Active wellbore cooling during deep drilling circulation via microencapsulated phase change materials: Energy redistribution mechanism and inverse design

Deep geothermal and hydrocarbon drilling requires effective thermal management as conventional surface cooling becomes insufficient at extreme depths. This study proposes an active wellbore cooling strategy using microencapsulated phase change material (MPCM)-based fluids. A transient multiphase-flow and heat-transfer model is developed to resolve interphase velocity slip, phase-change kinetics, and wellbore-formation heat transfer. The baseline thermo-hydraulic solver is validated against field data from 20 deep wells, achieving relative errors below 10%. Results reveal a “deep latent heat buffering and shallow sensible heat release” energy redistribution mechanism. Under baseline conditions, latent heat accounts for only ∼1.5% of total wellbore heat but induces thermodynamic clamping, reducing the bottomhole circulating temperature by up to 21.3 °C and yielding an active cooling enhancement ratio (ACER) exceeding 100%. Parametric analysis reveals a non-monotonic relationship between cooling performance and phase change temperature (PCT), demonstrating that the spatial precision of latent heat absorption is more critical than the total energy storage magnitude. Under the investigated design envelope, a ∼20 °C PCT sub-cooling design heuristic relative to the baseline BHCT is established for preliminary screening. Global sensitivity analysis identifies PCT and latent heat as dominant factors, while MPCM concentration and shell-to-MPCM thickness ratio influence cooling through the derived effective PCM-core loading. Finally, multi-objective optimization under rheological constraints breaks the single-variable thermodynamic floor. By synergistically pairing PCT with latent heat, the optimized framework pushes the cooling limit to 29.7 °C (ACER = 159%). This work provides a mechanistic and inverse-design framework for MPCM-based active wellbore cooling in extreme-depth energy drilling.