Transient Wellbore Heat Transfer Modeling and Flow Quantification Using Fiber-Optic Thermal Slugs for Geothermal Applications

Distributed Fiber Optic Sensing (DFOS) has become a key technology in geothermal development, providing continuous wellbore temperature monitoring with high spatial and temporal resolution. Beyond temperature tracking, DFOS enables detection of transient thermal responses that can reveal fracture connectivity and flow behavior in Enhanced Geothermal Systems (EGS). Interpreting such responses requires numerical modeling; however, conventional wellbore heat transfer models are limited either by steady-state thermal assumptions that fail to capture short-term dynamics or by fully coupled simulators that are computationally expensive and mathematically complex. To overcome these limitations, this study develops a transient wellbore heat transfer model with three key features: (i) incorporation of transient heat transfer through a thermal-analogy approach, (ii) simplification of fluid flow to steady state, avoiding the need for full mass–momentum–energy solvers, and (iii) direct representation of formation conduction without requiring a reservoir fluid-flow model. The model, formulated on the theoretical framework of Eickmeier et. al and implemented in a finite-difference scheme, was validated using Distributed Temperature Sensing (DTS) data from circulation tests. Results demonstrate strong agreement between modeled and measured thermal slug velocities and reveal that thermal slugs propagate slightly slower than the fluid front—contrary to the conventional assumption that both velocities are equal. Leveraging this insight, the model was applied to DFOS measurements from the Utah FORGE site to establish a new framework for flow quantification in fractured zones. In particular, thermal slug velocities derived from the RFS–DSS change rate were used within an inverse problem formulation to infer fracture connectivity and estimate distributed flow rates. This approach departs from the conventional assumption that thermal and fluid velocities are identical and instead provides a more physically consistent interpretation of DFSO data. By directly linking observed transient thermal responses to fracture-scale flow behavior, the method enhances the reliability of DFOS-based reservoir characterization and offers a practical tool for monitoring and managing EGS operations.