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