After cementing the production casing, the completion crew runs a final string of pipe called the tubing. The well fluids flow from the reservoir to the surface through the tubing. Unlike casing, tubing is not cemented in but hangs from the wellhead at the surface. Tubing is smaller in diameter than casing-the outside diameter ranges from about 1 to 4-1/2 inches (about 25 to 114 millimeters). Rig operators nearly always run tubing in a well because oil and gas produce better through small-diameter tubing than through large-diameter casing, the way a river flows faster through a narrow channel than through a wide bed. In addition, servicing crews can remove tubing if it becomes plugged or damaged. They cannot easily remove casing because it is cemented in the well.
A packer is a ring made of metal and rubber that fits around the tubing. It provides a secure seal between everything above and below where it is set. It keeps well fluids and pressure away from the casing above it. Since the packer seals off the space between the tubing and the casing, it forces the formation fluids into and up the tubing. One reason to isolate the casing is to keep sand and sediment in the produced fluids from eroding it in the same way sandblasting etches glass. Sulfur and other naturally occurring chemicals in the reservoir fluids are another enemy of casing. They cause the metal to corrode. Tubing can erode and corrode too, but it can be pulled and repaired or replaced, since it is not cemented in the hole. In addition, corrosion-resistant tubing is cheaper than corrosion-resistant casing because tubing is smaller.
The sealing element of a packer, the packing element, is a dense synthetic rubber ring that expands against the side of the casing. A packer may have one packing element or several separated by metal rings. Slips hold the packers in place. A slip is a serrated piece of metal that grips the side of the casing. Most packers also have a circulation valve that allows fluids to pass through it. Although most of the time a packer must seal fluids off from the annulus, sometimes the crew opens the valve and uses a circulating fluid to set or retrieve the packer. Retrievable and permanent packers are the two main types of packers.
Another device frequently installed in the tubing string near the surface is a subsurface safety valve. The valve remains open as long as fluid flow is normal. When the valve senses something amiss with the surface equipment of the well, it closes, preventing the flow of fluids.
Safety valve