I do not believe most of the increased production is from old wells. Shale plays a significant part of the new production.
http://www.mywesttexas.com/top_stories/article_eedababa-60f5-11e1-a5dd-001871e3ce6c.html
http://www.freerepublic.com/focus/f-news/2859541/posts
http://oilshalegas.com/wolfcampshale.html
One more reason why rail tank cars are scarce as hen’s teeth.