A "smart" predictive computer simulator of rod pumping systems called RODSTAR has been developed. RODSTAR combines expert rod pumping knowledge with state-of-the-art wave equation and pumping unit modeling algorithms. RODSTAR can simulate steel or fiberglass rods and any pumping unit geometry including long stroke non-beam pumping units.
Unlike conventional rod pump system design programs that rely on a trial and error approach, RODSTAR can automatically calculate the strokes per minute (spm), plunger size, and rod siring design to obtain the "target" production entered by the user. Although expert users can enter their own designs, taking advantage of the program's expert capabilities minimizes the time and expertise required to design rod pumping systems. RODSTAR includes a large pumping unit data base and is completely menu driven. A context sensitive expert help system warns the user of rods that need slim hole couplings, pump sizes that are too big for the tubing, pumping speeds that are too high, or designs that may result in severe under travel.
RODSTAR can automatically design steel or fiberglass rod strings.
When designing a steel rod string, RODSTAR "decides" what rod taper is needed and automatically calculates the length of each section for equal stress loading. When designing a fiberglass rod string RODSTAR calculates the length of the bottom steel section and selects the appropriate diameter and length of the fiberglass portion. The program ensures that the fiberglass rods are not in compression and avoids designs with excessive rod stretch. It also calculates energy consumption, electricity cost per barrel of produced fluid, and can model low or high slip prime movers. One of RODSTAR's unique capabilities is modeling pumping systems for shallow, high rate wells.
It is estimated that with RODSTAR the average user can obtain working rod pumping system designs ten times faster than with conventional non-expert computer programs.
When designing a rod pumping system one must specify the equipment, strokes per minute, and stroke length required to get the desired production at the lowest possible cost. In an effort to select the best system design for a well, the designer is mostly concerned with the following parameters:
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Production rate
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Capital cost
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Rod loading
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Gearbox loading
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System efficiency and power costs
Ideally, the designed system will give the highest present value profit after tax, considering capital and operating costs. However, because of the data, expertise, and time required to achieve this goal with conventional rod pump design software, most system designs are far from ideal.
In most cases production rate is of highest priority, followed by rod loading, gearbox loading, and energy cost. If electricity cost is high, it can be lowered by using a larger pump and a slower pumping speed. However, a large pump will increase rod loading and gearbox torque. Also, a large pumping unit must be used and this will increase capital cost. On the other hand, a smaller pump demands a faster pumping speed or longer stroke to make the desired production. These increases power consumption but reduces the required pumping unit gearbox size.
