Evaluation of Thermal Limits and Failure Mechanisms of ESP Motors in High-GOR

Abstract

Electric Submersible Pumps (ESPs) are widely applied in oil production, but their reliability is challenged in high gas‑oil ratio (GOR) environments where gas entrainment reduces cooling efficiency and accelerates thermal stress. This paper presents a comprehensive evaluation of ESP motor thermal limits and failure mechanisms based on field evidence from well PRH‑010 in Ecuador. Field measurements indicated motor surface temperatures of 286–310 °F under normal operation, with insulation degradation occurring at ~350 °F and shaft instability above 550 °F. Although measured surface temperatures remained below design limits, internal hotspot temperatures at the stator‑rotor interface were estimated to exceed critical thresholds (>350 °F), driving insulation failure. Cooling efficiency deterioration is observed as GVF exceeds approximately 15–20%, with significant degradation observed in PRH‑010 at ~17.8%. Severe degradation occurs beyond 30% GVF, particularly under low annular velocity conditions (<1.0 ft/s). Metallographic inspections confirmed adhesive wear, shaft overheating (blue discoloration on 80% of the surface), fissures in rotor bearings, and short circuits between stator and rotor windings. Chemical analysis identified the shaft material as AISI 4130 steel, rotor bearings as Stellite 12 alloy, and bushings as Cu‑Pb‑Sn bronze, all affected by thermal stress. The progression of failure can be explicitly described as: low production rate → low velocity → poor convection → rising temperature → increased winding resistance → higher I2R losses → runaway heating → insulation breakdown and shaft instability. By integrating thermodynamic balance modeling, hydraulic performance curves, and metallurgical evidence, this study uniquely defines operational thresholds and provides practical recommendations for ESP design and monitoring in high‑GOR environments.