Since the pay zone is sealed off by the production string and cement, perforations must be made in order for the oil or gas to flow into the wellbore. Perforations are simply holes that are made through the casing and cement and extend some distance into the formation. The most common method of perforating incorporates shaped-charge explosives (similar to those used in armor-piercing shells). Shaped charges accomplish penetration by creating a jet of high-pressure, high-velocity gas.
The charges are arranged in a tool called a gun that is lowered into the well opposite the producing zone. Usually the gun is lowered in on wireline. When the gun is in position, the charges are fired by electronic means from the surface. After the perforations are made, the tool is retrieved. Perforating is usually performed by a service company that specializes in this technique.