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) Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 2 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- Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 3 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 Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 4 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 Page 5 ≥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. Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 6 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 Page 7 ≥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. Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 8 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 Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 9 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. Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 10 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.
