Different types of forces NOT accounted for by the wave equation are 1) mechanical friction, 2) piston force acting on the polished rod due to tubing back pressure and 3) true vertical rod weight. Mechanical friction will be discussed from 1) over-tight stuffing box, 2) down hole sticking due to a severe dogleg in the wellbore profile and 3) friction from paraffin along a section of the rod string. The application of these external mechanical forces acting on the rod string impacts measured surface loads, down hole stroke length and plunger velocity, plus the calculated rod loading at the pump or other locations along the rod string.
Damping coefficients are used to subtract out fluid damping as a function of velocity along the rod string using the wave equation. Unaccounted for mechanical friction cannot be modeled by adjusting the damping factors in the wave equation. Mechanical friction impacts both the shape of the pump card and the measured surface dynamometer card loads versus position and, as friction on the rods goes up then the surface load range also changes. Field measured dynamometer data will be used to show examples of these different types of forces NOT accounted for by the wave equation.
Dog Leg Severity (DLS) had been used for many decades as recommendations to drill oil and gas wells and to provide "trouble free" operating conditions. Many of these recommendations were historically based on vertical, shallow (<5000 ft.) deep wells. But as wells continued to be drilled deeper, the recommendations were still applied. With the current drilling and operating practices of deviated and/or horizontal wells, these recommendations may no longer be applicable. Additionally, the deviation measurement interval (degrees/100 ft.) also may no longer be accurate when trying to match downhole problems using existing rod string design software. Furthermore, as wells have become deeper and many now also exclusively are drilled as deviated/ horizontal, side loading (SL) may be a more appropriate condition to be used to determine problems. This presentation will provide new recommendations for drilling wells that should provide better, longer term, less problematic operating wells.
Technical papers presented at the SWPSC on these topics will be provided in PDF format to interested participants of the Lunch and Learn.