Examination of the Enhanced Thermal Conductivity Effect by the Induced Convective Heat Flux in a Fractured Thermal Reservoir on Closed-loop Geothermal Energy Production Technology

Abstract

Closed-loop geothermal energy recovery technology is one of the promising methods to produce the heat energy from thermal reservoirs. This technology is independent of geothermal reservoir permeability, greatly reducing the exploration risk, and there exist no associated environmental and scaling issues that are usually accompanying the production of geothermal fluids to surface. A key limitation in this technology though is that the heat in a closed-loop system can only be produced from reservoir by heat conduction across the wellbore. The overall thermal conductivity is normally considered constant as the fluids in the reservoir are assumed static during the temperature change. However, there will happen some locally induced convective flow in the near wellbore region due to fluids density change caused by surrounding temperature reduction. This kind of induced heat convection will enhance the heat transfer process in the near wellbore region and eventually improve the heat production performance of a closed-loop system. This study builds two dimensional and three dimensional models to demonstrate that the induced convective flow will enhance the overall thermal conductivity in fractured or highly permeable thermal reservoirs. The results show that a significant convective flow induced velocity improvement can be observed in the region near the horizontal wellbore, with the spherically shaped convective flow likely spreading outskirts, mainly in the vertical downward lower region, for the cases with higher permeability. The naturally induced convective flow velocity has a significant increase for the permeability range of 1.0 D ~ 10.0 D. Note that this result also suggests that only heavily fractured thermal reservoir becomes sufficiently supportive to take the advantage of density-driven convective flow.