6 Artificial lift
Production wells are free flowing or lifted. A free flowing oil well has enough
downhole pressure to reach suitable wellhead production pressure and
maintain an acceptable well flow. If the formation pressure is too low, and
water or gas injection cannot maintain pressure or are not suitable, the well
must be artificially lifted. For smaller wells, 0.7 MPa (100 PSI) wellhead
pressure with a standing column of liquid in the tubing is measured, by a rule
of-thumb method, to allow the well to flow. Larger wells will be equipped with
artificial lift to increase production, even at much higher pressures. Some
artificial lift methods are:
3.
6.1 Rod pumps
Sucker rod
pumps, also
called donkey
or beam
pumps, are the
most common
artificial lift
system used in
land-based
operations. A
36
motor drives a reciprocating beam, connected to a polished rod passing into
the tubing via a stuffing box. The sucker rod continues down to the oil level
and is connected to a plunger with a valve.
On each upward stroke, the plunger lifts a volume of oil up and through the
wellhead discharge. On the downward stroke it sinks (it should sink, and not
be pushed) allowing oil to flow though the valve. The motor speed and
torque is controlled for efficiency and minimal wear with a pump off controller
(PoC). Use is limited to shallow reservoirs down to a few hundred meters,
and flows up to about 40 liters (10 gallons) per stroke.
3.6.2 ESP
A downhole pump inserts
the whole pumping
mechanism into the well. In
modern installations, an
electrical submerged pump
(ESP) is inserted into the
well. Here, the whole
assembly consisting of a
long narrow motor and a
multiphase pump, such as a
progressive cavity pump
(PCP) or centrifugal pump,
hangs by an electrical cable
with tension members down
the tubing. Ill: Wikipedia.org
Installations down to 3.7 km with power up to 750 kW have been installed. At
these depths and power ratings, medium voltage drives (up to 5kV) must be
used.
ESPs work in deep reservoirs, but are sensitive to contaminants such as
sand, and efficiency is sensitive to gas oil ration (GOR) (where gas over 10%
dramatically lowers efficiency
