Growth of coiled tubing workover operations during the past decade has increased the demand for improved downhole motors for use in cleanout and milling operations.
To meet that demand, engineers have developed a reduced-length positive displacement motor (PDM) with an "equidistant" power section stator designed to improve motor performance and reliability at higher operating temperatures.
Conventional stator, Fig.1a
The new power section replaces the variable-thickness elastomer stator (Fig. 1a) with one manufactured primarily of a precision milled contoured internal steel stator surface, which is laminated with a thin, uniform-thickness layer of elastomer (Fig.1b).
Equidistant technology stator, Fig. 1b
Like other PDMs, the new motor works on the reverse of the Moineau pump principle, converting hydraulic energy into mechanical energy. The term "equidistant" refers to the uniform-thickness elastomer compared to the variable-thickness elastomer stator of the conventional motor.
Reduced stator deformation and less heat buildup in the equidistant stator result in higher volumetric and mechanical efficiency, raising the motor's performance output, and making it tolerate higher wellbore temperatures, pressures, and fluid solids.
Two case studies illustrate the motor efficiency improvement.
Tough scale removal
TotalFinaElf SA discovered Dunbar field in 1973, in 476 ft of water on Block 3/14a of the UK North Sea. The field contained original estimated reserves of 193.4 million bbl of oil and 1 tcf of gas.
The company began producing the field in 1994, with peak production occurring in 1997, and to date has drilled 28 wells.
Infield lines provide multi-phase transportation to the Alwyn North process plant. The Ninian pipeline system transports crude oil to the Sullom Voe terminal, and the Frigg pipeline system sends the natural gas to the St. Fergus terminal.
TotalFinaElf drilled the D03 well as a subsea tieback and producer in the frontal panel of the reservoir from the Santa Fe-135 semisubmersible. Completed with 41/2-in., 13.5 lb/ft tubing and 7-in., 35 lb/ft liner with a maximum deviation of 42°, the D03 began producing in December 1994 from the Ness-A formation.
In February 1995, after depleting the Ness A, the company set a plug in the well's isolation packer and perforated the Ness B, Tarbert Basal, and UMS formations.
For most of 1998, the well produced 1% basic sediment and water (bs&w). In August of that year, however, the percentage increased to 5%. A well test in September 1999 confirmed 4.4% bs&w and 135 b/d of water.
A production log run in December 2000 encountered an obstruction at 14,210 ft below the rotary table, indicating that barium sulfate scale had built up in the tubing string and liner.
The operator had to perform the scale removal operations with coiled tubing from a lower deck of the platform, restricting available height for tool string deployment.
Crews performed the scale removal job with a 2.88-in. OD version of the reduced-length equidistant technology motor. Well fluids are known to be hostile to conventional workover motors, with a history of causing damage to the rotor and stator.
The motor and a 3.61-in. mill, which the service company called a "Metal Muncher Turbo Mill," successfully removed the hard scale from the tubing. A three bladed, DB-style underreamer deployed on the same motor, successfully removed the scale from the liner.
Total job time for both trips was less than 14 hr. A post-job inspection revealed that no damage had occurred to the equidistant stator.
Cutting exotic material
Machar field, operated by BP and located in 276 ft of water on Block 23/26a of the UK North Sea, is an oil accumulation that was developed in three stages, beginning in May 1995.
BP estimated the field's original recoverable reserves at 145 million bbl of oil and 127 bcf of gas. Sulfur content of the oil and gas is 0.13%.
The well Machar-18, drilled in 1992, served intermittently as a producer and a water injector until it was scheduled for abandonment in September 2001.
The abandonment procedure called for a new cutting assembly to be deployed on 1¾-in. coiled tubing, the flow rate would be limited to 71.4 gpm, and treated seawater would be used as the circulating fluid.
As part of the abandonment program, BP required onshore trials to qualify an effective cutting system for the well's 7-in., 26 lb/ft Super 13-Cr tubulars, which contain 5% nickel.
On previous jobs, standard PDM motors and hydraulic cutting tools could not achieve consistent cuts of this material. Insufficient motor horsepower had resulted in stall-outs during critical points of the cutting operation, which frequently resulted in broken knives and incomplete cutouts.
For the onshore trial, the service company suspended and anchored a joint of the Super 13-Cr tubular in a support frame.
Crews connected the cutting assembly to the pump unit using 2 in. high-pressure hose, with a flow meter installed in the line, and lowered the assembly into the tubular to the point where technicians could visually observe the cut and collect cuttings samples.
For the trials, the company used a 2.88-in. OD equidistant technology workover motor run in conjunction with a 3¾-in. OD multistring cutter dressed with Superloy knives to perform a series of cuts in the Super 13-Cr material.
The trials achieved an average cut time of 47 min, with no motor stalls occurring throughout the entire test sequence. Only minimal wear was imparted to the cutter knife tips, but none were damaged or broken.
