Selection of Artificial Lift Systems for Deliquifying Gas
WellsPage 10
2.4d Hydraulic Pumping
Hydraulic pumping systems represent one of the most flexible
lift technologies capable of producing fluids. As such, they are
frequently used in applications where other lift technologies have
failed. These systems consist of a subsurface pump powered by a
high pressure liquid that is pumped from the surface.
This section presents the operating principals, operating
limits, and system requirements for hydraulic artificial lift
systems. Recommended practices, operating considerations, and costs
are discussed. This discussion will be limited to lift systems used
for gas well deliquification.
The two predominant types of hydraulic subsurface pumps are
piston pumps and jet pumps. Accordingly they will be the focus of
this document although other hydraulic technologies are included
for reference.
2.4d.1 System Description Hydraulic Piston Pumps
The surface and subsurface equipment for a typical hydraulic
lift system are shown below:
Hydraulic Lift System (Courtesy of Weatherford
International)
Surface pump systems deliver high pressure liquid to power the
subsurface pump. They consist of high pressure pumps, prime movers,
fluid conditioning equipment, manifolds and valves. These surface
systems can be skid mounted or permanent installations.
Surface Pump System (Courtesy of Weatherford International)
The pumps are typically multiplex piston pumps but can also be
any type of pump that is compatible with the liquid power fluid
being used, and can generate the necessary pressure and flow rate.
The most common power fluid is water, although conditioned produced
oils are frequently used. Note: It is absolutely necessary that a
surfactant be used if water is the power fluid and the downhole
pump is a piston pump. The reason is that water has no lubricating
properties. It is also possible to use other liquids such as diesel
but they are normally cost prohibitive. The prime movers are
typically electric where line power is available. Gas and diesel
engine driven surface pumping systems are common where electricity
is not reliable or available.
The typical subsurface assembly consists of a piston pump or jet
pump landed in a seating assembly above a retrievable standing
valve and packer. The pump can be run and retrieved either by
wireline, or by gravity and power fluid circulation (free style).
When the power fluid is delivered to the subsurface pump through
the production tubing, the pressure of the power fluid holds the
subsurface pump against the seating assembly. When the power fluid
is delivered through a second tubing string or through the annulus,
the subsurface pump is locked into a profile in the seating
assembly, lock mandrel, or sliding sleeve (Note: The second method
applies to jet pumps and not to piston pumps).
System ConfigurationsThe common configurations for hydraulic
pump systems can be described in terms of how the subsurface pump
is deployed and retrieved.
Free Pump Casing Return The pump is deployed within the
production tubing by gravity and/or by power fluid circulation.
Power fluid pumped down the production tubing to the subsurface
pump causes the subsurface pump to lift production fluid comingled
with discharged power fluid up the annulus between the tubing and
casing. A packer below the pump isolates the annulus so that it can
be a return flow path. The pump is retrieved by just reversing the
direction of flow of the power fluid.
Free Pump Parallel Return This configuration is similar to the
Free Pump Casing Return configuration except a second tubing string
is used to return the comingled production and power fluids to
surface. This arrangement leaves the casing annulus open for
production of gas. As for the previous pump, this one is also
retrieved by just reversing the direction of flow of the power
fluid.
Fixed Pump Casing Return The pump is attached to a tubing string
and run into the well. The power fluid is delivered through the
tubing, and comingled production and power fluids are returned in
the casing annulus. It is retrieved by removing the tubing to which
it is attached.
Fixed Insert Conventional An insertable style pump is attached
to tubing (coiled or stick pipe) and run into the well inside of
production tubing. Power fluid is delivered through the coiled
tubing with the comingled power and production fluids produced in
the annulus between the coiled tubing and production tubing. This
leaves the casing annulus available for gas production.
Wireline Pump Standard Circulation The subsurface pump is run
into the production tubing using the power fluid or on wireline,
seats and seals in a downhole seating assembly. A jet pump is also
able to set in a sliding sleeve or gas lift mandrel. A packer below
the pump isolates the casing annulus so that it can be used to
return comingled power and production fluids to surface. Power
fluid pumped down the production tubing holds the pump in place.
Retrieval is accomplished by using wireline.
Wireline Pump Reverse Circulation The subsurface pump is run
into the production tubing on wireline, latched and sealed in a
sliding sleeve or mandrel. Power fluid is pumped down the casing
annulus or a parallel string of tubing and the comingled power and
production fluids are pumped up the production tubing. This
arrangement keeps well fluids off of the casing but can expose the
casing to high injection pressures.
