A Single-Well EGS Configuration Using a Thermosiphon

Abstract

This paper describes the investigation of a single-well enhanced geothermal system comprised of a downhole thermosiphon and a novel completion design using a downhole heat exchanger. The thermosiphon is a device that takes advantage of the gravity head difference between liquid flowing down the annulus and vapor flowing up the tubing. Without a connection to a large reservoir volume, a downhole heat exchanger will rapidly deplete the heat near the wellbore and cannot generate useful energy output for very long. Therefore to enhance heat extraction, the effectiveness of a second annulus outside the casing was investigated. A working fluid would circulate by natural convection down the outer annulus and upwards through fractures created in the reservoir. This configuration has the advantages that a second injection well is not required, the need for downhole pumping is avoided by the thermosiphon effect, and the fractures carrying fluid through the reservoir makes the system sustainable. A limitation is that reliance on free convection to circulate fluid through an EGS reservoir does not result in large flow rates, which lowers the productivity. We constructed a coupled finite-difference wellbore and reservoir model representing the fluid mechanics, fluid phase behavior and heat transfer. We investigated the energy output of the system and the feasibility of sustainable deployment.