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Salt formations are utilized to store hydrocarbons, hydrogen, compressed air, and nuclear waste disposal. The stability and integrity of these salt structures are crucial for ensuring environmental and operational safety. This is governed by the combined effect of Thermal-Hydraulic-Mechanical-Chemical (THMC) processes. It is necessary to classify the phenomena involved in each process and identify the parameters that significantly influence them to comprehend the coupled mechanisms. While field operations provide valuable insights, they must offer detailed knowledge of the coupled processes. Analytical and numerical methods are essential to adequately describe THMC processes and bridge this knowledge gap. This study presents the coupled THMC processes in energy storage stage, gas storage, and waste disposal in salt caverns. The objective is to identify the key parameters associated with each process. Additionally, the study investigates available codes and software that simulate these processes and phenomena. Their advantages and drawbacks are discussed. The outcome of this study will help to design a comprehensive framework that integrates software and codes, lab experiments, and field investigations. This framework will facilitate the enhancement of environmental and operational safety measures. [Display omitted] • THMC processes in salt caverns: Energy storage, gas storage, and waste disposal. • Key parameters: Identifying crucial factors in each process. • Codes and software: Analyzing advantages and drawbacks of simulation tools. • Knowledge gap: Bridging field operations and detailed understanding of coupled processes. • Environmental and operational safety framework: developing a comprehensive approach for safety. [ABSTRACT FROM AUTHOR]

Geologic repositories for radioactive waste are designed as multi-barrier disposal systems that perform a number of functions including the long-term isolation and containment of waste from the human environment, and the attenuation of radionuclides released to the subsurface. The rock laboratory at Mont Terri (canton Jura, Switzerland) in the Opalinus Clay plays an important role in the development of such repositories. The experimental results gained in the last 20 years are used to study the possible evolution of a repository and investigate processes closely related to the safety functions of a repository hosted in a clay rock. At the same time, these experiments have increased our general knowledge of the complex behaviour of argillaceous formations in response to coupled hydrological, mechanical, thermal, chemical, and biological processes. After presenting the geological setting in and around the Mont Terri rock laboratory and an overview of the mineralogy and key properties of the Opalinus Clay, we give a brief overview of the key experiments that are described in more detail in the following research papers to this Special Issue of the Swiss Journal of Geosciences. These experiments aim to characterise the Opalinus Clay and estimate safety-relevant parameters, test procedures, and technologies for repository construction and waste emplacement. Other aspects covered are: bentonite buffer emplacement, high-pH concrete-clay interaction experiments, anaerobic steel corrosion with hydrogen formation, depletion of hydrogen by microbial activity, and finally, release of radionuclides into the bentonite buffer and the Opalinus Clay barrier. In the case of a spent fuel/high-level waste repository, the time considered in performance assessment for repository evolution is generally 1 million years, starting with a transient phase over the first 10,000 years and followed by an equilibrium phase. Experiments dealing with initial conditions, construction, and waste emplacement do not require the extrapolation of their results over such long timescales. However, experiments like radionuclide transport in the clay barrier have to rely on understanding long-term mechanistic processes together with estimating safety-relevant parameters. The research at Mont Terri carried out in the last 20 years provides valuable information on repository evolution and strong arguments for a sound safety case for a repository in argillaceous formations. [ABSTRACT FROM AUTHOR]

CO2 geo-sequestration into saline aquifers and an enhanced geothermal system (EGS) are two of the most effective solutions to reduce CO2 emissions into the atmosphere. The significance of thermal-hydrological-mechanical-chemical (THMC) interactions is well identified in these two processes: fluid and heat flow, solute transport within a three-phase mixture; stresses and displacements related to geomechanical effects; non-isothermal effects on fluid properties and reaction processes; and equilibrium and kinetics of fluid-rock and gas-rock chemical interactions. In this paper, a novel numerical model is developed to describe the THMC processes mathematically. One mean stress formulation, which is simplified from the geomechanical equations with full stress tensor, is employed to solve mean stresses and displacements. Chemical equilibrium and kinetics are considered to quantitatively simulate geochemical reaction associated with solute transport. A sequentially coupled computational framework is proposed and used to solve reactive transport of water, gas components, and chemical species in subsurface formation with geomechanics. It is designed to keep a generalized computational structure for different THMC processes. A practical case with complex chemical compositions is presented to analyze the THMC processes quantitatively on the coupled effects of geochemistry and geomechanics during CO2 geo-sequestration. In addition, a practical EGS case is presented to address the mutual effects of each THMC process quantitatively, especially the mutual effects of stress and chemical reaction. Heat transfer affects mean stress and geochemical reactions; mean stress impacts the solute transport and successive chemical reactions; and geochemical reactions are shown to have significant impact on the mean stress, pressure, or temperature. [ABSTRACT FROM AUTHOR]

