Pumps & Systems, April 2008
Artificial lift technology selection is highly dependent on the conditions of the wellbore and the specific application parameters of a given well. Electrical submersible pumping (ESP) systems employing centrifugal pumps are the preferred artificial lift technology for wells with higher flow rates and deeper well depths. ESP systems are used to lift fluids for a variety of industries, including oil and gas, mining, municipal water and geothermal. Rod lift systems-the most common form of artificial lift-use sucker rods to bring fluid to the surface and are used primarily in lower flow wells.
However, there are applications in the petroleum industry when typical centrifugal or rod pumps are not applicable due to a variety of conditions in the wellbore. In wells with high gas content in the fluid, sand laden fluids or thick viscous oil, the performance of ESP systems can be degraded. In deviated wells, mechanical wear between the sucker rods in a rod lift system and the production tubing is a significant problem.
For these problematic applications, technology exists that combines a submersible motor and a progressing cavity pump (PCP). A PCP is a positive displacement pump, made up of a single external helical shaped rotor turning eccentrically inside a double internal helical shaped stator of the same minor diameter and twice the pitch length of the rotor. This geometry creates "cavities" that are pushed up the pump with each turn of the rotor. Most commonly, these pumps are connected to the surface with sucker rods and are driven by a surface motor.
Because the entire rod string rotates within the tubing string, rod/tubing wear can be a serious issue among deviated wells. This can bore out big holes in the tubing, causing well fluid to slip back down the annulus and seriously hinder production. To solve this problem, Centrilift developed a method to incorporate ESP motors with PCPs to create the electrical submersible progressing cavity pumping (ESPCP) system. With this combination, PCPs are now able to match ESPs in depth and deviation capabilities.
The ESPCP system is made up of seven components: the submersible motor, gear-reduction unit, seal section, flex-shaft assembly, pump, variable speed drive and submersible cable.
Submersible Motor
ESP motors, ranging from 10-hp to 2,000-hp, are used on ESPCP systems. The motors are designed and manufactured with features to maximize run life in harsh downhole conditions. Optimized geometries and slot fill, as well as shaped rotor bars, enhance motor efficiencies and a patented T-ring in the bearing system maximizes motor life by providing radial support, maximized oil flow, anti-spin and optimized bearing performance during thermal cycling.
Gear-Reduction Unit (GRU)
Because of physical constraints with PCPs, they must rotate at a slower rpm than ESP systems. Standard electrical motors rotate at approximately 3,600-rpm at 60-Hz, but a PCP's operating range is typically 100- to 500-rpm. A double planetary gear designed unit can reduce the speed down 9 to 11 times its original RPM. The design allows it to retain more than 98 percent of the motor's efficiency.
This GRU also fulfills another requirement of PCPs-the higher torque needed to overcome the interference fit between the rotor and the stator. The GRU's design allows for the reduction of speed and the increase in output torque, allowing the motor to work easier throughout the life of the system.
Seal Section
The seal section protects the GRU and the motor from well fluid, equalizes the motor oil to the wellbore fluid pressure and supports the pump shaft thrust load. The short axial span between the bar stock components in the seal design also enhances rotational stability and minimizes vibration.
Flex-Shaft Assembly
As mentioned above, the PCP moves in an eccentric motion while the motor, GRU and seal move in a concentric motion. Therefore, there is a need for that eccentric motion to be transferred to the concentric motion while dampening the vibration it causes. A flex-shaft assembly couples the seal shaft and the pump rotor to compensate for the eccentric rotation and vibration.
Pump
PCPs are most effective in viscous oil, sand laden fluids and gassy wells. They are also suitable when emulsion is a concern, due to its geometry and gentle pumping action. Another benefit is that PCPs require smaller motors than ESPs, lowering energy costs.
Variable Speed Drives
Variable speed drives (VSD) provide a means to control the downhole ESPCP system. The ability to alter the settings on the downhole system allows operators to adjust production output as downhole conditions change. The VSD protects the pumping system by sensing changing conditions such as flow or pressure that could potentially cause damage. Alarms and limits can be programmed in the VSD, allowing for automatic step changes to optimize production and operating conditions.
Variable speed drives incorporate a graphical control system (GCS), which is a user-friendly interface to program and launch software features designed to maximize the life of the pumping system. The GCS interface also records, logs and communicates data from the drive and any downhole monitoring system to a laptop, SCADA or satellite system.
Cable
Cable connecting the downhole motor to the surface control system is available in a wide variety of configurations to most effectively match the fluid properties and wellbore geometries. Cable is made in both round and flat designs. While round cable has the best electrical properties, there are cases where the well diameter requires flat cable for maximum clearance and minimal damage. Cables have solid copper conductors, either EPDM rubber or polypropylene insulation, and coated galvanized steel armor. Some configurations include a lead sheath to enhance run life.
ESPCP Application Benefits
-
Initial costs are lower compared to a conventional ESP system
-
Improved volumetric and overall pump efficiency versus an ESP
-
Reduces operating expenses
-
Eliminates mechanical wearing of tubing and rods in deviated or horizontal installations
-
Allows for smaller motors to drive the system
-
Improves reliability and optimizes motor and pump speeds
-
Eliminates the need for multiple oil systems and seals for the motor and gear reducer
-
Provides efficient gas separation in deviated or horizontal wells
-
Allows the maximum flow rate at the lowest pressure drop
For oil fields with higher workover rates and/or difficult rig locations, ESPCPs can be deployed using a through tubing conveyed (TTC) deployment system. This technology allows the pump to be retrieved by wireline while leaving the other components downhole. In abrasive conditions, for example, the pump is typically the most failure prone component in a system, and with the TTC system, frequent pump change-outs can be achieved without the high costs of a work-over rig.
Case History
A Rocky Mountain operator was interested in replacing an existing rod driven progressing cavity pumping system in a horizontal coalbed methane gas well with an ESPCP system to increase run life and reduce workovers, as well as lessen the overall footprint of the surface equipment in an environmentally sensitive area. In a CBM well producing 100 percent water, the deviated wellbore was causing significant rod and tubing wear due to the lack of lubrication for the pump.
The ESPCP system increased production 350 million cubic feet of gas a day from the CBM well and system run life improved by more than threefold, providing additional operating cost savings. One manufacturer installed its ESPCP system to enhance overall PCP system run life and eliminate the surface drivehead. For added savings and ease of operation, the ESPCP system was deployed with a TTC deployment system. TTC deployment reduced the cost of pump change outs by more than $200,000.