Multileg Closed-Loop Geothermal Systems

Closed-loop geothermal systems are thought to be attractive because of their controlled nature, their relatively small footprint, and their scalability outside the geography of conventional geothermal sites. By adding multiple legs to the hydraulic system, the performance can be enhanced with minimal additional surface footprint.

With multiple parallel legs, splitting the flow equally among the legs becomes critical in system optimization. This is complex and requires the careful selection of hole size, especially in the manifold and flow junctions. It is made even more difficult when we factor in the effects of trajectory uncertainty and how the actual drilled path may differ from the design path.

To satisfy this conundrum, we built a flow simulator to model these systems. The model uses 1D thermal conductivity in the formation with laminar or turbulent flow in the wells. We model the impact of temperature on the fluid density and the thermal-driven flows in the system.

We show that the flow rate variation between parallel flow paths can be as much as three times higher in some legs. This also impacts the temperature in the produced fluids.

With very small errors in the trajectory, the flow in some legs can stall, or even be reversed. In addition, starting the flow in all the parallel legs can be especially difficult.

We also discuss how the trajectory could be changed to make the flow more stable, the performance more consistent, and operating the system considerably less challenging, thus reducing performance uncertainty and making the economics more attractive.