The Moineau Principle is the fundamental concept behind the Progressing Cavity Pump design. It originated in the 1920s when French aeronautical engineer Rene Moineau sought to enhance engine performance by developing a compressor. The pump consists of two components, the rotor and the stator. The rotor is a metal piece with a single machined “lobe” running down its length, while the stator is a metal tube with two spiral rubber “lobes” molded inside.
When you insert the rotor into the stator, it creates sealed “capsulisms” along the length of the pump, and when the rotor turns, it displaces fluid from one end to the other. As such, the progressing cavity pump is categorized as a type of positive displacement pump. Its spiral shapes are based on hypocycloids, which are curves that are generated as a point on the circumference of a smaller circle rolls around the inside of another circle.
The progressing cavity concept relies on the use of two dissimilar profiles with varying “leads” to create spiral-shaped cavities along the power section’s length when the rotor is assembled into the stator. In a progressing cavity power section, its rotor will have one less lobe than its stator counterpart. This arrangement results in one cavity being completely sealed off by the stator profile, another cavity remaining completely open with the rest of the cavities left somewhere in between in a semi-sealed state.
The cavities are referred to as “capsulisms” and operate on a principle similar to pumps. However, in this case, fluid is supplied from the mud pumps at the rig to flow into the motor. As the fluid fills the voids left by the cavities and becomes pressurized, it compels the rotor to rotate in the opposite direction of the rotor’s spiral lead. This is how the power section converts hydraulic energy from flow rate and pressure into mechanical energy of RPM and torque.
A progressive cavity pump (PCP) is a specialized technology that gained prominence in the 1980s for pumping heavy oils in challenging environments due to its ability to handle viscous and abrasive fluids at high pressure in downhole applications. After some initial technical challenges related to the pump drive were resolved, PCPs began being used for oil production, notably in Canada for handling sand and viscous crude.
Over the past two decades, PCP technology has expanded to handle various fluids at higher pressures and volumes, often replacing donkey and centrifugal pumps. In the 1970s, PCPs were adapted for use as downhole motors. Utilizing this technology, positive displacement motors have become essential to drilling operations and well planning.
A positive displacement motor, often referred to as a PDM or mud motor, comprises of three primary sections: the bearing section, transmission section and power section. The bearing section takes on the axial and radial loads on the motor during drilling operations. The transmission section centralizes the rotor’s eccentric motion and transmits energy to a drive mandrel, which then turns the bit.
The power section generates RPM and torque utilizing the flow of drilling fluid. The bearing and transmission sections are typically produced by downhole tool companies that either specialize in motors or have a suite of other downhole tools. These companies then purchase the power section from a vendor that specializes in the design and manufacture of progressing cavity power sections.
In modern drilling, positive displacement motors are used worldwide in various drilling environments and applications, enabling capabilities beyond standard rotary drilling. This technology has revolutionized the drilling industry, facilitating higher performance straight hole drilling, improved directional drilling control, and even horizontal drilling.
The power section industry has its own set of standardized terms, but there’s often confusion due to variations in terminology used by different manufacturers. To clear this up, let’s establish the following key terms:
Stage: the distance one lobe covers to complete a 360° revolution over the stator’s length
Lead: the distance one lobe covers to complete a 360° revolution over the rotor’s length
Pitch: the measurement from the crest of one rotor lobe to an adjacent crest.
Stage and lead can sometimes be confused as they are similar but not equal dimensions. Where they differ is determined by the lobe ratio between the rotor and stator. For example, in a 1:2 lobe design, the rotor's lead length is one-half that of the stator's stage length. In a 6:7 lobe setup, the rotor’s lead is six-sevenths of the stator’s stage.
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Fit is typically determined by choosing one stator from a group of various stator profile sizes to pair with a rotor. The fit must be chosen to provide the best balance of performance and stator life to achieve the objectives of the application. An ideal power section fit provides enough compression to limit RPM slip during operation but not so much compression that it leads to failure due to excessive stress in the elastomer.
Too much compression constricts the rotor’s movements and causes friction between the rotor and the elastomer. This friction generates unnecessary heat and increases wear on both the rotor and elastomer.
Too little compression allows the drilling fluid to bypass the seal lines in the power section without imparting energy on the rotor. Without this energy, the bit will lose RPM, leading to reduced ROP.
Since the elastomer expands with temperature, the fit chosen in the motor shop, at room temperature, is looser than what the power section will experience downhole during operation. Power section manufacturers and motor providers have a wealth of experience in selecting fits for different applications.
The formulas below show how to calculate fit based on the measurements taken from the rotor and stator.
Odd Lobe Rotor
Rotor C to V - Stator Minor = Fit*
Even Lobe Rotor
Rotor Minor +2ecc - Stator Minor = Fit*
*A negative result indicates a fit with clearance. A positive number indicates a tight or compressed fit.
The downhole environment is complex, and many factors can affect power section performance, including the fit of the power section’s rotor and stator profiles, fluid compatibility, downhole temperatures, and drilling dysfunction.
