Experimental investigation and performance analysis of a packed-bed thermal energy storage system for oilfield produced fluid heating

Addressing the challenges of high heating costs for oilfield wellhead produced fluids and the inherently low thermal conductivity of phase change material (PCM), this study introduces an expanded graphite/paraffin composite phase change material (CPCM) with enhanced heat transfer performance. An innovative tank-type encapsulation design is proposed, enabling industrial-scale production and practical engineering implementation of the CPCM. Based on this design, a packed-bed latent thermal energy storage (PBLTES) system suitable for heating produced fluids at oilfield wellheads is constructed. Field tests are conducted to evaluate the charging and discharging performance of the system, together with a 24 h static thermal insulation test, and its economic feasibility is systematically assessed. The experimental results indicate that the CPCM increases the thermal conductivity by a factor of 4.22 relative to pure paraffin. Under ambient temperatures of 30 °C and 15 °C, the PBLTES tank attains charging capacities of 460.4 MJ and 444.6 MJ, respectively, with corresponding discharging capacities of 429.5 MJ and 414.0 MJ, resulting in discharging efficiencies of 91.1% and 86.3%. During discharging, the outlet temperature of the produced fluid remains above 50 °C, meeting on-site heating requirements. In a 24-h insulation test, the tank temperature decreases from 84.8 °C to 81.3 °C at 30 °C and from 85.1 °C to 80.1 °C at 15 °C, with heat losses of 36.1 MJ and 51.6 MJ, respectively. The economic analysis shows an initial investment of 11900 $ and annual electricity savings of 3310 $, with a payback period of 3.6 years.