Closed Loop For hydraulic piston pumps, the power fluid can be
kept in a closed loop separated from the produced fluid by using a
second tubing string to return the segregated power fluid to
surface. However, it is generally more cost effective to comingle
the power fluid and then separate fluids as needed at the surface
rather than complicate the well completion with a closed loop
system. It is not possible to have closed loop jet pump systems
because the power fluid and produced fluids become comingled during
the jet pumping process. For these reasons closed loop systems are
rarely used.
2.4d.2 Hydraulic Piston PumpsHydraulic piston pumps are similar
to sucker rod pumps except the reciprocating pump piston is driven
by an internal hydraulic engine section. This engine section
converts the continuous flow of the power fluid into reciprocating
motion. The power fluid causes the piston in the engine section to
stroke. At each end of the piston stroke a valve shifts to redirect
the power fluid to drive the engine piston back in the reverse
direction. The result is continuous reciprocation of the engine
piston and pump piston.
PowerFluidWellFluidsReturnFluid
Hydraulic Piston Pump (Illustration courtesy of Weatherford
International)
Because the pump section of hydraulic piston pumps is
essentially a sucker rod pump, hydraulic piston pumps have many of
the same advantages and limitations as sucker rod pumps. They
provide strong draw down of fluids and have good volumetric
efficiency. They tend to be insensitive to temperature. However,
they are precision devices with close tolerance components and are
not tolerant of gas, sand and particulate matter. Unlike sucker rod
systems, hydraulic piston pumps do not require a rod string, so
they avoid issues related to rod-tubing wear.
Typical RangeMaximum*
Depth5,000 to 10,00017,000
Volume50 to 500 BPD4,000 BPD
Temperature100 to 250F500F
Deviation15-25/100 Build Angle
CorrosionGood
Gas HandlingGood
Solids HandlingPoor
Fluid Gravity8 API
ServicingHydraulic or Wireline
Prime MoverGas engine or Electric
OffshoreGood
System Efficiency40% to 50%
(Courtesy of Weatherford International)
In general, overall reliability for hydraulic piston pumps is
good except in abrasive fluids. Life expectancy of hydraulic piston
pumps should be similar to sucker rod pumps in similar
circumstances since the pump components exposed to well fluids are
similar. However, the increased precision and smaller valve
components used in hydraulic piston pumps requires that particles
be removed from the power fluid to prevent premature wear of the
engine end. More importantly, because all subsurface hydraulic
pumps can be easily retrieved and replaced, problems with hydraulic
pumps will have less of an impact on production than would failure
of other types of lift pumps.
2.4d.3 Hydraulic Jet PumpsJet pumps operate based on venturi
nozzle principles whereby the kinetic energy of a high pressure/low
velocity fluid is converted to low pressure and high velocity as
the flow area passes through the nozzle. This is in response to the
decreasing area of the fluid passages. Production fluid then
comingles with the power fluid as they enter the throat of the jet
pump. It accelerates with the power fluid, and then becomes
pressurized as the comingled fluid decreases in velocity in the
diffuser.
Hydraulic Jet Pump (Illustration courtesy of Weatherford
International)
Jet pumps have no moving parts so they have no mechanical wear
and are not susceptible to gas locking thereby making them
extremely reliable. They are tolerant of moderate to severe volumes
of sand and particulate matter, corrosive fluids, and high
temperatures. Contrary to intuition, jet pumps do not cause
emulsions because there is insufficient time for the emulsion to
form. Jet pumps often work where other lift technologies fail.
Free gas is the primary physical challenge for jet pumps. Too
much free gas can choke the inlet of the throat. This condition
results in the formation of cavitation bubbles which can damage the
throat when they finally collapse. Free gas problems are
exacerbated when operating at pump intake pressures below the
bubble point of the reservoir fluids being produced. To prevent
problems with free gas, a sufficient flow area in the throat must
be provided which provides a flow path for the gas through the
throat.
A pump intake pressure that is too low will also result in
cavitation issues (pumping off the well). The requirement to
maintain a minimum pump intake pressure limits the amount of draw
down that can be achieved with jet pumps.
Overall system efficiency is lower than for positive
displacement pumps. This coupled with hydraulic transmission losses
usually require more power to drive jet pumps than some other lift
technologies.
Typical RangeMaximum*
Depth5,000 to 10,00020,000
Volume300 to 1,000 BPD>35,000 BPD
Temperature100 to 250F500F
Deviation