O. Lynn Rowlan, Carrie Anne Taylor and Ken Skinner
Echometer Company
Clint Haskins, Fluid Finder
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 in 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