Sucker-rod pumping units can be operated over a range of pumping speeds. It is expressed in Strokes per Minute (SPM). Obviously, this assumed that the up and down stroke of a unit forms one complete stroke cycle. Sucker rod pumping speed can be calculated manually using a stopwatch or automatically by the mean of a pump-off controller (POC). This article will detail the manually pumping speed calculation procedure.
Manually Pumping Speed Calculation Procedure:
Start the stopwatch at the bottom of the stroke and measure the time it takes for the unit to complete 10 strokes.
Stop the stopwatch at the end of the 10 strokes when the PU is back at the bottom of the stroke. Make a note of the number of seconds it took.
Convert the number of seconds into minutes (divide by 60).
Divide 10 (the number of strokes) by the time in minutes to obtain the pumping speed expressed in strokes per minute (SPM).
A good understanding of counterbalance is vital to the successful operation of surface sucker rod pumping units. Poor counterbalance practices can cause early failure of the gear reducer gearing and will result in excessive energy cost.
Non-counterbalanced lever system:
The figure above represents a simple non-counterbalanced lever system. On the up-stroke, by pulling down on the end of a beam, a man is lifting a bucket full of water having a combined weight of 150 lbs.
Note that the upstroke effort of the man is a substantial 150 lbs.
The problem of heading (flow off and pump) is often encountered in gassy wells. This heading effect which can blow the tubing dry occurs as follows:
Gas expansion in the tubing as oil from the reservoir travels towards the surface (due to gas pressure decrease).
Formation of a gas “plunger” that can push the liquid above it out of the tubing and into the flow line at high speed. As the gas forces the liquids out of the tubing, the pressure in the tubing decreases rapidly and the gas expands even more.
This heading behavior of reservoir fluids causes cycles of high production followed by low or no production.
When heading process starts, the expanding gas pushes the liquid into the flowlines and increases production for a short time. In the meantime, the liquid leaving the tubing is replaced by more and more free gas. Eventually, the tubing is blown dry and production stops until the tubing fills with liquid again.
The constant up-and-down movement of a sucker rod creates excessive friction between rods and tubing which can result in premature rod and tubing failures due to excessive wear. If left unchecked, this generally requires a costly intervention to make repairs.
To extend the sucker rod run life, one of the widely used techniques is the use of rod rotators. A rod rotator is a mechanical device installed on the polished rod between the carrier and the polished rod clamp. It incrementally rotates the rod with each stroke. A rotating mechanism with an actuator lever arm is connected to the walking beam with a metal string. As the surface unit moves up and down, it pulls and releases this metal string so that, moves the actuator lever arm up and down. The rotation mechanism is activated and this slowly rotates the polished rod and the rod string below.