Geomechanics for Energy and the Environment最新文献

In the process of integrating supercritical CO₂ (ScCO₂)-enhanced shale gas recovery and geological sequestration, the mechanical properties of shale can be impacted by ScCO₂ under high-temperature and high-pressure conditions. This can affect wellbore stability, production efficiency, and the safety of sequestration. To address this issue, this study investigated the interactions between shale and three types of fluids: ScCO₂, water, and a combination of ScCO₂ and water. Experiments were conducted at high pressure (15 MPa and 45 MPa) and high temperature (100 °C). Changes in shale's mechanical properties before and after immersion were analyzed using uniaxial compression tests and acoustic emission monitoring. The main cation content, microstructure, and element minerals of shale's solution after immersion were also studied. The results show that immersion in ScCO₂ and related fluids deteriorates shale's mechanical properties. Immersion in ScCO₂ has the least effect on shale strength, followed by the change in shale strength caused by immersion in water, and shale strength is the lowest after immersion in a combination of water and ScCO₂. ScCO₂ imbibition promotes the occurrence of micro-cracks, while immersion in water makes shale's matrix loose, forming a pore network structure that is most significantly affected by a combination of water and ScCO₂. For unsoaked and water-immersed shale samples, the acoustic emission events mainly occur during the unstable crack propagation stage, while the acoustic emission events in shale samples treated with ScCO₂ are more dispersed. Compared with previous dynamic pressure immersion experiments, the strength of shale after static pressure immersion increases by 10–30 MPa. This study aims to provide a more comprehensive understanding of the alterations in the mechanical properties of shale when subjected to high temperature and high-pressure immersion conditions. The findings provide valuable data for shale gas extraction and carbon sequestration.

The present study introduces a numerical approach to simulate thermally induced fracture slip using a grain-based distinct element model. As part of DECOVALEX-2023 Task G, we verified the model through benchmarks, explored the thermo-mechanical processes under various conditions, and validated the model against laboratory experiments on both saw-cut and tensile-splitting fractures. In this method, the rock sample was represented by a group of polyhedral grains, such as random Voronoi diagrams or tetrahedra. The thermo-mechanical behavior of the grains and their interfaces was calculated using the distinct element method. Additionally, a novel method to determine micro-parameters of grains and contacts based on an equivalent continuum approach was proposed. The main emphasis was placed on simulating the temperature evolution, thermal stress development and fracture displacements under thermo-mechanical loading. The benchmarks demonstrated the model’s ability to replicate fracture behavior under various conditions, in good agreement with analytical solutions, capturing the phenomena of fracture slip and opening. In the modeling of laboratory experiments, a comparison between the experimental results and the numerical results revealed that the model reasonably reproduced the heat transfer within the rock specimen, the horizontal stress increment depending on boundary condition, and the progressive fracture shear failure. Although discrepancies existed regarding the onset of fracture slip and the magnitudes of stress and displacement, the model demonstrated qualitative consistency with the experimental findings. By tracking the contact area variation, we also found that the model effectively mimicked the mechanism of asperities shear-off, irreversible damage and normal dilation that occur during the peak stage.

In the 2023 phase of the international collaborative DECOVALEX modeling project, Task E focused on understanding thermal, hydrological, and mechanical (THM) processes related to predicting brine migration in the excavation damaged zone around a heated excavation in salt. Salt is attractive as a disposal medium for radioactive waste because it is self-healing and is essentially impermeable and non-porous in the far field. Investigation of the short-term, near-field behavior is important for radioactive waste disposal because this early period strongly controls the amount of inflowing brine. Brine leads to corrosion of waste forms and waste packages, and possible dissolution of radionuclides with brine transport being a potential transport vector to the accessible environment.The Task was divided into steps. Step 0 included matching unheated brine inflow data from boreholes at the Waste Isolation Pilot Plant (WIPP) and matching temperature observations during a Brine Availability Test in Salt (BATS) heater test. Step 1 included validation of models against a thermo-poroelastic analytical solution, and two-phase flow around an excavation. Finally, Step 2 required all the individual components covered in steps 0 and 1 to come together to match observed brine inflow behavior during the same BATS heater test.There were a range of approaches from the teams, from mechanistic to prescriptive. Given the uncertainties in the problem, some teams used one- or two-dimensional models of the processes, while other teams included more geometrical complexity in three-dimensional models. Task E was a learning experience for the teams involved, and feedback from the modeling teams has led to changes in follow-on BATS experiments at WIPP. The primary Task E lessons learned were the impact of hydrologic initialization methods (wetting up vs. drying down), the difference between confined and unconfined thermal expansion, and the large changes in permeability associated with heating and cooling.

