Selection of the Proper Pumping Mode
The pumping mode of a sucker-rod pumping system is defined as the combination of pump size, polished-rod stroke length, pumping speed, and rod string design.
the optimum design is based on the value of lifting efficiencies and the one with the maximum ηlift is selected.
Maximizing the lifting efficiency coincides with the case of setting the polished rod power, PRHP, to be a minimum.
for a given hydraulic power
lifting efficiency and PRHP are inversely proportional.
A pumping system design made by this principle results in minimum operation cost and in a maximum of system efficiency.
If the best mode is selected, the energy input at the polished rod is only slightly greater than the pump’s hydraulic power, ensuring a lifting efficiency of over 94%.
The worst mode, on the other hand, requires almost three times as much energy as the best one for lifting the same amount of liquid from the same depth.
by increasing the pump size the attained maximum lifting efficiency values increase for all tapers.
Therefore, use of bigger plungers with correspondingly slower pumping speeds is always useful and results in lower energy requirements.
Also use of the heavier rod strings increases the power requirement for smaller pump sizes.
OPTIMUM COUNTERBALANCING OF PUMPING UNITS
ideal counterbalance conditions are desired that can have many beneficial effects on the operation of the sucker-rod pumping system:
Gearbox size can be reduced when compared to an unbalanced condition,
The size of the required prime mover is smaller, and
The smoother operation of a properly balanced speed reducer lowers maintenance costs and increases equipment life.
the mechanical Cyclic Load Factor (CLF). It can be calculated from the variation during the pumping cycle of the net torque on the reducer as the ratio of the root mean square and the average net torques:
These methods try to find the maximum counterbalance moment satisfying one of the following criteria:
The peak motor currents are equal during the up-, and downstroke,
The peak net torques on the up-, and downstroke are equal,
The required mechanical powers for the up-, and downstroke are equal, or
A minimum of the cyclic load factor is achieved.
The CLF value valid for this case is greater than the minimum CLF achieved by the recommended optimization model.
PITFALLS IN ROD STRING DESIGN
Rod string design aims at the determination of:
• The rod sizes to be used in the string,
• The lengths of the individual taper sections, and
• The rod material to be used.
The two basic problems in rod string design concern:
(1) how rod loads are calculated, and
(2) what principle to use for the determination of taper lengths.
At the time of design, rod loads are not known, and they also depend on the taper lengths that are about to be determined. Therefore, one has to rely on approximate calculations to find probable rod loads that will occur during pumping
Design Principles
Early rod string design methods utilized the simplifying assumption that the string was exposed to a simple tension loading. An examination of the rod loads during a complete pumping cycle, however, shows that the rod string is under a cyclic loading.
The nature of the loading is pulsating tension because the whole string is under tension at all times, but rod stress levels change for the up-, and the downstroke.
sucker-rod strings should be designed for fatigue endurance.
CONCLUSIONS
The pumping system’s energy efficiency depends primarily on the amount of downhole power losses.
Maximum system efficiency is ensured by achieving a maximum of lifting efficiency.
The proper selection of pumping mode can ensure maximum lifting efficiency and thus a most energy-efficient sucker-rod pumping system.
Optimum counterbalancing of pumping units has many beneficial effects.
Minimizing the CLF is the preferred method for finding optimum counterbalanceconditions.
Available rod string design procedures can have many pitfalls. Proper designs should provide a uniform fatigue loading of all rod taper sections.