Advantages, limitations
Drilling motors and workover motors vary somewhat in design. To provide directional capabilities, a drilling motor contains a deflection device, typically an adjustable-bent housing, between the power section and bearing assembly.
The drilling motor also must allow for higher bending stress and cyclic loading than a workover motor; its components therefore require higher tensile strength.
Operators use workover motors in cased-hole environments, deployed on coiled tubing or small threaded pipe that generate considerably less weight on bit (WOB) than motors used in drilling operations normally experience.
Positive displacement motors have proven to be the most reliable and easy-to-operate for workover motor applications. The PDM's ability to convert hydraulic energy to mechanical energy, however, is not completely efficient because of leakage and other energy losses.
Torque and bit speed define the PDM output power, with the physical properties of the stator elastomer determining the maximum power output.
As load or WOB increases, differential pressure and torque also increase, causing the hydraulic forces inside the power section to deform the rubber profile until the sealing lines between the cavities open.
The resulting leakage causes the bit speed to drop until the motor finally stalls. To prevent stalling, the stator elastomer must provide sufficient hardness to withstand the deforming hydraulic forces. Conversely, the elastomer must also provide ample flexibility to seal properly, and it must provide enough fatigue strength to withstand the cycling loads.
Increasing flow rates produce increasingly faster rotor speeds, resulting in higher sliding velocity between rotor and stator, increased elastomer wear, more heat from friction, and repeated flexing of the rubber by the rotor and hydraulic forces.
A phenomenon called "hysteresis heat generation" results in heat build-up. Because rubber acts as an insulator, the heat cannot dissipate to the annulus. Heat build-up plus downhole temperature can undermine the elastomer's bond to the stator tube and also can degrade the elastomer's physical properties.
The rubber can become brittle and unable to flex under the cyclic loading of rotor and pressurized fluid. Cracks initiate and grow, and finally rubber chunking damages the stator.
Equidistant technology
Meeting the growing demand for improved PDM performance requires increasing the motor's mechanical and volumetric efficiencies while minimizing energy losses. Improving the rubber compound on the stator of conventional motors has produced only incremental improvements.
PDM technology studies have shown that motor efficiencies would increase significantly if the lobes of the stator were made primarily of a solid, nonelas tomeric material.
The biggest challenge, however, was finding a machining method to create lobes in a tube over the length of a typical stator. The equidistant stator has a stator tube with an internal profile that requires less rubber compound when molding the stator.
Fig. 1 shows the cross section of a conventional stator (1a) and one featuring equidistant technology (1b). The lobe, which was formerly molded completely of rubber, now is backed up by the internal profile of the stator tube.
During operation, the equidistant power section's lobes provide more resistance than lobes in a conventional motor. Even at high differential pressures, the stator profile does not lose its sealing capability between the cavities.
Thus, the motor maintains greater volumetric efficiency resulting in improved motor performance.
The equidistant technology motor experiences less heat buildup in the thin elastomer layer than in the thick rubber lobe of a conventional stator, resulting in improved mechanical efficiency and fewer energy losses.
While the equidistant design generates less heat, it also transfers heat faster to the drilling fluid in the annulus because of better thermal conductivity. As a result, equidistant motors using the same elastomer compounds as in conventional motors, operate effectively at higher temperatures.
Even though the highest recommended operating temperature is 400° F., tests have shown that the equidistant motor can operate in temperatures up to 420° F. with no damage or swelling to the rubber lining in the stator.
Performance comparison
Fig. 2 compares the performance of a 27/8-in. conventional workover motor with that of a 27/8-in. equidistant motor. The X-axis indicates rotor speed and the Y-axis is the output torque.
The vertical lines show constant flow rate power curves—in this case showing the power curves for 60, 90, and 120 gpm.
The horizontal lines show the power curves for constant differential pressures of 200, 400, 600, 800, and 1,000 psi across the motor.
Both power sections have the same lobe configuration and stator pitch length but slightly different profiles. The conventional motor has 5.5 stages and a larger chamber volume, compared with the equidistant stator with 4 stages.
Due to the difference in chamber volume, the conventional motor runs slower than the equidistant motor at the same flow rate, yet produces more torque at the same differential pressure.
Because of its improved efficiency, the equidistant motor produces considerably more output power than its conventional counterpart from the same input energy.
In addition, the equidistant motor can withstand more load, resulting in even more torque output.
Finally, since the equidistant motor only has 4 stages compared with 5.5 at the same stator pitch length, it delivers more power at less profile length.
The author
Mark McGurk is Thru-tubing Fishing product line manager with Baker Oil Tools, currently based in Houston. He has 12 years' industry experience, including the UK and Far East. McGurk holds an HNC in electrical engineering from Dundee College of Further Education, Dundee, Scotland.
