Closed-Loop Geothermal Wells

Closed-loop wells also known as deep borehole heat exchangers (DBHE) are an unconventional approach to exploit geothermal energy and overcome the typical limits of hydrothermal systems. Theoretically, closed-loop wells can extract heat from a geothermal reservoir regardless of its permeability; they function like a downhole heat exchanger by circulating a working fluid inside the casing only without any exchange of fluid between rock and formation. Energy recovery is via conductive heat-transfer, across the well walls, from the rock formation to the working fluid that circulates inside the well.

There are three (3) basic types of closed-loop well configurations namely a) co-axial wells where fluid is pumped down the annulus and produced back to surface via an insulated tubing, b) multiple-string wells where downward circulation is via another tubing rather than through the annulus and c) U-shaped wells where two connected wellbores are used, one to pump down the working fluid and another one to recover it back to surface. See figure below (adapted from H Zhu et al, 2020).

Closed loop wells are an area of active technology development and whilst some pilots have been executed, very few if any closed-loop wells currently operational.

The benefits of closed-loop wells are the absence of fluid exchange between formation and wellbore and hence no dependence on permeability, no production chemistry issues such as scaling, etc etc. The main drawback of closed-loop well technology is the slow and inefficient heat recovery compared to conventional geothermal due to the slowness of conductive heat-transfer processes. This slowness, in turn, relates to the relatively low specific-heat of rock formation (especially if the rock is tight) compared to the much higher specific-heat of the working fluid. Unless circulation rates are very low, the rate of thermal recharge (i.e., heat transfer from areas further away from the well) will therefore be much slower than the rate of heat withdrawal immediately around the well. Consequently, a heat-sink will inevitably develop around a closed-loop well as long as working fluid is being circulated. Combating heat-sink development is a major challenge for a closed-loop geothermal development.

Optimization of the well configuration and completion design is a key part of concept select for a closed-loop project. To maximize heat transfer, the wellbore volume at target depth should be as large as possible which means, the larger casing the better and the longer the wellbore the better. Obviously, these gains need to be balanced against the increased drilling cost and risk of larger and longer wells. For co-axial wells, choosing the optimum tubing size and insulation thickness is another key factor. Choosing a flowrate that optimally balances the desire to maximize instantaneous thermal-power versus the desire to minimize thermal decline over time, is another key aspect. Finally, production-operation options can help to mitigate (to some extent) reservoir cooling of the near-wellbore area: cyclic production (to allow thermal recharge of the reservoir in between episodes of circulation and heat withdrawal), reversing flow direction (in U-shaped wells) or switching laterals (in complex multilateral U-shape configurations). The optimum wellbore concept and production strategy could be very different for a geothermal development that aims to deliver power at peak-demand periods versus a project that strives to deliver a stable base-load.

AEGeo has developed a series of tools that can help the screening of different closed-loop well configurations and identify concepts that are optimum for a given subsurface setting and project requirement. This includes optimizing casing and tubing size for co-axial wells, determining the range in optimum flowrate, modelling the temperature profile along a wellbore and also modelling the impact of cyclic production. See snapshots below.