Effects of deep geothermal injection and extraction well modeling approaches on flow and heat transport in multilayer aquifers

Geothermal reservoir modeling often focuses on fractured or single-layer systems, even though multilayer porous aquifers offer an additional opportunity for geothermal energy extraction. This study examines how specific modeling assumptions influence the flow field and thermal evolution in stratified geothermal systems. It focuses on a representative multilayer aquifer of the Bückeberg Formation in the North German Basin. The targeted interval contains stacked sandstone units separated by claystone between depths of 1200 and 1400 m with reservoir temperatures around 70 °C and injection rates of several tens of litres per second.

A three dimensional numerical model is developed in the open source software OpenGeoSys to evaluate groundwater flow and heat transport in this layered system. As the fluid viscosity is temperature-dependent, the resulting flow field evolves over time. To investigate the associated water distribution around the well, injection and extraction are represented using three numerical approaches. First, a pipe based implementation is used to explicitly model the flow through the wells so that the distribution of injected and produced water between the sandstone layers is not prescribed but governed by the geological and hydraulic properties of the multilayer aquifer. Further, two imposed injection concepts are applied for comparison: a line based source term and a source term defined on a cylindrical borehole surface. Moreover, the approximation of constant viscosity is assessed by comparison with simulations using both constant and temperature-dependent viscosity for the three well implementations. 

Preliminary results show that temperature-dependent viscosity noticeably alters the flow field and affects the evolution of production temperature. These tendencies confirm the relevance of viscosity formulation when analysing thermal behaviour in multilayer geothermal systems. The ongoing comparison of injection and extraction well approaches extends these findings by including the influence of inflow and outflow patterns on the flow field near the wells, where flow paths in multilayer aquifers are inherently more complex. This highlights the importance of choosing appropriate inflow and outflow conditions for modeling the thermo-hydraulic response of stratified reservoirs.