Prevention of Thermal Pollution of Groundwater near Open Loop Geothermal Energy Systems

Open loop geothermal energy systems (GES) extract groundwater through an extraction well, pass the water through a heat exchanger to remove or add heat, then reinject the cooled or heated water into the aquifer. Because the reinjected water has a different temperature than the ambient groundwater, reinjection creates a thermal plume that travels downgradient and can be harmful to downstream water users or ecosystems. The heat from the thermal plume dissipates as the plume travels through the aquifer, so after sufficiently long distances have been traveled, the water has returned to the ambient temperature of the aquifer. Thus, the sites chosen for GES should be located sufficiently far from temperature-sensitive ecosystems. We propose the adjoint method for identifying allowable locations of GES. With the adjoint method, information is propagated upgradient from the location of the ecosystem, such that only one adjoint simulation is required to evaluate the appropriateness of any location as a site for GES. The application of the adjoint method in this context requires the assumption that the water extraction and reinjection have a minimal influence on the flow field. In this work, the errors introduced by that assumption are evaluated, and a correction is developed to reduce the errors introduced by that assumption. The approach is applied and tested for a field site in Finland, where the ambient groundwater velocity is very low and therefore the extraction and reinjection of water at the GES substantially alters the flow field.