A comparative evaluation of the parallel flow and spherical reservoir models of HDR geothermal systems

A terminology is developed to link two conceptual semi-analytical models of HDR geothermal energy extraction through the common criterion of reservoir volume. The first is a parallel fracture model (PFM) that represents thermal recovery from an arrangement of prismatic blocks, thermally isolated from the geologic host medium. The second is a spherical reservoir model (SRM) that admits energy recovery from both a central production zone and the surrounding geologic body. Behaviour of the two different, but complementary, systems are governed by the dimensionless variables of thermal drawdown, T_D, circulation rate, Q_D and time, t_D, with the PFM further conditioned by a diffusion length ratio representative of the fracture spacing. Thermal response of the PFM exhibits a worst case threshold for thermal recovery at small magnitudes of circulation rate Q_D, corresponding to the system progressing in near thermal equilibrium. A close correspondence exists between the thermal response of the PFM for low Q_D and that of the SRM for high Q_D where the influence of external heat supply is not apparent. Circulation tests conducted in existing reservoirs return magnitudes of Q_D and diffusion length ratios that suggest they are operating close to thermal equilibrium as predicted from the PFM. With this determined "a priori", the assumptions made in the SRM are not violated, suggesting the significant contribution that external heat supply may make to the gross energy recovery. As predicted by the SRM, energy recovery remains practically unbounded for relevant magnitudes of Q_D with this effect being noticeable well within the projected reservoir lifetime.