Page 215 - Introduction to Petroleum Engineering

202 WELL COMPLETIONS
10.6 WELLbORE AND SURFACE HARDWARE

All of the previous sections deal with connecting the formation to the wellbore.
In addition, wellbore and surface hardware are needed to complete the well and then
produce oil, gas, and associated water. Wellbore hardware includes production
tubing, nipples, subsurface safety valves, packers, and pumping equipment. Surface
hardware includes the wellhead, the Christmas tree, a pump driver, a separator,
storage tanks, and pipelines. In the following discussion, we briefly describe produc-
tion tubing systems and then pumping or artificial lift systems and introduce surface
facilities.
Production tubing consists of many 30 to 40‐foot lengths of pipe joined together.
Production tubing extends from the wellhead at the surface to the producing zone.
The weight of the tubing is supported by the wellhead. A short piece of pipe, or
landing nipple, is placed at or near the lower end of the tubing. The inside dimensions
of the landing nipple are machined to fit with tools and other hardware used during
workovers and other operations later in the life of the well. A packer may be installed
at the lower end of the tubing to seal the annular gap between the tubing and the
casing. In some wells, subsurface safety valves are installed in the tubing near the
surface so that fluid flow can be stopped in the event of damage to surface valves and
other equipment.
Pumping, or “artificial lift,” equipment is needed in many wells to lift liquids to
the surface because reservoir pressure is not sufficient on its own. Common methods
for artificial lift include sucker rod pumps, electric submersible pumps (ESP), gas
lift, or progressive cavity pumps (PCPs). These four methods are described here.
The rocking motion of a pump jack (also known as horsehead, nodding donkey,
grasshopper, etc.) is often encountered in oil country. The pump jack raises and
lowers the sucker rod that drives a piston pump near the bottom of the tubing.
In ESP, multiple impellers are mounted on a shaft driven by an electric motor.
Power for the motor is provided by an electric cable that runs along the side of the
tubing to the surface. Submersible pumps can be used in oil or gas wells to pump
liquid volumes at high rates. They are also common in coal gas production, offshore
production, and environmentally sensitive areas where the footprint of surface
facilities needs to be minimized.
In some wells, gas is injected at the surface into the tubing–casing annulus. The
gas flows through “gas‐lift” valves into the tubing to help lift the liquid to the surface.
The gas mixes with the liquid (oil or water) and reduces the density of the gas–liquid
mixture. If the density is low enough, the reservoir pressure may be able to push the
mixture to the surface.
Invented by Rene Moineau in 1930, PCPs consist of a helical steel shaft or rotor
that fits inside a helical rubber stator. When the rotor turns, cavities between the rotor
and stator advance along their axis. Liquid inside the cavities is forced toward the
surface. A PCP mounted near the end of production tubing is typically driven by a
motor mounted on the wellhead and connected to the PCP rotor by a steel shaft.
Although PCPs are used to lift liquids to the surface, they also function as downhole
motors in drilling operations.