The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing, and may have a flange [..]

Unprocessed natural gas produced from a reservoir containing oil; natural gas produced with oil from oil wells. Sometimes called "Braden- head gas," "oil well gas," "wet gas," or "solution gas." 

The portion of the casing that protrudes above the surface and to which control valves and flow pipes are attached.

A fitting attached to the top (head) of the casing in a producing oil or gas well (usually at or just above ground level). The casinghead regulates the flow of oil or gas, helps separate oil from gas, allows the pumping of oil from an oil well, and facilitates periodic cleaning out of the well. See Christmas tree