Last week members of a team at Oil field giant Baker Hughes were awarded a patent for a drive-controlled downhole pump system configuration where oil well performance is optimized through the use of voltage and torque sensing and control to precisely time the restart of a well when an interruption has occurred. The result is a higher performing well.

While the patent is wide ranging (as many are), the essential benefit of the technique is to make incremental improvements to system availability and to lower the time between failures across a wide range of scenarios. This would not be possible without the drive.

For example, according to the patent, "...if the pump system has enough torque to restart at the identified speed, it can be immediately restarted. If the system does not have enough torque to restart immediately, the drive continues to match the motor speed until the motor speed falls below a threshold level at which the pump system has sufficient torque to restore forward rotation of the pump and support the column of fluid in the well. The pump can then be restarted. Alternatively, the drive can continue to match the motor speed until it is determined that the motor has stopped, at which point the pump can be restarted. It should be noted that, if the drive sweeps through the range of frequencies at which the pump motor may be spinning and does not detect any drops in output current, it can be assumed that the pump has stopped spinning and can be restarted normally."

The point is that we've reached an exciting time in the development of drives. System engineers understand the base economics, the usefulness and the potential of drives, and are now using them as tools to improve overall system performance and make machines more flexible and productive.

So it's not always just about energy.

For those who would care to learn more about the Baker Hughes patent, here are some highlights.

The application challenge is explained in the "related art" section:

Crude oil is typically produced by drilling wells into oil reservoirs and then pumping the oil out of the reservoirs through the wells. Often, the oil is pumped out of the wells using electric submersible pumps. Electrical power is provided to electrical drive systems at the surface of the wells and these drive systems provide the required electrical power to the downhole pumps.

While downhole pumps are designed to operate continuously, they are subject to interruptions that can result from a number of different causes. For example, changes in well conditions (e.g., the appearance of gas in an oil well) may cause the pump to stop operating. Interruptions or variations in the power supplied to a pump's drive system may also cause operation of the pump to be interrupted. Even if these interruptions in the operation of the pump are relatively short, they may nevertheless be very disruptive, particularly when the pumps are submersible pumps operated in deep wells.

These interruptions may be very disruptive because submersible pumps, which must fit in a well and must therefore be long and narrow, have very little inertia. Consequently, when there is a change in conditions which causes an interruption, these pumps slow down or stop very quickly in comparison to pumps which have more inertia, such as surface pumps. The deceleration of the pump is even more pronounced in deep wells due to the large fluid column above the pump. Normally, when the operation of the pump is interrupted for longer than about half a second, the pump will have begun to spin in reverse.

Typically, there is some speed below which the pressure produced by the pump is insufficient to support the column of fluid. When the rotation of the pump falls below this speed, the fluid starts to fall back through the well and through the pump, dramatically increasing the torque required to resume forward rotation of the pump. While it is possible to match the speed of the pump motor, slow its reverse spin and start it spinning forward again, this often requires a great deal of torque. The torque that can be generated by the pump system may be limited by such factors as the output of the drive for the pump motor, the impedance of the cable carrying the power downhole, etc., so restarting the pump motor may require more torque than the system can generate. It is therefore typically necessary to stop the pump and wait for the column of fluid to drain from the well before the pump can be restarted. The time required for the fluid column to drain back into the formation may take a few minutes in some cases, while in other cases it may take more than an hour.

Normally, when it is necessary to restart a pump, an operator waits for a predetermined period and then restarts the pump. The wait period is typically determined by adding the amount of time necessary for the fluid to completely drain from the well and a safety margin (for example, an additional 25%.) Because each well may normally produce hundreds or even thousands of barrels of oil in a day, the cost associated with the delay between the pump stopping and being restarted can be very high. There is therefore a need to minimize the delay between the time the pump stops and the time the pump is restarted.

And the drive-enabled solution is explain in the summary of the invention (although there are many disclosures. We've just highlight one.)

This disclosure is directed to systems and methods for using variable speed drives to restart downhole submersible pump motors that solve one or more of the problems discussed above. In one particular embodiment, a downhole electric submersible pump deployed in a well is controlled using a variable speed drive. The variable speed drive includes a control system which is configured to detect interruptions in the operation of the pump system. If the control system detects a power interruption or some other interruption that requires the restart of the pump motor, the control system determines the reverse rotational speed of the pump motor and restarts the motor when this speed is sufficiently low. In this embodiment, the control system is configured to reduce the output voltage of the variable speed drive and sweep through a range of output frequencies to determine the frequency at which the current drawn by the motor is lowest. This is the frequency at which the apparent impedance of the motor is highest, indicating that the frequency of the variable speed drive's output matches the speed of the motor. The reverse rotational speed of the motor is known from this frequency, so the control system determines whether the speed is low enough that the pump system has sufficient torque to restart. If the speed is low enough, the motor is restarted. Otherwise, the control system continues to monitor the speed of the motor and restarts the motor after its speed is determined to be sufficiently low.