Controlling Run Time May Not Prevent Rod Buckling

Controlling Run Time may Not Prevent Rod Buckling

Lynn Rowlan, Echometer James F. Lea, PL Tech

Abstract

Rod Buckling is thought to occur when a well pumps off and pounds fluid (Pump capacity exceeds inflow and PIP is very low). Fluid pound is the sudden opening of TV and rapid unloading of rods, which is thought to result in mechanical jarring of equipment and reduced equipment life. Fluid pound is thought to occur when the plunger hits the fluid in the pump barrel. The term Fluid Pound may not be exactly correct because the TV always opens before hitting fluid, maybe just a 1/16th inch away from the gas free liquid level in the pump barrel when "fluid pound” occurs. The industry, as yet, does not have a model to simulate so called “fluid pound”. The load at the pump is set to zero in a predictive wave equation model unless the user adds some default compression load at the pump. However, the predictive programs can predict rod compression up from the pump as the situation commonly described as “fluid pound” occurs. Diagnostic programs usually do not predict rod compression at the pump with incomplete fillage and “pound” but they also show rod compression uphole.

Damage to the rods and tubing is usually due to up-hole compression where rods slap against the tubing (loosening the couplings) and wear against the tubing. Sucker rod pin failures occur due to rod slap loosening the couplings. Compressive forces in the rods above the pump apply a force to increase the pressure inside the pump barrel to be greater than the pump discharge pressure. Rod buckling up the hole occurs when the pump slows down to compress the fluid in the pump. The sucker rod pump slows just as bad if the gas is in the 100-200 psi range, and rod-on-tubing wear can be worse at higher intake pressures. If gas interference is present, then expect increased rod-on-tubing wear, due to the rods buckling to compress gas in pump.

To mitigate rod buckling maintain a high volumetric pump fillage using a POC to control runtime, but analyze the buckling tendency, because buckling could be high even at pump fillages higher than expected. If at all possible keep the pump filled with liquid, eliminate any gas interference, set the pump Intake below the perforations and use weight bars to absorb any compressive loads.