Power section efficiency and durability are influenced by how tight or loose its stator and rotor pieces fit together. The power section fit is driven by several factors including the downhole temperatures, rotor condition, and the reaction between the drilling fluid and stator elastomer.
Fluid compatibility between mud and the stator elastomer is crucial; adverse reactions can lead to issues, affecting downhole performance and fit.
Downhole temperatures, driven by geology and sometimes exacerbated by friction, can age the stator elastomer and reduce its working life. Countermeasures include de-rating, optimal fit selection, and fluid management.
Reduced stator life often results from drilling dysfunction, including torque/RPM swings and vibration. Mitigation involves choosing the right motor configuration for the application, fit optimization, implementation of optimal drilling parameters with adjustments at critical points, and using proper connection and sliding practices.
All of these factors have the potential to significantly impact power section performance and life. Following best practices reduces elastomer cyclical loading, extending its life.
In the context of power sections for downhole motors, several critical factors influence performance and efficiency during drilling operations. Understanding these factors is essential for optimizing drilling processes and achieving desired penetration rates.
RPM (Revolutions per Minute): RPM, measuring number of rotations over one minute, depends on the rotor's geometry, including the number of lobes and pitch/lead. A higher flow rate supplied by rig pumps leads to increased RPM. Flow rate is often reported in units of gallons per minute (gpm), liters per minute (lpm) or cubic meters (m3) per minute. Differential pressure also affects RPM as its behavior depends on the integrity of the seal between the rotor and stator profiles. That seal is what holds the fluid and pressure, causing the rotor to turn. A broken, worn, or leaking seal will cause a drop in RPM, and as the pressure increases, the leaking will also worsen, furthering a reduction in RPM. In a worst-case scenario, a compromised seal will cause RPM to drop to zero, creating a stalled condition.
Torque: Torque is defined as a twisting force over length acting on an object. In drilling operations, the differential pressure created by the power section translates into torque at the bit. The higher the pressure, the higher the torque output from the motor. Note that effective differential pressure is different than off-bottom pressure (OFB). The higher the OFB value is, the less effective differential pressure there is available to drill with. A thorough understanding of off-bottom pressure (OFB) and effective differential pressure can help explain why a motor might stall despite a limited amount of differential pressure. To address this, high-performance motors with added stages have been designed to provide more differential pressure capability.
Horsepower: Horsepower results from the interplay between RPM and torque. While higher differential pressure theoretically leads to increased horsepower, the power section’s design limitations mean that there is a point where fluid will begin to leak or slip past the seal line between the stator and rotor. When RPM and torque are reduced, they no longer produce enough force to overcome the load on the bit, and the motor is more likely to stall. Power output (HP) can be calculated using the formula:
(RPM x Torque (ft-lbs)) / 5252 = Power output (HP).
Temperature: Temperature of the intended drilling application is a key consideration during a power section’s design. Factors contributing to temperature variations include the power section's geometry, particularly the number of lobes, pitch, and lead designs. Temperature can impact the thermal expansion of rubber components and affect the overall fit, hence the design and production of multiple stator sizes for a given model. Additionally, static or bottom-hole circulating temperature variations in well bores can influence power section performance.
Hysteresis: Hysteresis represents the heat generated by the load, or stress, applied to the stator’s elastomer as the rotor’s surface passes over it during operation. Think of repeatedly bending a paperclip until it breaks. The fatigue failure of the paperclip is hot when it breaks. That heat is the result of hysteresis.
Lean Six Sigma is a process improvement approach blending the tools and techniques from Lean and Six Sigma methodologies. Lean primarily seeks to eliminate waste from processes in order to maximize value to customers. Six Sigma’s approach reduces variation in those processes to maximize quality control.
At PV, Lean Six Sigma (LSS) is more than a process improvement approach or project management methodology—it is a way of thinking. Not only do we apply LSS philosophy in our manufacturing processes, we use it in product design & development, transportation, accounting and in the office, as well.
The fit between the rotor and stator in downhole motors is critical for performance and durability. It can be influenced by various factors, including design considerations and environmental variables. Achieving the right fit is essential to minimize RPM drop as differential pressure increases while preventing excessive heat buildup in the stator to extend its service life.
There are two approaches to manage fit:
1. Design and Manufacturing Stage: The fit can be addressed during the design phase by considering factors such as well temperature and thermal expansion of materials. By anticipating the elastomer’s thermal expansion, optimal compression can be designed to accommodate temperature variations.
2. Fit Management at Maintenance Facilities: To assess fit, motor shops use measurements, typically minor diameters for stators and major diameters for rotors. However, this approach provides limited insight into the overall fit along the entire profile. Fit should be considered comprehensively across the profile to prevent fluid bypass, which can lead to motor performance issues.
The standard fit guideline is that a tighter fit generally yields better performance but may reduce the equipment’s service life. In the past, a larger amount of compression was used, resulting in high horsepower with less RPM drop. However, this limited the equipment’s service life. Nowadays, manufacturers balance compression to limit RPM drop at high differential pressures while maintaining lower elastomer heat generation for extended stator life.
