This work investigates the aging behavior of Polyetheretherketone (PEEK) when exposed to potassium formate brine at elevated temperatures, conditions commonly encountered during downhole drilling and completion operations. PEEK has emerged as a high-performance thermoplastic for oilfield environments due to its stable aromatic structure, excellent chemical resistance, high thermal stability, and superior mechanical strength. These characteristics make PEEK attractive not only for sealing components but also as structural parts and potential replacements for corrosion-susceptible metals, such as carbon steel. Potassium formate brine is widely used as a high-density, environmentally compliant drilling and completion fluid, particularly in deep, high-temperature wells where conventional brines and water-based systems may degrade. To evaluate the long-term durability of PEEK in such harsh environments, an accelerated aging study was conducted using controlled autoclave tests at elevated temperatures. Injection-molded specimens (ASTM D638 Type V) of unfilled PEEK and short carbon fiber reinforced PEEK were aged at 200, 225, and 250 °C in potassium formate brine for durations up to ~42 days. After each exposure interval, specimens were evaluated for physical, mechanical, thermal, and chemical changes. Characterization included hardness measurements, weight change, tensile testing, DMA, DSC, TGA, and FTIR analysis. Results indicate that for unfilled PEEK, tensile strength at break increased while elongation at break decreased with aging duration at all temperature levels. At consistent exposure times, higher aging temperatures resulted in a greater increase in tensile strength and a stronger reduction in ductility. DMA measurements showed corresponding increases in storage modulus, consistent with embrittlement trends observed mechanically. DSC testing revealed slight increases (several degrees Celsius) in glass transition temperature with longer exposure time and higher temperature. FTIR spectra showed no significant chemical changes between aged and unaged conditions. Collectively, increases in modulus and tensile strength, reduced elongation, and slight Tg shifts suggest minor thermally induced crosslinking reactions in the alkaline brine system, along with possible crystal rearrangement or annealing effects due to prolonged exposure above the glass transition temperature. Overall, the study demonstrates that PEEK maintains strong structural stability under high-temperature potassium formate aging conditions. Despite modest stiffening and reduced ductility, the material remains robust, supporting its continued suitability for sealing and structural components in harsh downhole environments.
