Method to Calculate Location of Downhole Sticking


O. Lynn Rowlan and Carrie-Anne Taylor
Echometer Company

Mechanical Friction in a sucker rod lifted well acting on the rods is not modeled by the wave equation, resulting in the excess frictional loads and horsepower being displayed in the plot of the load versus position at any point along the rod string from the surface to the pump.  When the plunger stops at positions other than at the beginning, top, and end of the stroke, then sticking down hole is frequently the cause.  Coefficient of Rod Stretch, Kr, for a single rod of area A and is defined as the required load in pounds applied to the rod of length L to stretch the entire rod string equal to 1 inch.  The rod stretch spring constant, Kr, can be used to identify the depth to a severe dogleg in the well that is causing the downhole sticking.  The location of the severe mechanical force being applied to the rod string can be accurately determined, by finding the depth to where the measured Kr is equal to the Kr calculated.  Severe doglegs in the upper sections of any well should be avoided because the increased mechanical friction forces due to the dogleg severity will result in increased failures and high operating cost.