Hydraulic fracturing:

An operation in which a specially blended liquid is pumped down a well and into a formation under pressure high enough to cause the formation to crack open, forming passages through which oil can flow into the wellbore.

Sand grains, aluminum pellets, glass beads, or similar materials are carried in suspension into the fractures.

When the pressure is released at the surface, the fractures partially close on the proppants, leaving channels for oil to flow through to the well.

Compare: explosive fracturing.