Abstract: Studies on enhanced geothermal system (EGS) are important for developing geothermal power generation. As an EGS typically involves multi-regional and multi-physical coupling and the in-well flows are turbulent, it is necessary in EGS numerical simulation to handle regional coupling correctly, represent in-well turbulence reasonably, and achieve sufficient computational accuracy and efficiency. In this study, the processes of porous flow and heat transfer associated with EGS are systematically investigated based on a multi-region and multi-physics coupled 3D finite element model. The obtained results are given as follows. 1) The natural coupling of temperature, pressure, and velocity fields between different EGS regions can be successfully realized in terms of correctly posed connection conditions. 2) Different turbulence models for in-well flows predict essentially consistent in-well pressure variation. The turbulence pressure drop across the well depth is predicted to be about 4 times as large as the laminar pressure drop but 3 orders of magnitude smaller than the overall pressure drop from the injection well to the production well, resulting in insignificant influence of in-well turbulence on the EGS heat extraction process. 3) With the vertical variations of EGS structure and thermophysical properties, the natural convection inside the reservoir, and the effects of in-well turbulence all being negligible, an EGS is dominated by horizontal porous flow and horizontal convective heat transfer, and thus may be approximately simulated by 2D modeling.

Key words: geothermal energy, enhanced geothermal system(EGS), finite element model, domain coupling, in-well turbulence