A numerical sensitivity study of how permeability, porosity, geological structure, and background hydraulic gradient control the lifetime of a geothermal reservoir

The utilization of geothermal energy comes with a high economic risk. Many of the parameters that control the quality of a geothermal reservoir are heterogeneously distributed and therefore difficult to predict in a rock volume. We carried out a numerical sensitivity study using a large number (1027) of 4D numerical models of a geothermal doublet. We systematically varied each parameter (e.g., porosity, permeability, background hydraulic gradient (BHG)) and tested the longevity of the simulated reservoirs, by calculating the time until the temperature of the produced fluid fell to 100°C. We increased complexity of the models by introducing permeability anisotropy and contrast to simulate layering, fracture anisotropy, and a fault-zone. Our results confirm the positive effect of porosity on heat capacity. However, they show also that the BHG, together with permeability, if sufficiently high, can outperform the other parameters. In the more complex models, certain configurations of the parameters constitute tipping points, i.e. small modifications of the parameters decide between extreme longevity and early thermal breakthrough. For example, highly permeable zones, as is common in faults, can provide high initial yields, but also channel the fluid flow and thus can restrict the exploitable reservoir volume. The BHG can be outperformed by small variations in permeability contrast caused by layering and fracture anisotropy. Even low permeability contrasts (103) or fracture-induced permeability anisotropies (101) can channel fluid flow and thus significantly restrict the utilizable reservoir volume. In such cases, the initial yields may be high, but the lifetime of the reservoir is short.