Modelling an unconventional closed-loop deep borehole heat exchanger (DBHE): Sensitivity Analysis on the Newberry Volcanic Setting

Abstract

Geothermal energy is a baseload resource that has the potential to contribute significantly to the transition to a low-carbon future. Alternative (unconventional) deep geothermal designs are thus needed to provide a secure and efficient energy supply. Current Enhanced Geothermal Systems (EGS) are under technical review as a result of the associated low recovery factors and risk of induced seismicity in connection with reservoir stimulation operations, and Supercritical EGS (SEGS) concepts are still under early research and development. The Newberry and Icelandic Deep Drilling Projects (NDDP and IDDP) aid these developments to drill deeper into very hot temperature zones. An in-depth sensitivity analysis was investigated considering a deep borehole closed-loop heat exchanger (DBHE) to overcome the current limitations of deep EGS. Using the DBHE, cold working fluid is pumped down in the outer annulus and rises to the surface via natural convection or is pumped up via an inner tubing. A T2Well/EOS1 model previously calibrated on an experimental DBHE in Hawaii was adapted to the current NWG 55-29 well at the Newberry volcano site in Central Oregon. A sensitivity analysis was carried out, including parameters such as: the working fluid mass flow rate, the casing and cement thermal properties and the wellbore radii dimensions. The results allow an assessment of key thermodynamics within the wellbore and provide an insight into how heat is lost/gained throughout the system. This analysis was performed under the assumption of sub-critical conditions. Requirements for further software development are briefly discussed, which would facilitate the modelling of unconventional geothermal wells in supercritical systems.