The "holy grail" of the prog-ressing cavity (PC) pump world is within reach. The concept of a uniform-walled pump stator has been cons-idered for many years, but manufac-turing technology limitations hindered any movement - until recently.
Over a year ago, the manufacturing technique, which would set the "concept" free and start the process toward a reality, was developed. A "uniform-walled stator" was being used in the transfer pumping business when the oil and gas industry noticed it and applied it to the production, downhole pump market.
Since that time, several companies have actively initiated field testing of downhole pump systems with this technology. One company reported a system in the ground for eight months, with no problems encountered. The young nature of the technology, and long installations with minimal problems, offer little or no performance data.
Manufacturing technique
The previous method of installing rubber elastomer onto a stator tube resulted in a serrated tooth profile, seen when looking at the tube in cross section. This created added mass within the raised elastomer lobes, and resulted in severe heat retention (low dissipation) and hysteresis effects, causing premature elastomer failures in high temperature situations.
The new manufacturing technique almost eliminates these effects by coating the elastomer evenly onto a pre-lobed surface. This lobed surface is directly compatible with the choice of rotor lobe configuration and allows for uniform heat distribution and dissipation along the length of the stator.
Advantages of a uniform-walled stator range from increased pumping efficiencies to better solids tolerance, and higher pressure and temperature operating tolerances. The more uniform distribution of elastomer inside the stator tube has decreased the interference (gap between the stator and rotor surfaces), resulting in more uniform fluid flow between the rotor and stator surfaces, increasing solids tolerance, pumping efficiencies, and high pressure/high temperature (HP/HT) tolerance.
Directional applications
Those who participate in the directional drilling business will see this as an opportunity to breathe new life into the conventional mud motor. Around for more than 30 years, the mud motor has dominated the directional drilling market as the tool of choice in everything from drilling simple vertical wells to high angle extended reach wells. It has yielded recently to advancements in the rotary steering tool market.
Rumors circulating around the industry are suggesting 1.5-2.0 times the horsepower from a motor power section of the same length as today's conventional mud motor. Looked at from another point of view, a power section one half the length of the currently manufactured power section can generate the same horsepower.
This implies that shorter motor lengths are possible for the same horsepower, reducing the need for motors with extended power sections - used periodically in situations where extra horsepower and/or torque is required.
Being more resistant to HP/HT effects, the often-experienced problem of "stator chunking" will be reduced. Expect this HP/HT market to be one of the first applications for implementation of this technology. There are countless examples of conventional motor runs being cut short because of simple stator chunking, and lost motor power.
Although this will not help the "running out of weight" problem often encountered when drilling in extended reach situations with conventional motors, it will allow for more aggressive directional drilling in the upper angle building and turning sections of a well construction plan. This brings up the issue of drill bits, and how to deal with the higher bit torques generated by these new power sections.
Roller cone bits especially, may have a problem with this new motor configuration. There will definitely be a learning curve, while drillers and bit manufacturers evaluate just how much these new power sections will generate bit wear. Polycrystalline diamond compact (PDC) bits may benefit from these new power sections. Increased motor torque capability will favor PDC bits over roller cone bits because of two reasons:
- Increased bit torque favors PDC bits, by design.
- Increased bit torques will accelerate roller cone bit wear, and increase the potential for failure.
Expect a drilling motor alternative with these new power sections sometime in late 2000 - and many smiles on the faces of directional drillers all over the world.
