http://www.laweekly.com/news/what-went-wrong-at-porter-ranch... has a very clear diagram of what's happening. The inner casing of the well sprang a leak. The gas is blowing all the way back down the outer casing of the well, then exiting adjacent to the casing. So, while the leak is concentrated to an area directly adjacent to the well, it's not a matter of simply capping the pipe.