Ambient groundwater flow may influence the performance and thermal recharge of deep low-enthalpy geothermal systems by altering the heat transport in the subsurface. While this effect has been studied extensively for shallow aquifer thermal energy storage systems, its effect at depths relevant for low-enthalpy geothermal production remains less understood. Additionally, at these depths the ambient groundwater flow velocities are generally unknown and difficult to quantify. This thesis investigates how ambient groundwater flow affects the technical and economic performance and the thermal recharge of deep low-enthalpy geothermal doublet systems.

Representative groundwater flow velocities for clastic sedimentary reservoirs in the Netherlands were first estimated using formation pressure measurements between depths of 1-3 km from the Southern North Sea pressure database. Hydraulic gradients between hydraulically connected wells were combined with estimates of permeability and dynamic fluid viscosity to derive average Darcy velocities. The estimated average groundwater flow velocities reached values up to approximately 0.020 m/day.

In order to assess the influence of ambient groundwater flow, these estimates were imposed in numerical reservoir simulations using the Delft Advanced Research Terra Simulator by applying lateral pressure gradients to represent background flow. Simulations were performed for one homogeneous reservoir model and five heterogeneous fluvial reservoir models, using multiple background groundwater flow directions and magnitudes. The influence of ambient groundwater flow was evaluated based on production temperature, system lifetime, thermal recharge time, reservoir temperature distributions and economic performance.

The results show that even relatively small ambient groundwater velocities can substantially influence the behaviour of geothermal systems. Ambient groundwater flow aligned with the production-induced flow, accelerates thermal breakthrough and therefore shortens system lifetime, whereas opposing flow delays thermal breakthrough and extends system lifetime. These variations in production temperature and system lifetime also affect the economic performance of geothermal systems, as scenarios with extended lifetimes generally result in higher Net Present Value and lower Levelized Cost of Heat, while shortened lifetimes reduce the economic viability of the project.

Ambient groundwater flow also strongly influences thermal recharge by displacing the cold water plume and altering the relative contribution of advective and conductive heat transport processes through which the reservoir thermally recovers. The simulations further demonstrate that geological heterogeneity strongly controls how ambient groundwater influences geothermal production and thermal recharge.

These findings highlight the importance of considering ambient groundwater flow when evaluating the technical and economic performance and thermal recharge of deep low-enthalpy geothermal systems.