Advancing Geothermal Efficiency: Evaluating the Impact of Vacuum Insulated Tubular Designs and a Novel Premium Connection Insulation on Thermal Performance and Power Output

Abstract This paper presents a field-based evaluation of Vacuum Insulated Tubing (VIT) performance in a coaxial closed-loop geothermal test well drilled in Navasota, Texas, in 2022. The study focuses on quantifying heat loss at uninsulated premium connections and assessing the impact on thermal efficiency and power output. The well was completed with a 4-inch × 2-7/8-inch VIT string to a depth of 11,369 ft (3,465.3 m), where the bottomhole static temperature reached 132.2°C (270°F). High-resolution Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) systems were deployed along a cemented 6-inch casing, enabling continuous thermal monitoring during circulation of filtered fresh water at 75 and 150 gallons per minute (GPM). This field data was critical in identifying localized thermal losses at connection points and served as the foundation for calibrating a numerical model of the wellbore. The analysis revealed that uninsulated connections significantly increased effective thermal conductivity, reducing the system's ability to retain heat. Simulations incorporating advanced insulated connections demonstrated substantial improvements in thermal retention and energy recovery. The close agreement between measured and modeled data validated both the VIT's core design and the modeling approach. This work underscores the value of high-fidelity field data in diagnosing performance limitations and guiding design improvements. The findings support the adoption of insulated connections and optimized joint lengths in future VIT deployments and provide a validated framework for modeling thermal behavior in closed-loop geothermal systems.