Coupled fluid-structure model for predicting fatigue life in sucker rod pumps under high-viscosity conditions

The sucker rod pump (SRP) is widely used to artificially lift heavy oil in wells operating under cold or thermal production because of its high efficiency and reliability. However, rod fatigue and failure often result from viscosity-induced loads. Commercial design tools solve the wave equation to forecast rod life but overlook the transient, developing viscosity effects, leading to inaccurate predictions, rod float, unexpected high polished rod loads, and early failures. This study proposes a coupled fluid-structure model for transient fluid frictional forces on SRP subsurface parts under high-viscosity conditions. A benchmark study against existing models and a validation study against a field case show reliable results of the proposed model. Additionally, a sensitivity analysis is performed to explore the impact of flow and design parameters on the stresses at the top of the rod, the tendency to buckle, and fatigue life. Results show that increasing liquid viscosity from 1 to 100 mPa.s reduces the rod’s fatigue life by an average of 30%. Conversely, increasing viscosity to 1,000 mPa.s lowers the top-of-section stress by up to 200% during the downstroke and causes immediate failure. This work enhances understanding of the transient interactions not captured by standard design methods, improving predictions in high-viscosity wells.