The theory of Long Stroke Pumping Units (LSPU) and the associated benefits have been generally accepted throughout industry for many years. Unfortunately there is usually a stigma associated with theory in that it sometimes differs greatly with actual results. What looks good on paper may not necessarily reflect reality. More often then not theory must be supported with actual field test results to give it real credibility. This paper will reiterate/expand on some of the more prolific benefits a LSPU offers and provide actual performance comparison analysis between conventional and LSPU, thus supporting the theory.
The benefits associated with LSPU translates to production cost reduction. Mechanical advantages, such as fewer pumping cycles without sacrificing production (increases rod pump, sucker rod and pumping unit life), higher rod pump efficiency, higher rod pump gas compression ratio, larger pumps, ncreased production, more production using less power, and increased tubing wear area are some ofthe more inherent benefits that can be expected or realized when using LSPU,
Actual field data comparing long stroke Rotaflex and conventional pumping units currently operating in the US. will be presented. Types of analyses will include overall system efficiency and maintenance cost.
Production cost reduction as it pertains to rod pumping artificial lift systems can be achieved by increasing the overall system efficiency and/or reducing maintenance costs.
When considering overall efficiency it is sometimes easier to think of a pumping unit as a black box' with electricity going in at one end and produced fluid out the other end. Overall system efficiency then is the ratio of output vs. input and represented as follows:
Nsys = HHP/IHP2
where: Nsys = overall efficiency of pumping system HHP = hydraulic horsepower used for fluid lifting IHP = electrical power input in horsepower
Nsys is optimum when HHP is as close to IHP as possible. In other words the energy losses in the overall system are kept to a bare minimum and/or the system is optimized.
It is important to note that any economies gained by optimizing the overall efficiency of a pumping system can be quickly overshadowed by excessive maintenance and repair costs. The major components of such costs include surface and subsurface equipment replacement (e.g. pumping unit failure, sucker rod failure, sucker rod pump failure and tubing failure). h is imperative that the actual well parameters are accurately known and that the selection of the right pumping equipment is made to help keep such costs down.
The mechanical and operational advantages that a LSPU offers allows both Nsys to be optimized while at the same time minimizes maintenance and repair costs.
In 1985 a mechanical LSPU was designed and tested3. Over the past ten years the original design has evolved into a product line entitled Rotaflex offering four models with stroke lengths from 7.31 m to 7.77 m (288 ° to 306 °) and maximum pumping speed of 5 S.P.M. (Fig. 1).