Progressing cavity pumps are susceptible to an undesirable, damaging phenomenon known as stick-slip. Stick slip, or rod wrap, as it’s often called, causes downhole speed oscillations that can ultimately lead to rod failures and premature wear on pumping system components.

Any driven mechanical system exhibits some degree of compliance due to the elastic nature of the connection between the drive and load. The connection can stretch, bend, and/or twist, allowing the driving end to become out of phase with the reaction at the other end. In systems with stick-slip, friction compounds the problem as moving surfaces stick together, preventing motion until they suddenly break free and slip.

In a progressing cavity pump, a rod string extends hundreds or thousands of feet from the motor at the surface to pump itself downhole. Such a long shaft inherently has considerable springiness, both axially and radially, that allows it to store a large amount of energy. As the rod rotates, static frictional forces within the pump prevent the rotor from moving relative to the stator, and the rod string winds up. When the torsional force becomes high enough, the pump rotor breaks free and spins at a very high speed until the energy in the shaft is dissipated. Once the energy is released, the relative velocity between the surface and downhole drops to the point where friction again takes over and causes the pump to stick. Further relative motion is prevented until the rod is sufficiently wound again. This cycle repeats continuously, resulting in large swings in pump torque and speed and intermittent motion.

Figure 1 shows stick-slip oscillations in a PCP used for dewatering a coal-bed methane well in Wyoming. In the chart, motor torque varies from 25 to 55 ft-lb, which translates after gearing to 97 to 213 ft-lb at the rod with a mean torque of 155 ft-lb. The oscillation is clearly evident and measures ±58 ft-lb with a period of 3.4 seconds. Given the 3,156-foot depth of the pump, this oscillation represents approximately 15 revolutions of the rod string. The rod is turning at 206 rpm.

Oscillations caused by stick-slip in a progressing cavity pump. Hydraulic pumping systems also exhibit the characteristic stick-slip signature, only oscillations are measured in pressure rather than current or torque.

Operating PCPs in a cyclic stick-slip manner is both detrimental and inefficient. Stick-slip places undue strain on the working components of the system and ultimately causes premature failure within the pump, rod string, and tubing.  It also substantially reduces the pumping capacity of the pump. Pumping does not occur during the portion of the cycle when the rotor and stator are stuck to one another. Additionally, there may be little or no pumping while the pump rotor is rapidly accelerating and decelerating after breaking free due to cavitation of the fluid and other effects. Given the incompressibility and inertia of the fluid column, it is likely that much of the energy is converted to heat within the pump and fluid. This heat, in turn, causes the rotor and stator to swell, increasing the required breakaway torque and further hampering normal operation.

What solutions are there to stick-slip? One approach is to use a larger rod string to reduce the amplitude of the oscillations. By increasing the rod string from 7/8” to 1” on the well shown above, the resulting oscillations were diminished by about an order of magnitude. This can substantially increase the life expectancy of the rod.

Figure 2 shows the difference. Rod torque varies from 125 to 139 ft-lb with a mean torque of 132 ft-lb. The oscillation has been reduced to ±7 ft-lb and a period of 1.4 seconds. This represents about 1.5 revolutions of the rod string.

While increasing the diameter of the rod may reduce the amplitude of the oscillations, retrofitting a well with a larger rod string is a significant expense in terms of material, labor, and downtime. Furthermore, the approach may only move the problem to the pump.

The best solution is to avoid operating the pump in this manner altogether. The ultimate goal is to control the pump itself, not merely the polished rod. As such, a new control strategy that actively damps these large oscillations and controls the speed of the pump is required.

To learn more about stick-slip and PCPs, please contact us.