Hydraulic fracturing is a well-stimulation technique in which rock is fractured by a hydraulically pressurized liquid. A high-pressure fluid is injected into a wellbore to create cracks in the deep-rock formations through which natural gas, petroleum, and brine will flow more freely. When the hydraulic pressure is removed from the well, small grains of hydraulic fracturing proppants hold the fractures open once the deep rock achieves geologic equilibrium.