http://www.google.co.th/url?sa=t&rct=j&q=cost%20of%20us%20shale%20wells%2F&source=web&cd=4&ved=0CD0QFjAD&url=http%3A%2F%2Fwww.stgs.org%2Farticle.pdf&ei=SvVvT6PgMs7HrQfZ8uyfDg&usg=AFQjCNEX5SSlPrwY1LSXqXHUB7QdYuQHAw