The importance of thermal-hydrological-mechanical-chemical (THMC) interactions is well recognized in the procedure of CO 2 geo-sequestration. Geo-mechanics and geo-chemistry may have significant effects on the aqueous phase composition, porosity and permeability of the formation, which in turn affect flow and transport processes. Using a mean stress formulation, geomechanical effects are considered such as stresses, displacements, and rock deformation in CO 2 sequestration. Chemical equilibrium and kinetics are taken into account in the mass balance equation, which is able to quantitatively simulate fluid flow, solute transport and geo–chemical reaction in the operation of CO 2 geo-sequestration. Since rock strength decreases with increasing amounts of reactive minerals, substantial dissolution/precipitation of rock composition may lead to significant changes in the mechanical behavior, a generic computational scheme has been developed to take the mechanical and chemical coupling effects into account. Based on these theories, a novel mathematical model of the THMC processes is developed in this paper. A fully coupled computational framework is proposed and used to simulate reactive transport of water, CO 2 gas and species in subsurface formation with geomechanics. The novel frameworks are designed to keep a generalized computational structure for different THMC processes. The coupled THMC simulators focus on: (1) fluid and heat flow, solute transport in a three-phase mixture, (2) stress and displacement related to mean stress, (3) non-isothermal effects on fluid properties and reaction processes, and (4) the equilibrium and kinetics of water-rock and gas-rock chemical interactions. A practical reactive transport examples with cold supercritical CO 2 injection into saline aquifer has been proposed to analyze the THMC processes quantitatively. It is indicated that the geochemical reactions do not have significant impact on pore pressure, mean stress and temperature. The thermal energy transport with low temperature significantly affects the mean stress and geochemical reactions in the saline aquifer. Low temperature accelerates equilibrium dissolution of gas and mineral, but slows down kinetic dissolution/precipitation of minerals, such as anorthite and kaolinite. [ABSTRACT FROM AUTHOR]

The significance of thermal-hydrological-mechanical-chemical (THMC) interactions is well recognized in the operation of CO 2 geo-sequestration. Geo-mechanical and geochemical effects may significantly affect aqueous phase composition, porosity and permeability of the formation, which in turn influence flow and transport. The TOUGHREACT simulator has the capability to quantitatively simulate fluid flow, solute transport and geochemical reaction in CO 2 geo-sequestration using sequential coupling. Using a mean stress formulation, geomechanical effects such as stresses, displacements, and rock deformation in CO 2 sequestration have been simulated by the recently developed TOUGH2_CSM. Based on these simulators, in this paper a novel mathematical model of the THMC processes is developed. A sequentially coupled computational framework is proposed and used to simulate reactive transport of water, CO 2 gas and species in subsurface formation with geomechanics. The novel frameworks are designed to keep a generalized computational structure for different THMC processes. The coupled THMC simulators focus on: (1) fluid and heat flow, solute transport within a three-phase mixture, (2) stresses and displacements related to the mean stress, (3) non-isothermal effects on fluid properties and reaction processes, and (4) the equilibrium and kinetics of fluid-rock and gas-rock chemical interactions. A set of partial differential equations is presented to model the THMC processes of the fluid and heat flow, solute transport in aqueous and gaseous phase, mean stress, and geochemical reactions under both equilibrium and kinetic conditions. A 2D reactive transport model with complex chemical compositions is presented to analyze the THMC processes quantitatively on the coupled effects of geochemical reaction and geomechanics during CO 2 geo-sequestration process. The model is able to analyze the long term fate of CO 2 and the efficacy of different trapping mechanism (structural, residual, solubility and mineral trapping) with respect to key minerals. The modeling algorithm can also be used to simulate CO 2 EOR and other secondary and EOR processes in petroleum reservoirs. [ABSTRACT FROM AUTHOR]

• Self-propping of the asperities contributes to the permeability enhancement during hydro-shearing. • Thermal stress induced shear-slip was observed in saw-cut fracture at elevated temperature. • Asperities degradation of rough Barre granite fracture surface was higher at elevated temperature. • Asperities degradation aided in mineral dissolution from Barre granite fracture surface at elevated temperature. • New micro-cracks observed in rough surface Barre granite at elevated temperature contributed to permeability enhancement. Injection-induced shear stimulation (commonly known as 'hydro-shearing') is often implemented in Enhanced Geothermal Systems (EGS) to enhance the reservoir permeability. Hydro-shearing occurs when a critically stressed pre-existing fracture (or fault) slips at an injection pressure below the minimum principal stress in the reservoir, which opens the fracture due to self-propping of asperities on the fracture surface. In this study, six triaxial flow through tests were conducted on rough fracture and saw-cut granite specimens at temperatures of 20°C, 65°C, and 130°C in order to investigate the impact of hydro-shearing on fracture response to coupled thermal-hydrological-mechanical-chemical (THMC) processes. Hydro-shearing results in a permanent shear-slip in a rough fracture, which opens the fracture due to self-propping of asperities and contributes to retainable permeability enhancement. Thermal stress induced shear-slip was observed in case of saw-cut fracture tested at 130 °C. Thermal stress at elevated temperature could have contributed to the mechanical crushing of asperities and weak fractures, which resulted in higher amount of gouge particles. Increase in temperature also resulted in an enhanced mineral dissolution rate in post-hydro-shearing stage compared to pre-hydro-shearing stage. X-Ray micro CT image showed new micro-cracks propagated from rough surface during hydro-shearing at 130°C, which could contribute to the fracture permeability during post-hydro-shearing stage of the specimen. Gradual decrease in fracture permeability observed during post-hydro-shearing affects the long-term production of the reservoir. The result of the experiments presented here helps to strengthen the understanding of the fracture response to coupled THMC processes during EGS stimulation through hydro-shearing. [ABSTRACT FROM AUTHOR]