A Pump-Off Detector System
Authors: Patterson Maurice M. 
Published: December 2007

The development, theory, and testing of a system for detecting pump-off are presented. The device uses load and velocity of the polish rod to calculate the energy put into the pumping system. As the well pumps, this energy is calculated for each stroke. Changes in energy levels can be related to pump-off. Results of field tests verify the capabilities of the system.

Introduction

When the pump of a rod-pumped system only partially fills with fluids during the upstroke, the partially fills with fluids during the upstroke, the traveling valve will open only after the piston strikes the surface somewhere during the downstroke. This condition, called fluid pound or pump-off, increases the wear on the pump and the rods, and decreases pump efficiency. Fig. 1 shows the input energy or pump efficiency. Fig. 1 shows the input energy or surface dynamometer card for a typical well before and after pump-off. Ideally, a rod pump is sized for a particular well so that its capacity closely matches the particular well so that its capacity closely matches the well potential. As the well declines, the pump capacity normally is reduced to avoid continuous operation in a pumped-off condition. Sometimes this is done by changing the pump diameter or reducing the strokes per minute. Another way to reduce rod pump capacity is to operate the pump intermittently. pump capacity is to operate the pump intermittently. Pumping units using electric motors can be equipped Pumping units using electric motors can be equipped with a timing device to vary pumping cycles. The most common device is a percentage timer. With this tool, a well is set to operate for some percentage of 15 minutes. Then, the device is turned off for the remainder of the period. The theory is that fluid will build up in the well during the off time and will be pumped down to minimum operating level during the pumped down to minimum operating level during the run period. Such systems have limitations because all wells do not reach equilibrium at the same time for each 15-minute cycle. For this reason, pump-off controllers have been tried in many rod-pumped installations in the U.S.Basically, five types of systems can detect pumpoff. One system detects the fluid level in the annulus pumpoff. One system detects the fluid level in the annulus and shuts down the well if the fluid level drops below a predetermined amount. Another type is a flow/no flow system that attempts to measure the flow at the surface and shut down the well if flow is zero or drops below some established value. Vibration sensors have been used for years to detect the motion of the pumping unit when fluid pump-off begins in the well. Sometimes these are adequate; however, certain surface conditions, such as a heavy truck going by or high winds, can cause the vibration sensors to shut down the unit prematurely. One of the most common types of pump-off detector is the motor current-sensing device. Such systems measure the difference between motor current peaks, the current at some point in the downstroke, or the average current In many cases the motor current device is adequate; however, with the use of high-slip motors and out-of-balance pumping units, current detectors have limited use. Rod loading is affected directly by what occurs inside the pump. Therefore, several pump-off detectors have been used based on the rod loading or the displacement and velocity of the rod. Many investigators calculate rod load at a particular point in the downstroke or the derivative particular point in the downstroke or the derivative of rod load with time to determine pump-off.

JPT

P. 1249