What role does sand play during hydraulic fracturing?

In the context of hydraulic fracturing, sand serves as a critical propping agent, or proppant.1

During the fracturing process, a mixture of water, chemical additives, and sand is pumped into the reservoir at high pressure to create and enlarge tiny fissures in the rock formation. Once the high pressure is released and the water flows back out of the well, the sand remains trapped within these newly created fractures. By physically holding these fissures open, the sand prevents the rock from closing back up, thereby maintaining a permeable network that allows oil and gas to flow more easily into the wellbore.21