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)
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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 man-
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drel, or sliding sleeve (Note: The second method applies to jet pumps and
not to piston pumps).
System Configurations
The 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
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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 Pumps
Hydraulic 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.
Power
Fluid
Return
Fluid
Well
Fluids
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.
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Typical Range
Maximum*
5,000 to 10,000’
17,000’
Volume
50 to 500 BPD
4,000 BPD
Temperature
100º to 250ºF
500ºF
Depth
Deviation
15º-25º/100’ Build Angle
Corrosion
Good
Gas Handling
Good
Solids Handling
Poor
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≥8º API
Fluid Gravity
Servicing
Hydraulic or Wireline
Prime Mover
Gas engine or Electric
Offshore
Good
System Efficiency
40% 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 Pumps
Jet 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.
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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.
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Depth
Volume
Temperature
Typical Range
Maximum*
5,000 to 10,000’
20,000’
300 to 1,000 BPD
>35,000 BPD
100º to 250ºF
500ºF
Deviation
<25º/100’ Build Angle
Corrosion
Excellent
Gas Handling
Good
Solids Handling
Good
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≥6º API
Fluid Gravity
Servicing
Hydraulic or Wireline
Prime Mover
Gas engine or Electric
Offshore
Excellent
Efficiency
10% to 30%
(Courtesy of Weatherford International)
Reliability for hydraulic jet pumps is excellent if attention is given to avoiding
cavitation conditions. Jet pumps typically are resistant to moderate to severe
amounts of sand and particulate matter. As with hydraulic piston pumps, jet
pumps can be easily retrieved and replaced so problems with jet pumps will
have far less of an impact on production than would failure of other types of
lift pumps.
2.4d.4 Other Hydraulic Pumps
Other hydraulic pump systems typically involve special purpose subsurface
assemblies.
Various configurations of hydraulically driven rotary pumps, such as the
ClydeUnion Hydraulic Submersible Pump (HSP), have been developed, but
none have seen wide acceptance in the oil and gas industry because of cost
and issues related to reliability. The ClydeUnion HSP consists of a multistage hydraulic turbine power section which drives a multistage centrifugal
pump section. The HSP is equivalent to an electrical submersible pump that
is driven by hydraulic power fluid rather than by electricity. While it has eliminated the problems associated with the cable and its connection to the motor, the HSP producing end still has the same characteristics and issues as
the typical ESP. It is capable of producing high volumes but is susceptible to
being damaged by sand and other particulates in the produced fluid as well
as being adversely affected by ingested free gas.
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There is also a pump which requires three strings of tubing. Two are for the
power fluid and the third sting is for the produced fluid. The two power fluid
strings are alternately pressurized and depressurized in order to force the
engine piston, which is on the bottom of the pump, up and then down, i.e. a
closed power fluid pump. The pump piston is on top and is connected to the
lower piston by a middle rod. This piston brings in the produced fluid and
then forces it into the production string. This style of pump has a depth limit
as the rod connecting the bottom engine piston to top producing piston is always in compression. It is fixed pump and the tubing must be retrieved in order to retrieve the pump.
An emerging hydraulic lift technology involves a subsurface bladder pump
whereby pressurized hydraulic power fluid alternately pressurizes and collapses one or more subsurface bladders to displace fluid to the surface. This
system would be relatively tolerant of particulate matter but would have limitations related to production volume, temperature, and depth.
2.4d.5 Operating Requirements
Hydraulic pump systems require clean power fluid, typically water but any
liquid can be used. If clean power fluid is not available, then appropriate
preconditioning fluid treatment process equipment is required. Well site
planning should accommodate the surface area required for the fluid processing equipment as well as the surface power fluid pump system.
Hydraulic lift systems employing jet pumps require more power than other lift
systems due to their inefficiencies and the hydraulic flow losses in high flow
rate systems. However, the increased power consumption must be weighed
against the advantages of system flexibility, reliability, serviceability, and increased “up time” common for such systems.
The same is not true for hydraulic lift systems employing piston pumps. They
have a high system efficiency and low fluid flow rates. They are also known
to be flexible, reliable and serviceable.
2.4d.6 Cost Considerations
The capital acquisition cost (CAPEX) of hydraulic systems is predominately
related to the surface pumping system and fluid processing equipment. The
subsurface components are secondary costs. Operating costs (OPEX) are
typically dominated by energy costs to power the system because maintenance and intervention costs are minimal.
Many smaller hydraulic systems are portable and available as rental units.
Rental units are particularly useful for temporary high lift rates such as initial
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well deliquification, frac flow-back, and well kick-off after servicing. After the
temporary use, the rental unit can be removed and redeployed as needed.
2.4d.7 Recommended Practices
Design for Free Gas
In order to avoid breakout of free gas in jet pump applications it is always
best to operate above the bubble point of the formation fluids if possible. A
throat with sufficient area is should be installed to prevent the chocking of its
inlet and thereby creating the conditions necessary for the formation of cavitation bubbles. These bubbles can also be formed whenever there is too little
pressure feeding the pump. While this varies by each application, one rule of
thumb is to have a pump intake pressure equivalent to 100 feet of vertical
height of formation fluid above the pump for each 1000 feet of pump TVD.
For conventional hydraulic piston pump applications, the same techniques
used to limit gas ingestion in sucker rod pumps should be used for hydraulic
piston pumps. However, a third flow path to surface will be required if gas is
to be separated using a gas anchor or similar separation techniques. (Reference free pump parallel return and fixed insert conventional configurations
in section 2.4d.1)
Special gas handling hydraulic piston pumps are available which produce the
gas with the production liquids without requiring a third flow path. Details are
available from the pump supplier.
Design for Highly Deviated Wells
For free pump casing return configurations the pump can be deployed by
gravity and power fluid flow by pumping down the production tubing. Pump
deployment is only limited by the location of the packer that seals off the tubing to casing annulus. The pump can be retrieved by reverse flow by pumping down the annulus.
Pumps can be deployed and retrieved at 12-15/100 ft of deviation for the
longer piston pumps and 25/100 ft for jet pumps (which tend to be about
50% of the length of a piston pump).
While a piston pump can pass through doglegs, it is important that it be set in
a straight section of the curve that is equal to or greater than its length. It
should never be operated in with the middle rod in a curved condition as this
will lead to a premature and unnecessary failure.
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2.4d.8 Trouble-shooting
Hydraulic piston pump failures are similar to failures common to sucker rod
pumps since the pumping sections are similar.
Jet pump problems are most often related to cavitation or normal wear. The
parts that are most often affected are the throat (see below) and the nozzle.
Note: Nozzles can have abrasion/erosion damage but never cavitation damage.