A drilling rig bores a hole down through the dirt and rock to the oil reservoir. This hole is usually between 5 and 36 inches wide. The drill itself is pushed down by the weight of the heavy metal piping above it, which pumps down drilling fluid called "mud." This fluid can be just water, water with air bubbles, or water with polymers. The mud creates the ideal conditions for the drill by sweeping up debris as it is pumped back up to the surface. As the drill goes deeper, new sections of piping are attached.
Completion and Casing
Completion involves the finishing touches made to the borehole and piping to control the flow of oil into the well. The simplest approach, called "barefoot," is to not do anything. In the open hole approach, a liner is made with many small holes, and then set across the production area, granting an intervention conduit and borehole stability. Sometimes concrete is poured into the space between the pipe and the borehole to achieve this stability. This is called a closed hole.
The top of the well is set with a set of valves, known as a production tree. The production tree is there to control the flow of oil and the pressure inside the well. During much of its productive life, this is all an oil well needs. The interior pressure of the Earth is enough to push oil up the well. When the oil reservoir begins to run out, more active measures are needed to keep the oil flowing. This usually involves a "workover" of the old well, which means either replacing or widening the old well, or drilling a new well into the same reservoir. Then water, gas, or carbon dioxide is pumped into the oil reservoir to increase the pressure and push more oil up.
Oil that has come up through a well is usually transported to a collection point via pipeline. This is the most economical means of moving a bulky commodity like oil, given that both the well and the collection point are fixed.