directional drilling of a borehole in a formation, the system comprising:

a rotatable drill string having a central fluid passage for the passage of drilling fluid;

a rotatable drill bit connected to an end of the drill string, the drill bit comprising an intermediate space for receiving the drilling fluid from the drill string and at least two nozzles for ejecting the drilling fluid, each nozzle being in fluid communication with the intermediate space;

a first rotor section arranged within the central fluid passage of the drill string, the first rotor being directly drivable by the drilling fluid and being rotatable with respect to the drill string in a first direction and at a first rotational speed (ω 2/1 );

a flow diverter connected to and rotatable in conjunction with a downhole end of the first rotor section for diverting the drilling fluid in a predetermined direction with respect to an axis of the drill string;

a second rotor section being directly drivable by the drilling fluid and being rotatable with respect to the first rotor section in a second direction opposite to the first direction and at a second rotational speed (ω 3/2 ); and

a control unit for controlling the second rotational speed (ω 3/2 ) of the second rotor section with respect to the first rotor section, to thereby control the first rotational speed (ω 2/1 ) of the first rotor section with respect to the drill string;

the second rotor section being provided with at least one magnet;

the first rotor section being provided with at least one electrical coil arranged to magnetically couple with said at least one magnet; and

the control unit being connected to the at least one electrical coil for controlling the electrical load thereof.

2. The system of claim 1 ,

the first rotor section comprising a number of first blades arranged at a first angle (φ 1 ) with respect to the axis of the drill string to rotate the first rotor section in the first direction upon passage of drilling fluid; and

the second rotor section comprising a number of second blades arranged at a second angle (φ 2 ) with respect to the axis of the drill string to rotate the second rotor section in the second direction upon passage of drilling fluid.

3. The system of claim 2 , the second angle (φ 2 ) exceeding the first angle (φ 1 ).

4. The system of claim 2 ,

wherein the first blades extend into the central fluid passage of the drill string; and

wherein the second blades extend into the central fluid passage of the drill string.

5. The system of claim 1 , wherein the control unit is adapted to increase the electrical load of the at least one electrical coil to decrease the second rotational speed (ω 3/2 ).

6. The system of claim 1 , comprising at least a first bearing and a second bearing arranged within the central fluid passage of the drill string for decoupling rotation of the first rotor section from rotation of the drill string.

7. The system of claim 1 , the control unit comprising at least one orientation sensor for detecting an orientation of the control unit in the borehole.

8. The system of claim 7 , the control unit being adapted to provide control signals dependent on said orientation as detected, for the controlling of the second rotational speed (ω 3/2 ) of the second rotor section.

9. The system of claim 1 , wherein the control unit is integrated in the first rotor section.

10. The system of claim 1 , comprising an insert arranged within the intermediate space of the drill bit for receiving the fluid flow and directing the fluid flow to the at least two nozzles of the drill bit.

11. The system of claim 10 , wherein the insert is connected to the first rotor section and is rotatable with respect to the drill bit.

12. The system of claim 10 , wherein the flow diverter is integrated in the insert.

13. The system of claim 10 , the insert comprising a cylindrical body provided with a fluid channel for diverting the fluid flow, said fluid channel extending to a fluid opening which is eccentric with respect to the axis of the drill string.

14. The system of claim 10 , the insert being fixated in the intermediate space of the drill bit.

15. The system of claim 10 , the insert comprising at least two tubes, each tube having a first end adjacent to the flow diverter and a second end extending towards a corresponding one of the at least two fluid nozzles of the drill bit.

16. A drill string component for directional drilling of a borehole in a formation, the drill string component comprising:

a first rotor section arranged within a central fluid passage of the drill string, the first rotor section being rotatable with respect to the drill string in a first direction and at a first rotational speed (ω 2/1 ), the first rotor section being directly drivable by a stream of drilling fluid in the central fluid passage;

a flow diverter connected to and rotatable in conjunction with a downhole end of the first rotor section for diverting a flow of drilling fluid with respect to an axis of the drill string;

a second rotor section being rotatable with respect to the first rotor section in a second direction opposite to the first direction and at a second rotational speed (ω 3/2 ), the second rotor being directly drivable by the stream of drilling fluid in the central fluid passage; and

a control unit for controlling the second rotational speed (ω 3/2 ) of the second rotor section with respect to the first rotor section, to thereby control the first rotational speed (ω 2/1 ) of the first rotor section with respect to the drill string;

the second rotor section being provided with at least one magnet;

the first rotor section being provided with at least one electrical coil arranged to magnetically couple with said at least one magnet; and

the control unit being connected to the at least one electrical coil for controlling the electrical load thereof.

17. A method for directional drilling of a borehole in a formation, the method comprising the steps of:

rotating a drill string having a central fluid passage for the passage of drilling fluid and a rotatable drill bit connected to an end of the drill string in the borehole, the drill bit comprising mechanical cutting means forming a bit face for extending the borehole upon rotation of the drill bit, an intermediate space for receiving the drilling fluid from the drill string, at least two nozzles for ejecting the drilling fluid, each nozzle being in fluid communication with the intermediate space;

pumping drilling fluid through the internal fluid passage of the drill string;

the drilling fluid directly driving and rotating a first rotor section arranged within the central fluid passage of the drill string with respect to the drill string in a first direction and at a first rotational speed (ω 2/1 ), the first rotor section being provided with a flow diverter connected to and rotatable in conjunction with a downhole end of the first rotor section for diverting the drilling fluid with respect to an axis of the drill string;

the drilling fluid directly driving and rotating a second rotor section with respect to the first rotor section in a second direction opposite to the first direction and at a second rotational speed (ω 3/2 ), the second rotor section being provided with at least one magnet; and

controlling the second rotational speed (ω 3/2 ) of the second rotor section with respect to the first rotor section to thereby control the first rotational speed (ω 2/1 ) of the first rotor section with respect to the drill string by controlling an electrical load of at least one electrical coil of the first rotor section, the electrical coil magnetically coupling with said at least one magnet.