"Frac sand" is a high-purity quartz sand with very durable and very round particles. It is a pressure-resistant material produced specifically for the petroleum industry. It is used in the hydraulic fracturing process (called "fracking") to produce petroleum fluids, such as oil, natural gas, and natural gas liquids, from rock units that lack sufficient pore space for these fluids to flow to the well.
Most frac sand is a natural material made from high-purity sandstone. Alternative products are ceramic beads made from sintered bauxite or small metal beads made from aluminum.
How is frac sand used?
Some underground rock units, such as organic shales, contain large amounts of oil, natural gas, or natural gas liquids that do not flow freely into the well. They do not flow into the well because the rock units either lack permeability (the pore spaces are interconnected) or the pore spaces in the rock are too small for these fluids to flow through.
The hydraulic fracturing process solves this problem by creating fractures in the rock. This is done by drilling a well into the rock, sealing off the section of the well that contains the oil, and then pumping high-pressure water into that section of the well. This water is often treated with chemicals and thickeners, such as guar gum, to form a thick gel. This gel helps the water carry suspended frac sand particles.
Large pumps on the surface raise the water pressure in the sealed portion of the well high enough to exceed the breaking point of the surrounding rock. When the rocks reach their breaking point, they snap and water rushes rapidly into the fractures, causing them to expand and extend deeper into the rock. Billions of sand particles are carried deep into the fractures by this sudden surge of water. A single well may require thousands of tons of frac sand to stimulate the well.
When the pumps are turned off, the fractures shrink but do not completely close - because they are propped up by billions of frac sand particles. This can only happen if enough sand particles are delivered into the rock to resist the force of closing the fractures.
The newly formed fractures in the rock open, supported by the strong sand particles, forming a network of pores that allow oil fluids to flow out of the rock and into the well. Frac sand is called a "proppant" because it "props" the fractures open.
Other materials used as proppants include ceramic beads, aluminum beads, and sintered bauxite. Frac sand generally has the highest level of performance and is currently the most commonly used proppant in the oil industry.