Numerical investigation on heat extraction performance of an open loop geothermal system in a single well

Abstract

With the increasing concern on drilling costs in geothermal industry, more attention has been paid to the open loop geothermal system (OLGS) in a single well. For the OLGS, a vertical wellbore is generally drilled to the geothermal reservoir. Then, the injection and production intervals are perforated in the upper and lower wellbore and an insulated inner tube is installed in the wellbore. The working fluid is injected into the reservoir from the injection interval, then extracts heat from the reservoir, and finally returns to the ground surface through production interval and insulated tube. Thus, the injection and production can be accomplished in a single well, which avoids drilling an injection well and reduces drilling costs. However, few studies were conducted to investigate the heat extraction performance of OLGS comprehensively. Hence, a 3D unsteady-state multiscale model considering fluid flow and heat transfer processes of the borehole and reservoir is established. The model is verified by field data and an analytical solution respectively in various computational regions. Subsequently, the results indicate that a large flow rate is beneficial to obtain higher thermal power. The injection-production interval affects the OLGS performance significantly, and a longer interval could avoid the early breakthrough of circulating fluid effectively. And the permeability plays a remarkable role in heat production, while the influence of porosity is negligible. Furthermore, the fractures distribution near the wellbore may result in a more serious thermal breakthrough. And it is crucial to employ the insulation tube with low thermal conductivity to reduce the heat loss. The temperature field analysis depicts that the OLGS has a larger impact scope than conventional BHE, which acquires greater heat extraction performance. The main findings can offer guidance for the field application and optimal design of the OLGS.