The development of unconventional resources such as shale oil, shale gas, tight oil, and tight gas depends on the use of stimulation techniques to access previously unproducible formations. Look at the graphic below to see how unconventional formations have permeabilities that are much lower than conventional reservoirs.
A rock unit with a permeability of one microdarcy has one tenth the permeability of a rock with a permeability of one millidarcy. (These Darcy units are named after our French friend Henry Darcy, of whom we wrote in another lesson.)
Some of these shales have permeabilities approaching that of cap rocks. That makes it quite challenging to convince these formations to give up their entrained hydrocarbons. Therefore, some kind of stimulation will be necessary.
The most widely-used stimulation method for unconventional resources is hydraulic fracturing. Hydraulic fracturing reduces the distance that oil or gas needs to flow through impermeable formations by creating a system of conductive fracture (i.e. open to flow).
We’ll spend most of the rest of this section learning about this effective stimulation technique.
Images: “Illustration” by Top Energy Training