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.