Short but tough, a new series of powerful workover motors delivers several advantages.

A new workover motor has demonstrated that it can produce more torque at higher temperatures than previous positive displacement motors. These claims are realized through a new design power section that uses a contoured stator body with a uniform seal thickness. Because its interior profile is contoured to form the pressure cavity, the amount of rubber required to form the seal is greatly reduced, improving the motor's temperature rating and sealing efficiency. With lower friction and less distortion, the seal enables the motor to operate at higher differential pressure, thus producing higher torque.

The new design motor features stator lobes that are machined out of the body metal, greatly reducing the amount of rubber. The rubber seal is actually of equal thickness throughout, improving its heat dissipation ability. The metal lobes provide the stiffness needed to reduce seal distortion and leaks.

The highly-efficient new design permits motors to be made as short as 5.95 ft (1.8 m) while maintaining superior performance over conventional 11 ft (3 m) motors. This allows deployment of the motors in restricted-height areas of production platforms.

Design affects motor efficiency

PDM efficiency has both mechanical and volumetric components. The mechanical efficiency is mainly a function of friction between the rotor and the stator. It depends upon torque and differential pressure. The volumetric efficiency describes the leakage between the rotor and the stator, and is function of motor speed and flow rate. Together, the mechanical and volumetric components are influenced by:

• Surface condition of rotor - the smoother the better
• Downhole temperature. This is a mixed bag - although volumetric efficiency increases and mechanical efficiency decreases with temperature (because of a tighter fit between rotor and stator due to thermal expansion of the seal), volumetric efficiency is degraded when the sealing elastomer becomes softer with increased heat.
• Rotor/stator profile - mechanical efficiency increases with the number of lobes.
• Rotor/stator fit - the tighter the fit, the better the volumetric efficiency, but the worse the mechanical efficiency.
• Mud properties - hydraulic losses increase with viscosity, the better the fluid quality (lower sand content) the higher the mechanical efficiency.

Conventional motor performance is challenged by the effectiveness of the rubber element. It must seal effectively throughout a wide temperature range so maximum torque can be transmitted. It must also have mechanical strength to withstand the hydraulic forces as well as flexibility and fatigue strength to withstand the cycling loads. Failure can usually be traced to a build up of heat that adversely affects the rubber, making it brittle. It can delaminate from the stator body or spall, producing rubber chunks in the motor cavity. Performance degradation preceding failure is usually caused when high loads cause the seal to leak, ultimately resulting in a stalling condition.

Testing the principle

In a comparison test with a conventional PDM of identical length, rotor/stator fit and chamber volume, at several constant flow rates and differential pressures, torque and speed show great improvement.

Field results

In three recent applications, the equidistant technology motors were used to mill 80 ft (24 m) of 27/8-in liner, a cast iron bridge plug and 400 ft (122 m) of cement, and remove hard scale from 14,000 ft (4,270 m) of tubing and liner under hostile fluid conditions. In each case the motors performed without problems, completing the jobs in 20 hours, nine hours and 14 hours, respectively.
With their higher temperature and power ratings, the new technology motors can perform applications previously considered to be out of range for conventional motors. They can deliver higher penetration rates with fewer stalls, and their shorter length has expanded the envelope of interdeck applications on production platforms.

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