Once the hole has been drilled, it must be "completed" or prepared for extraction. Usually, this involves perforating the casing so oil or gas can enter the tube. Sand or gravel is often added to the bottom most reservoir to act as a screen.
Fluids -- often high-pressure water or an acid -- are then pumped through to clean and fracture the rock, encouraging it to begin releasing hydrocarbons. The main reservoir is then sealed off and connected to the surface with smaller diameter tubing.