A brief historical review starts from the double porosity approach of Barenblatt in 1960; various versions of it are still used today despite the difficult evaluation of the exchange coefficient between the two media. Later, the homogenisation theory was applied, but it stopped before solving the macroscopic equations. A decisive step was achieved in 1980 by viewing a fracture network as a set of disks randomly distributed in space. This was the starting point of our own works with additional ideas such as the crucial role of the dimensionless density for convex fractures and the use of systematic numerical experiments in order to rationalize the results.

Today, with the powerful computers at our disposal, very complex configurations can be addressed, but the theoretical and methodological gains are limited.

Therefore, the basic issues have not changed for real networks. They can be summarized and illustrated by our simple approach. First, what are the most salient features of the measurements made on oucrops and wells? Some of them provided a basis to the theoretical predictions of quantities such as percolation and macroscopic permeability, but alternative approaches could certainly be developed. Second, how can we reconstruct, i.e., generate realistic networks based on these measurements? Statistical techniques have been extensively used, but more advanced techniques such as AI should be envisioned. Third, experimental results are usually scarce and often not easily compared with numerical predictions.

Despite and/or because all these difficulties, this field is expected to remain very active in the future.

The HotBENT experiment at the Grimsel Test Site investigates the coupled thermal and hydraulic behaviour of a bentonite barrier subjected to high temperatures. The setup includes four electric heaters embedded in a bentonite buffer, with temperatures reaching up to 200 °C. Four “enhanced geosphere pressurization boreholes” using natural Grimsel groundwater support and enhance hydration of the bentonite buffer via surrounding fractured granite. This configuration induces complex interactions between drying, vapour transport, and re-saturation processes.

We present results from 3D fully coupled thermo-hydraulic simulations conducted using OpenGeoSys. The model incorporates temperature-dependent transport properties, bentonite-specific retention behaviour, and hydration from the surrounding granite. Although fractures are not explicitly represented, the surrounding rock acts as a boundary condition influencing water inflow into the bentonite.

Experimental data are used for calibration and validation. The study highlights how temperature gradients affect saturation, vapour movement, and pressure development in low-permeability media. We also discuss numerical challenges related to material nonlinearity and boundary implementation.

These results enhance our understanding of thermo-hydraulic behaviour in engineered barriers and their interaction with fractured host rocks — even when treated as continuous domains — supporting predictive modelling efforts in subsurface systems.

Geothermal energy, as one of the renewable energy types, has great potential to balance the variable energy supply from photovoltaic and wind energy types. This, however, requires the development of an Enhanced Geothermal System (EGS). Injecting cold water into deep geo-reservoirs induces thermoelastic deformation and changes the fracture apertures, which may significantly affect the EGS lifetime and energy performance. In this case, this study develops a fully coupled thermal-hydraulic-mechanical model to investigate the impacts of temperature, pressure, and aperture heterogeneity on the alterations of fracture aperture. Then, considering the reservoir lifespan, the research takes a further step into the influence of heterogeneous aperture variation on EGS production temperature and long-term energy performance. Results show that aperture significantly varies once a temperature breakthrough occurs in the production well for both homogeneous and heterogeneous aperture fields. The thermoelasticity-induced aperture variations enhance water flow rate, leading to a considerable increase in reservoir heat production rate, but a shorter EGS lifespan. Furthermore, temperature variation shows a more substantial influence on aperture alteration compared to pressure change. Additionally, a sensitivity analysis indicates that the 30-year total energy output is proportional to the injection/production pressures and injection temperatures. However, the fracture with either excessively large or small initial apertures leads to worse EGS energy performance.

High-temperature aquifer thermal energy storage (HT-ATES), with its high storage capacity and energy efficiency and its compatibilities with renewable energy sources, has generated widespread interest. One main criterion for a feasible HT-ATES is the thermal recovery efficiency, i.e., how much of the invested heat can be recovered. The heat lost during the HT-ATES is mainly due to the heat conduction and the density-driven buoyancy flow, which are more significant with HT-ATES compared to the conventional low-temperature ATES. Thus, understanding the fluid displacement and thermal transport processes during HT-ATES is essential for assessing the performance of HT-ATES. A group of key dimensionless parameters regarding the thermal recovery efficiency for HT-ATES are identified in this study. The numerical model is set for a typical HT-ATES based on the geological in the Burgwedel region and the designed operational parameters. Over one thousand cases are simulated for a sweep of the key parameters for multiple cycles and storage volumes, and the resulting recovery efficiency for each case is obtained. The hot water injection and displacement processes and the correlation between the recovery efficiency and the key parameters are investigated. The correlation functions are built to estimate the thermal recovery efficiency, which can be used for a quick assessment of potential HT-ATES sites when the properties of the aquifer are known.