Post-service characterisation of glass fibre reinforced Nylon 6 composites aged in coal seam gas wells

Over 300 metric tonnes of glass fibre-reinforced Nylon 6 composites used annually in coal seam gas (CSG) wells in Australia, in the form of composite sucker rod guides, end up in landfill at the end of their service life. The recyclability of Nylon 6 composites is constrained by limited understanding of how high pressure, high temperature (HPHT), and chemical or physical contamination affect the degradation and stability of the composite during field exposure. This study investigates the physical, thermal, and chemical stability of 25% (by weight) glass fibre-reinforced Nylon 6 (Nylon 6-GF composite) after 24 months of service in CSG wells. Samples were collected from the top, middle, and bottom sections of sucker rod strings from two different wells. Results showed that Nylon 6-GF composite surfaces exhibited an increase in roughness after use, particularly in vane regions (R a increased from 1.7µm in virgin samples to 11µm in field-exposed samples, with R p reaching up to 65.9 µm), while bulk regions remained relatively unchanged. No significant chemical degradation was detected, and the melting temperature of the exposed Nylon 6-GF composite remained consistent with virgin Nylon 6. However, a progressive reduction in crystallinity with increasing well depth was observed. Scanning electron microscopy (SEM) revealed only surface-level physical damage, without significant changes in fibre content but notable difference in fibre length distribution. Overall, results indicate that high pressure conditions, together with reduced diffusion due to reinforcing glass fibres, may reduce permeation driven degradation of Nylon 6-GF composite, supporting its potential for recycling and reuse within a circular economy.