Jonathan Martin, Black Mamba Rod Lift
Luke Beadry and Vladimir Pechenkin, DV8 Energy
Sucker rods are simple in form and function, however, they operate in a sophisticated engineered system over great lengths without direct visibility. Because of this, we as engineers must do our best in predictive efforts to provide the best configuration for the highly dynamic system.
Rod string design software utilizes complex math to compute stress loading throughout the system. Design software w/ deviation included incorporates well-bore geometry to estimate ancillary loads throughout the design, i.e.: side-load & drag load from rod-on-tubing contact.
An improved method of accurately evaluating system and rod string stresses has been developed and computed. This method combines the industry standard Gibbs wave equation and trusted rod-string loading computation, taking it a bit further, evaluating the deflection behavior of the sucker rods throughout the deviated wellbores, and computing additional stresses based on arc deflection.
In instances of deviation and molded rod guides, sucker rod behavior changes dramatically. Dog-leg severity combined with most molded rod guides creates intermittent sucker rod rigidity and assumed perfect alignment with the tubing. This then causes the ability of increased bending moments and bending stresses. Fiber loading1 around the circumference of the sucker rod is then increased, leading to regions of the rod body exterior being subjected to stresses far higher than intended.
By accurately computing bending stresses and adding them to the total system loading through the deviated well bore, accuracy in the predictive system is increased, creating additional awareness to the system on a detailed, per rod level.
Despite general assumptions, stress loading is not linear throughout each sucker rod taper. This is often understood but not quantified, visualized, or addressed in string design software outputs. The effort of this paper is to provide detailed engineering awareness to rod loading, openly discussing with operators the effects of minor rod deflection and its associated stresses, and how this then encourages pre-mature fatigue fractures.
dv8 Energy and Black Mamba Rod Lift have worked together to build a web-based rod string loading fact checking system which validates and visualizes string design loading throughout the pumping cycle, addressing the bending stresses created from deviation and rod deflection in three different states: slick rod, guided rod, and under a buckling state.