Soft stimulation technologies have been proposed as a means to reduce the breakdown pressure and mitigate the risk of induced seismicity during geothermal reservoir stimulation. Yet, the underlying mechanisms remain poorly understood due to the complexity of the coupled thermo-hydro-mechanical (THM) processes. In this work, a fully coupled THM model is developed to evaluate and compare the performance of different stimulation scenarios (monotonic, stepwise injection rate, cyclic injection rate or temperature, and stepwise combined with cyclic injection rate stimulation) on a synthetic, highly permeable reservoir with near-borehole clogging. Simulation results show that stepwise injection rate stimulation yields the most favourable outcomes, followed by the stepwise injection rate combined with cyclic injection rate stimulation. On the other hand, fatigue effects are seen to play a negligible role in the improved performance since the tensile stress at the fracture tip is relaxed with the continuous fracture growth. In addition, cyclic injection temperature stimulation is generally neither better nor worse than monotonic stimulation, but has slightly different characteristics, creating more local damage controlled by the period of the injection cycle. Cyclic injection rate stimulation can slightly reduce the peak pressure, compared with monotonic stimulation, but only when the injection rate is low. The reduction in peak pressure occurs due to the combination of thermally-induced stresses associated with cooling and incremental damage rather than any influence of fatigue. Stepwise or low-frequency cyclic injection rate stimulation are suggested rather than a high-frequency cyclic injection rate stimulation, while injection with cyclic temperatures is suggested when more local damage is wanted.

Rocks can undergo fatigue failure when subjected to cyclic mechanical, hydraulic, or thermal loadings, or a combination of these. Therefore, accounting for possible fatigue damage is important for subsurface engineering projects, such as the cyclic stimulation of geothermal reservoirs. However, existing models do not simultaneously account for degradation of both tensile strength and stiffness under varying-amplitude loading and coupled thermo-hydro-mechanical (THM) conditions. To address this, a new cohesive zone model is developed to account for the effect of fatigue on tensile strength and stiffness. The model is then used within the framework of zero-thickness interface elements to simulate the response of pre-existing or new fractures. Hydraulic and thermal processes are included in both the cohesive interface elements and the continuum elements, allowing the consideration of coupled thermo-hydro-mechanical processes. The fatigue damage variable is set to evolve with the number and magnitude of cycles according to Palmgren-Miner's rule. The proposed method is validated against three laboratory tests from the literature, including cyclic Brazilian test, cyclic hydraulic fracturing test and cyclic thermal stimulation test. All three validation results show that the fatigue damage or reduced breakdown pressure can be well reproduced. Mesh sensitivity based on the simulation of the Brazilian test, in which interface elements are inserted in-between all the continuum elements, highlights the influence of the mesh orientation and mesh density on the simulation results. In addition, stabilisation of the method is demonstrated by increasing the mechanical viscosity, which must be used with care to avoid predicting a longer fatigue life. The ability of the method to handle varying-amplitude cyclic loading is demonstrated by the simulation of a synthetic cyclic loading scheme based on the Brazilian test. The proposed method can be used to support the design of cyclic thermal stimulation campaigns for geothermal (or other) reservoirs, by being able to simulate the reduction in strength due to fatigue, and thus reducing stimulation pressures needed.

Clay-rich geological formations are considered as host rocks for deep geological disposal of radioactive waste. Over the long term, gas will be produced and will migrate through the surrounding geological formation. Gas transport mechanisms have been investigated in laboratory tests. However, the effects of material heterogeneity remain insufficiently explored. This paper presents a stochastic analysis of two-phase flow in clays under gas injection, incorporating spatially correlated porosity. The study evaluates the effects of sample size, gas injection pressure, and the choice between two-dimensional (2D) and two-dimensional (3D) conditions on the statistical outputs, including mean behaviour and variability. The results indicate that larger samples exhibit reduced variability in the degree of saturation and gas permeability due to enhanced averaging effects. Moreover, the variation in results is higher under high gas injection pressure compared to low gas injection pressure. In addition, the variability of results is significantly reduced in 3D simulations compared to 2D, with high-permeability regions more likely to form continuous pathways under 3D conditions, emphasising the necessity of accounting for 3D effects. The findings indicate that sample size is a critical factor in experiments, as it influences the number of tests required to achieve results within a desired level of accuracy.

The crucial interaction between lessons learned from the study of unsaturated soil mechanics and energy geotechnics was highlighted at the recent third edition of the International Symposium on Energy Geotechnics (SEG23), held in Delft, the Netherlands. This short communication summarises the discussion that revolved around handling the many issues raised by the current energy transition from fossil fuels to more sustainable and renewable resources, and the need to integrate unsaturated soil knowledge into energy geotechnics. The panel discussion at the symposium emphasised how crucial it is to use the fundamental concepts of unsaturated soil mechanics for a range of energy applications to be able to characterise key underlying multi-phase processes and enable efficient design. With representatives from around the world, the panel discussion's goal was to close the gap between theoretical research and real-world applications by fostering a dialogue between academics and industry, thereby advancing creative and sustainable geotechnical solutions. The understandings generated from this conversation highlighted the necessity of ongoing cooperation and knowledge sharing to propel area developments and successfully address the urgent energy and environmental challenges of our time.

