How helpful is Hotelling's Rule in determining the price of oil?
It would be easy enough to subject the question to simple quantitative analysis - get the time-series of historical oil prices and away we go.

Or, we can take a quick qualitative approach and get the same answer:

The Rule just sets out what the most economically efficient path would be. That's nothing at all to do with how things happen in reality: it's just how things behave in textbooks for beginners. For example, it relies on an assumption of perfect foreknowledge; whereas, in the real world, we have previously-unanticipated innovation that radically changes both industry's cost structures, and the amount of known reserves - as your fracking example illustrates.