In addition to the metal casing pipe, another smaller metal pipe is inserted down to the producing target formation inside the smallest casing pipe. It shows up in blue in this drawing. This smaller pipe is called the “tubing”. It is through this tubing pipe that the gas actually flows up to the surface. This tubing is used in addition to the casing because, for one reason, there is usually enough sand and other abrasives in the gas that it will erode holes in the casing pipe if the gas blows up out of the well only through the casing pipe. So tubing, which can be replaced, is used to convey the gas instead.