Some good news (for once) about the Gulf gusher. Relief Well One detected primary well yesterday

[Statistical]

Work on the first relief well, which started May 2, continues. The well reached a depth of 16,275 feet on June 23 before the drillstring was removed from the well to carry out the first 'ranging' run using wireline. During the ranging run, the MC252 well was successfully detected. Subsequent ranging runs will be needed to more precisely locate the well. Drilling and ranging operations will continue over the next few weeks towards the target intercept depth of approximately 18,000 feet, when 'kill' operations are expected to begin. The second relief well, which started May 16, is drilling ahead at a measured depth of 10,500 feet. Both wells are still estimated to take approximately three months to complete from commencement of drilling

http://www.bp.com/genericarticle.do?categoryId=2012968&contentId=7063132

A somewhat simplified explanation. First thing is BP isn't drilling the relief well. They have hired an company who specializes in solving difficult drilling problems (damaged wells, difficult geology, relief wells, etc). The lead engineer on relief well team has indicated he has done 40 relief wells and hit target on first attempt 40 of 40 times.

The relief well has to intersect the main well at ~18,000 ft below ocean floor. Obviously there is no way to see the well so how do you hit a target about a foot in diameter 3 miles under sold rock? Rather than poke around blindly they need something to identify the angle, direction, distance to main well so they can periodically adjust well shaft and "steer" into intersection point.

They do this by ranging. The main well is charged with an electrical current; electricity traveling in a wire generates a magnetic field. The sensor measures the magnetic field and a computer models the relationship between relief well and main well. From that they give drilling team a new "heading" (direction & angle).

The first ranging was conducted yesterday and the relief well "saw" the magnetic field and the drilling team has an updated target to drill towards.

Best analogy I can think of would be the game Marco-Polo. You are chasing blind so periodically you need to stop call Marco and then "aim" for the "Polo" response. Detecting the magnetic "Polo" is an important but geeky development and likely will be ignored by the media. Given the damaged condition of main well it wasn't 100% guaranteed that relief well would be able to "see" main well. The next step is to pull up sensor package, re-lower drill string and drill closer to the main well. They will keep alternating drilling and ranging until they hit main well.