The Variable Speed Submergible Pump has been used for several years to math, submergible pump's characteristics to a well's production capability. Past applications have included those where a bottom Past applications have included those where a bottom hole pressure measuring device was used to control the pump speed and therefore the pump intake pressure. The automatic pump-off controller, discussed pressure. The automatic pump-off controller, discussed in this paper, allows an operator to maintain minimum pump intake pressure without a bottom hole pressure sensor. The controller also uses special pressure sensor. The controller also uses special speed variation techniques to prevent or possibly break a gas lock condition.
This paper will discuss the development of the controller and the Physical principles which it is based on. Operational tests will b, described along with the problems encountered. Finally, the future use of this controller in new applications, such as with wells which produce sand, will be presented.
Since August of 1977 variable frequency static power converters have been supplying variable power converters have been supplying variable frequency, variable voltage power for deep well submergible pumps. The benefits of these applications have been described in previous publications by this author and others such as Kelley and Alcock. Through May of 1979 the majority of variable speed submergible Pump applications had been on wells with relatively low gas to liquid ratios (200 to 400 ft /bbl). In June of 1979 a Variable Speed Drive was placed on an existing submergible pumping installation which was cycling because of excessive gas. The well had a GLR of 600 to 700 ft /bbl. When the well was produced with the variable speed system at rates equivalent to the 60 Hz producing rate the pump continued to gas lock and was shut down on underload. It was recognized that a bottom hole pressure sensor which feeds its information to the pressure sensor which feeds its information to the drive would not solve the gas locking problem at the rates the producing company desired from the well. The pressure sensor would not measure any pressure change at the moment of the gas lock. Also the pressure would change with a relatively long time pressure would change with a relatively long time constant meaning that the Pump would be down on underload. It was these considerations which led to the development of an automatic pump-off controller to be used with the Variable Speed Submergible Pump Drive.
The hydraulic h.p. or rate at which the pump does work can be defined as follows:
Pump Horsepower = Pump Horsepower =If the assumption is made that the rate, head, and efficiency of the pump will change with relatively long time constants then it can be assumed that the Sp. Gr. is the cause of short term changes in the h.p. required by the pump. These Sp. Gr. changes are usually caused by changes in the amount of free gas in the produced liquid. This gas effect or gas interference will generally increase as the pump intake pressure is lowered. This effect will pump intake pressure is lowered. This effect will continue to increase until a "pumped off" or gas locked condition is reached.
The h.p. the pump requires will be reflected by the amperage the motor draws or specifically the real power portion of that current. It can then be assumed that the magnitude of the deviation of the real power has some relationship to the submergible pump's intake pressure. while the exact relationship cannot be defined, it can be generally stated that increasing amperage deviations are related to decreasing pump intake pressures. It is on this assumption that the "Auto-Amps" mode of the Automatic Controller's operation is designed.
While the assumption that amperage deviation could allow a controller to sense relative differences in pump intake pressure for control purposes, it does presuppose that minimum pump purposes, it does presuppose that minimum pump intake pressure is always the producer's objective. If the operator is producing from a reservoir where a relatively high pump intake pressure is necessary, then the probability of determining a suitable pump intake pressure based on amperage deviation would be very small.