Bio-mediated methods, such as microbially induced carbonate precipitation, are promising techniques for soil stabilisation. However, uncertainty about the spatial distribution of the minerals formed and the mechanical improvements impedes bio-mediated methods from being translated widely into practice. To bolster confidence in bio-treatment, non-destructive characterisation is desired. Seismic methods offer the possibility to monitor the effectiveness and mechanical efficiency of bio-treatment both in the laboratory and in the field. To aid the interpretation of shear wave velocity measurements, this study uses the discrete element method to examine the small-strain stiffness of bio-cemented sands. Bio-cemented specimens with different characteristics, including properties of the host sand (void ratio, uniformity of particle size distribution) and properties of the precipitated minerals (distribution pattern, content, Young’s modulus), are modelled and subjected to static probing. The mechanisms affecting the small-strain properties of cemented soils are investigated from microscopic observations. The results identify two mechanisms controlling the mechanical reinforcement associated with bio-cementation, namely the number of effective bonds and the ability of a single bond to improve stiffness. The results show that the dominant mechanism varies with the properties of the host sand. These results support the use of seismic measurements to assess the mechanical efficiency and effectiveness of bio-mediated treatment.

Geothermal energy extraction through deep mine systems offers the potential to reduce the cost of geothermal systems while meeting the cooling needs of deep mines. However, the injection of cold water into the subsurface triggers strongly coupled thermo-hydro-mechanical (THM) processes that can affect the stability of underground excavations. This study evaluates the impact of geothermal energy extraction on the temperature and stability of a deep mine. By quantifying the sensitivity of the mine temperature and stability to various parameters, we propose a scheme to optimize geothermal energy production, while achieving rapid mine cooling and maintaining stability. We first evaluate the impact of geothermal operations on mine temperature and stability through THM numerical modeling. The simulations show that poro-elastic stress quickly affects mine stability, while thermal stress has a more significant impact on the long-term stability. We then use Distance-based Generalized Sensitivity Analysis (DGSA) to quantify parameter sensitivity. The analysis identifies the distance between the mine system and the geothermal system as the most influential factor. Other important parameters include the injection rate, injection temperature, well spacing, coefficient of thermal expansion, permeability, Young's modulus, and heat capacity. Finally, we propose a DGSA-based optimization framework that accounts for subsurface uncertainty and validate the optimized results. Our results indicate that, with favorable geological conditions, a rational selection of system design parameters can enhance geothermal energy production while ensuring rapid mine cooling and stability. This study provides essential insights for the optimization of deep mine geothermal systems and supports effective decision-making.

Bio-cemented soils can exhibit various types of microstructure depending on the relative position of the carbonate crystals with respect to the host granular skeleton. Different microstructures can have different effects on the mechanical and hydraulic responses of the material, hence it is important to develop the capacity to model these microstructures. The discrete element method (DEM) is a powerful numerical method for studying the mechanical behaviour of granular materials considering grain-scale features. This paper presents a toolbox that can be used to generate 3D DEM samples of bio-cemented soils with specific microstructures. It provides the flexibility of modelling bio-cemented soils with precipitates in the form of contact cementing, grain bridging and coating, and combinations of these distribution patterns. The algorithm is described in detail in this paper, and the impact of the precipitated carbonates on the soil microstructure is evaluated. The results indicate that carbonates precipitated in different distribution patterns affect the soil microstructure differently, suggesting the importance of modelling the microstructure of bio-cemented soils.

Microbially induced carbonate precipitation is a promising ground improvement technique which can enhance the mechanical properties of soils through the precipitation of calcium carbonate. Experimental evidences indicate that the precipitated carbonate can display different distribution patterns. Crystals can develop at grain–grain contacts (contact cementing), connect soil grains that were initially not in contact with each other (bridging), precipitate on the grain surface (coating), or fill in the void space (pore filling). This paper investigates the role of the aforementioned distribution patterns on the mechanical behaviour of lightly bio-cemented soil samples using discrete element modelling. Bio-cemented samples with different distribution patterns and carbonate contents are built, and a series of drained triaxial compression simulations are carried out at different confining pressures. The results show that cementation in the form of bridging and contact cementing leads to obvious improvement in stiffness, strength and dilatancy. In contrast, cementation in the form of coating contributes only slightly to mechanical improvement, and pore filling exhibits negligible influence on the mechanical response of the material. The findings suggest that, to gain strength improvement in the most effective way, treatments should be tailored to precipitate calcium carbonate crystals in the form of bridging.

Geological Disposal Facilities (GDF) for radioactive waste will generally rely on clay-rich materials as a host geological formation and/or engineered barrier. Gas will be produced within the GDF, which can build up significant gas pressure and will activate the migration of gas through the clay materials via different transport mechanisms. These transport mechanisms are usually investigated in laboratory tests on small clay samples of a few centimetres. In this paper, a new Pneumo-Hydro-Mechanical (PHM) Finite Element model to simulate gas migration in saturated clay samples of this scale is presented. In the proposed modelling approach, continuum elements are used to represent the mechanical and flow processes in the bulk clay material, while zero-thickness interface elements are used to represent existing or induced discontinuities (cracks). A new triple-node PHM interface element is presented to achieve this. The performance of model is illustrated with synthetic benchmark examples which show the ability of the model to reproduce observed PHM mechanisms leading to propagation of cracks due to the gas pressure (gas fracturing).