It is worth pointing out that it is the exploratory, fracturing and 'setting up' of a well that demands the most work and labor. Once they're pumping, you don't need nearly the same amount of help.

After the pricing downturn, the number of exploratory/new wells and re-fracturing of old wells declined remarkably, so it's not like they actually 'need' as many people on the oil field. If oil goes back above, say 60$/barrel you'll start seeing more hiring/exploration in ND. Right now they can profitably pump extant wells, but new wells will wait for demand...