A coupled FSI-experimental framework for multi-factor degradation assessment of PDM stator elastomers: Synergistic effects of thermo-mechanical and chemical stresses

Abstract

Stator rubber degradation is a primary failure mechanism in positive displacement motors (PDMs), directly compromising drilling reliability and increasing operational costs. This study introduces a novel, integrated framework that couples high-fidelity fluid-structure interaction (FSI) simulations with quantitative swelling and aging experiments to enable a comprehensive, multi-factor assessment of elastomer degradation under realistic downhole conditions. In contrast to conventional studies that examine thermal, mechanical, or chemical effects in isolation, this work uniquely captures the synergistic interactions of thermo-mechanical stresses and chemical exposure across three nitrile-based rubber formulations-NBR1, NBR2, and HS-B-thereby enabling a physics-based, predictive evaluation of stator service life. Nonlinear finite element simulations in Abaqus incorporate Mooney-Rivlin hyperelasticity, thermo-mechanical coupling, and hysteresis heat generation, with boundary conditions reflecting actual field parameters (149.0°C, 200 psi, 2 Hz rotation). Results show that HS-B exhibits up to 40% lower thermo-mechanical stress and a peak temperature of 74.0°C-significantly lower than 86.0°C in NBR1. Swelling tests in water and xylene reveal HS-B's superior chemical resistance, with only 7.1% volume increase in hydrocarbon environments versus 17.1%–19.1% in conventional NBRs. After 168 h at 149.0°C, HS-B shows minimal degradation in tensile strength (−18.7%) and hardness (−4.6%), outperforming conventional rubbers. The key innovation lies in the quantitative coupling of multi-physics simulation and empirical swelling/aging data, enabling a robust, predictive framework for elastomer lifetime assessment. This integrated methodology not only identifies HS-B as the optimal stator elastomer but also provides actionable design insights for enhancing PDM durability under extreme downhole conditions. By bridging the gap between theoretical modeling and field-relevant material validation, the proposed approach supports virtual prototyping, informed material selection and the development of more reliable, cost-effective drilling systems.