Page 268 - Hydrocarbon

Well Dynamic Behaviour 255

Intermittent gas lift
Plungers.

The first five on the list are all pumps, literally squeezing, pushing or pulling the
fluids to surface, thus transferring mechanical energy to the fluids, albeit in different
ways. The gas lift systems add energy by adding light gas and thus lowering the
overall density of the produced fluids. A brief introduction to each of the systems
follows. Their schematics are shown in Figure 10.27.

10.8.1. Beam pump
These pumps are also commonly known as ‘rod pumps’ or ‘nodding donkeys’.
The beam pump has a subsurface plunger. The plunger is rocked up and down
by the movement of the walking beam on surface. The plunger has a check valve
(the travelling valve), whilst underneath the plunger there is a second valve
(the standing valve). As the plunger moves up liquid is sucked from the reservoir.
On the downstroke, the plunger refills, thus there is only pumping on the upstroke.
The walking beam is driven by an electric or reciprocating motor. The downhole
plunger and walking beam are mechanically connected by sucker rods. Different
plunger sizes (both area and length) allow for a large range of possible flowrates. For
a given plunger size, the flow rate can be further adjusted by altering stroke length
and pump speed. Even lower flow rates can easily be accommodated by cycling the
pump on and off. Finding the right balance between stroke length and pump speed
is the art of beam pump design. Sub-optimal designs lead to poor efficiencies and
excessive rod and pump wear. A ‘dynamometer’ is used to monitor the system. The
dynamometer shows the relationship between pump travel and load. Beam pumps
are very common on land wells, but are usually limited to a few hundred barrels per
day. Occasionally, designs can accommodate larger rates, especially if the surface
beam system is replaced with a hydraulic piston.

10.8.2. Progressive cavity pump

The progressive cavity pump consists of a rotating corkscrew like subsurface
assembly which is driven by a surface mounted motor – usually electrical. Beam
pump rods are used to connect the two. The flowrate achieved is mainly a function
of the rotational speed of the subsurface assembly. There is in principle very little
that can go wrong with progressive cavity pumps, although energy is lost by rod
wear in transferring the torque down the well – especially in deviated and deep
wells. For this reason, deeper pumps can employ a downhole motor similar to that
used in an ESP. Progressive cavity pumps excel in low productivity shallow wells
with viscous crude oils and can also handle significant quantities of produced solids.

10.8.3. Electric submersible pump

The ESP is an advanced multistage centrifugal pump, driven directly by a downhole
electric motor. The ESP’s output is more or less pre-determined by the type and