Method and system for precise drilling guidance of twin wells

1. A method to guide a drilling path of a second well in proximity to a first well comprising:

applying a varying electrical current to a conductive casing or liner of the first well;

drilling a second well along a drilling trajectory;

from the second well, sensing an electromagnetic field generated by the current in the first well, and

guiding the drilling trajectory of the second well using the sensed electromagnetic field;

wherein applying the current further comprises drilling a third well to a distal end of the first well and forming a conductive path between the distal end of the first well and along the third well to a source of the current.

2. A method to guide a drilling path of a second well in proximity to a first well comprising:

applying a varying electrical current to a conductive casing or liner of the first well;

drilling a second well along a drilling trajectory;

from the second well, sensing an electromagnetic field generated by the current in the first well, and

guiding the drilling trajectory of the second well using the sensed electromagnetic field;

wherein applying the current further comprises drilling a third well to a distal end of the first well, and placing into the third well a conductive fluid to establish a conductive path between the distal end of the first well and a source of the current.

3. A method to guide a drilling path of a second well in proximity to a first well comprising:

drilling a third well towards a distal section of the first well and establishing a conductive path along the third well to the distal section of the first well;

forming an electrical circuit comprising an electrical generator, a conductive casing or liner of the first well and the conductive path along the third well, wherein said generator applies a varying electrical current to the circuit;

from the drilling path of the second well, sensing an electromagnetic field generated by the current in the first well, and

guiding the drilling path of the second well using the sensed electromagnetic field.

4. The method in claim 3 wherein the applied current is an alternating current (AC).

5. The method in claim 3 wherein the first well is horizontal and the drilling path of the second well is horizontal along a portion guided by the sensed electromagnetic field.

6. The method in claim 3 wherein the electromagnetic field is sensed by a pair of orthogonal magnetic sensors in the second drilling path and said method further comprises determining a distance between the sensors and the first well and a direction from the sensors to the first well.

7. The method in claim 3 wherein the generator is an above-ground alternating-current electrical generator.

8. A drilling guidance system for guiding a drilling path of a second well in proximity to a first well, said system comprising:

a first conductive path extending a length of the first well;

a generator of electrical current connected to opposite ends of the first well to apply current to the first conductive path;

a magnetic field sensor in the drilling path of the second well arranged to detect a field strength and direction of an electromagnetic field generated by the current applied to the first conductive path, and

a third well extending from a ground surface to a proximity of a distal portion of the first well and a further conductive path along the third well, wherein the further conductive path is electrically connected to the generator and the distal portion of the first well.

9. The system in claim 8 wherein said further conductive path comprises an electrode electrically coupled to the distal portion of the first well.

10. The system in claim 9 wherein said electrode further comprises expandable spring contacts to engage the third well.