Electromagnetic ranging source suitable for use in a drill string

1. An electromagnetic ranging source that comprises:

a tubular body having a circumference, wherein the tubular body forms part of a drill string;

a ferromagnetic core conforming to the tubular body and extending at least halfway around the circumference to define a gap with circumferentially-spaced poles;

one or more current-carrying windings around the core to induce a magnetic field between the poles; and

a downhole power source that drives the one or more current-carrying windings, wherein the ranging source provides a time-varying magnetic field as a ranging beacon to guide drilling of a second well relative to a first well.

2. The ranging source of claim 1 , wherein the tubular body comprises a non-magnetic steel.

3. The ranging source of claim 1 , wherein the gap extends no more than 90° of the circumference.

4. The ranging source of claim 3 , wherein the gap has a circumferential dimension of at least 2 cm.

5. The ranging source of claim 1 , wherein the gap has a long dimension parallel to an axis of the tubular body.

6. The ranging source of claim 1 , wherein the gap has a long dimension that is angled with respect to an axis of the tubular body.

7. The ranging source of claim 6 , wherein the core is angled with respect to the axis of the tubular body.

8. The ranging source of claim 1 , wherein the downhole power source provides pulsed or alternating current.

9. The ranging source of claim 1 , wherein the downhole power source is a flow-powered electrical generator.

10. An electromagnetic ranging method that comprises:

deploying a first drill string in a first borehole, the first drill string having an electromagnetic ranging source having a ferromagnetic core conforming to a circumference of a tubular body and extending at least halfway around the circumference to define a gap with circumferentially-spaced poles, the source further having one or more current-carrying windings around the core to induce a magnetic field between the poles, wherein the ranging source provides a time-varying magnetic field as a ranging beacon;

deploying a second drill string with sensors in a second borehole; and

determining distance or direction between the first drill string and the second drill string based on magnetic field measurements obtained by the sensors in response to the ranging beacon; and

extending the second borehole based on the determined distance or direction.

11. The method of claim 10 , further comprising causing a downhole power source to drive the one or more current-carrying windings with pulsed or alternating current.

12. The method of claim 10 , further comprising circulating a fluid through the first drill string, thereby powering a downhole electrical generator that is coupled to the one or more current-carrying windings.

13. The method of claim 10 , further comprising rotating the first drill string to modulate the magnetic field.

14. The method of claim 10 , wherein said drilling a second borehole includes intersecting the first borehole.

15. The method of claim 14 , wherein prior to their performance, said assembling and drilling were contingent upon occurrence of a well-control problem in the first borehole.

16. The method of claim 14 , further pumping a kill fluid into the first borehole via the second borehole.