Casing fluid level is an important parameter in the use of electric submersible pumps (ESPs) for oil and gas production. A well operator continually faces a balancing act between production and protection. Production must be increased without lowering the fluid level to the point where the pump is starved of the fluid it needs for cooling. If the fluid is lowered too far, an underload or pump-off condition will shut the pump down, leaving the well productionless, potentially for hours, while the tube drains and refills.

Operators of ESP systems use various methods to gauge fluid levels. One of the more popular methods is the periodic use of an instrument like an Echometer to accoustically “shoot” fluid levels. This method requires manual intervention, is subject to human error, and does not provide the continuous measurement required for closed-loop control of fluid level. In critical applications, downhole pressure sensors may be used to overcome these limitations. Downhole gauges, however, are expensive and often disappointingly unreliable. The ideal solution would be an automated means of measuring fluid level that would eliminate the additional hardware altogther.

Unico ESP drives offer one such solution. They employ a unique, patented method of determining casing fluid level without the use of a downhole pressure sensor. The drives are designed for precise measurement of downhole motor speed and torque. Fluid level estimation software within the drive uses these precise measurements to periodically probe for pump differential pressure. In turn, that pressure is used, along with other well and pump parameters, to estimate pump fluid flow, pump intake pressure, and casing fluid level. In addition, the drive also provides closed-loop regulation of casing fluid level using its casing fluid estimate.

Field tests have been done to assess the performance of sensorless fluid level measurement. In one test, conducted on an ESP well in western Texas, fluid level measurements from the drive were compared against those of an Echometer.

To evaluate the consistency of the fluid level estimate at different speeds, pump speed was varied at 36 minute intervals from 64 Hz down to 58 Hz, then back up to 68 Hz in 2 Hz increments. The drive sampled the casing fluid level every nine minutes, and an Echometer shot was taken every 36 minutes at the end of each interval. The results are shown below. Casing fluid level from the Unico drive (pink) is compared to the Echometer equivalent liquid column (blue).

Casing fluid level, as measured by a Unico ESP drive, versus Echometer fluid level measurements at various pump speeds. The sensorless technique provides automated continuous measurement.