This paper describes a new and different approach to conventional beam pumping with a subsurface pump and sucker rod string. The beam pumping cycle takes the rod string from tension on the up stroke to compression on the down stroke. Minimizing the compressive forces on the down stroke, creating a higher minimum load aids in extending the service life of tubing, rods and pump. This can be done by the lower portion of the rod string being larger sucker rods or sinker bars or by proper sizing of the pump plunger or a combination of both along with proper stroke length and strokes per minute.
Data presented in this paper shows another means of keeping the rod string in tension by use of a different type of subsurface pump. The fluid load on the areas of the pump act to increase the down stroke load. This down stroke load tends to keep the lower end of the sucker rod string in tension rather than allowing it to go into compression. This paper presents the theory of a different kind of subsurface pump backed up by field data showing the benefits of this pump.
The pumping cycle of any subsurface sucker rod pump is basically the same; on the upstroke the rods are in tension with the plunger lifting fluid to the surface and on the down stroke the rods are in compression with the plunger falling to the bottom of the barrel ready for the next cycle. The stroke length can vary from 12" to 305". The number of strokes per minute can vary, the speed of up and down strokes may be different, but the cycle always produces a cyclic loading from tension to compression.
The upstroke or tension side of the cycle doesn't present the problems or opportunity for damaging effects on the tubing string, sucker rod string and subsurface pump as does the down stroke or compression side of the cycle.
This paper will therefore, discuss and describe means for minimizing the damaging effects of compression by helping the sucker rod string to be in tension particularly through the use of a ‘tension pump’.
We should note here there are two different types of "compression" involved with subsurface pumps. One is the compression ratio due to the pump plunger size, stroke length and unswept volume or waste space in the pump chamber due to extensions, fittings and cages. This ratio is important when pumping high gas/oil ratio wells. The other "compression" is the one we are addressing in this paper.
As previously stated, the pumping cycle takes the sucker rod string and subsurface pump through a cycle of tension to compression. The tension or upstroke side of the cycle creates the peak load of the installation while the compression or downstroke side of the cycle creates the minimum load of the installation. These peak and minimum loads are identified as PL and ML in pump calculations. The recommended maximum or peak load (PL) and stress for any sucker rod string is based on the modified Goodman Stress Diagram as shown on page 10 of API 11-BR, Reference 1. The modified Goodman Stress Diagram is recommended for determining the allowable range of stress and allowable sucker rod stress for any grade of sucker rod.
The peak and minimum load both enter into determining recommended sucker rod loads and stresses. Cyclic loading causes the rod string to have a peak and minimum load. If the minimum load could be increased bringing it close to the peak load, then the sucker rod string can support higher loading and stresses and provide longer service life. Let's suppose the PL and ML were the same, wouldn't the sucker rod string run forever since there would be no difference in up and down stroke load? In other words the ML/PL ratio would be 100%.