Argillaceous rocks are a suitable host rock for the deep geological disposal of exothermic High-Level Radioactive Waste (HLW) and Spent Fuel (SF). Excess pore pressures develop in this type of rocks when subject to increased temperatures that, in some circumstances, may lead to the fracturing of the rock. The paper explores this phenomenon by means of coupled numerical analyses carried out within a fully coupled thermo-hydro-mechanical (THM) framework. The general THM formulation is described as well as the anisotropic elastic and anisotropic elastoviscoplastic constitutive laws employed. Thermal extension triaxial tests are simulated as a check on the performance of the numerical formulation and to provide calibration data for the tensile strength of the material. Selected results from a comprehensive set of three-dimensional analyses of a large-scale field heating test, designed to study the possibility of thermal-induced failure in the rock, are presented and discussed. The analyses reproduce satisfactorily the observed patterns of behaviour. The effects of the constitutive law, material parameters and the presence of the excavation damage zone (EDZ) around the main drift and around the heater boreholes are studied. In particular, their effects on the state of the stress in the heated area are examined in the context of the potential for thermal fracturing of the rock.

Over the last few decades, the study of gas injection in porous media, particularly under multi-field coupled conditions, has emerged as a prominent focus within the field of geotechnical engineering. This article presents a comprehensive comparison of three numerical strategies, evaluating their impact on computational efficiency and result accuracy during Hydro-Mechanical (HM) coupled simulations of gas injection in clay-based geomaterials. This comprehensive comparison encompasses three numerical simulation methods for the mechanical sub-problem: The Standard Finite Element Method (SFEM), the Standard Finite Element Method with Selective Integration (SFEM+SI), and the Mixed Finite Element Method (MFEM). The Heat and Gas Fracking model (HGFRAC) is introduced to illustrate the computational characteristics of these methods. The results indicate that the effective application of SFEM is heavily dependent on a high-precision mesh. Convergence issues may arise when dealing with relatively coarse meshes. Nevertheless, these convergence issues can be effectively mitigated by incorporating either the Selective Integration method or the MFEM formulations. In terms of computational efficiency, it is evident that the SFEM+SI method demonstrates higher efficiency than SFEM and MFEM. However, it is noteworthy that the computed gas flow patterns of SFEM and SFEM+SI can be affected by the alignment of the mesh. With MFEM, displacements and strains are calculated as independent unknowns, enhancing result accuracy and achieving mesh independence.

For the disposal of heat-generating radioactive waste and spent fuel in claystone formations, support structures for underground excavations are essential for the operational safety of geological disposal facilities (GDF). Claystone formations of moderate strength, at considerable depths, are characterised by their squeezing, creeping, and, sometimes, swelling behaviour that results in continuous tunnel convergence over time. Under these conditions, a rigid support system can be subjected to very high loads, requiring a considerable thickness and/or high-performance concretes. Therefore, yielding support systems are being currently investigated as a promising alternative for GDF. This work presents a numerical study of the behaviour of a tunnel in a claystone formation with a yielding support system. The use of a compressible mortar between the rock and the lining is assumed, as a means of reducing loads and mitigating the effects of creep deformations. A key aspect of the analyses is that the host rock is characterised by a constitutive model that includes a number of features that are relevant for the satisfactory description of the hydro-mechanical behaviour of stiff clayey materials, such as mechanical anisotropy, creep, strain softening, and its ability to simulate localised deformations through a nonlocal regularisation. An elastoplastic constitutive model was also developed to represent the behaviour of the compressible mortar. Results provide relevant insights into the performance of the adopted yielding support system, particularly regarding the effect of time-dependent deformations and the additional relaxation of the rock on the fractured zone near the excavation.

In this study, different approaches in performance assessment (PA) of the long-term safety of a repository for radioactive waste were examined. This investigation was carried out as part of the DECOVALEX-2023 project, an international collaborative effort for research and model comparison. One specific task of the DECOVALEX-2023 project was the Salt Performance Assessment Modelling task (Salt PA), which aimed at comparing various models and methods employed in the performance assessment of deep geological repositories in salt. In the context of the Salt PA task, three distinct teams from SNL (United States), Quintessa Ltd (United Kingdom), and GRS (Germany) examined the consequences of employing different levels of abstractions when modelling the repository's geometry and implementing various features and processes, using the example of a simple hypothetical repository structure in domal salt. Each team applied their own tools: PFLOTRAN (SNL), QPAC (Quintessa) and LOPOS (GRS). These differ essentially regarding numerical concept and degree of detail in the representation of the underlying physical processes. The discussion focused on when simplifications can be appropriately applied and what consequences result from them. Furthermore, it was explored when and if a higher level of fidelity in geometry or physical processes is required.

This paper presents a combined laboratory and numerical investigation on the injection-induced permeability changes in pre-existing fractures. The analyses conducted were primarily based on the results of an innovative laboratory experiment designed to replicate the key mechanisms that occur during hydraulic stimulation of naturally fractured rocks and/or faulted zones. The experiment involved pressure-controlled fluid injection into a laboratory-scale pre-existing fracture within a granite block, which was subjected to true triaxial stress conditions. Rough and smooth fractures are investigated, and the results are discussed. Based on the experimental results, two contributing mechanisms were considered to describe the pressure-driven permeability changes in pre-existing fractures: (1) elastic opening/closure leading to a reversible permeability change, and (2) fracture sliding in shear mode, causing dilation and hence an irreversible permeability increase. With these assumptions, an aperture-dependent permeability function was adopted to couple the hydraulic flow with the mechanical deformations along the fracture. Subsequently, a 3D coupled hydro-mechanical model was developed to replicate fluid-injection tests conducted at various conditions, including different stress conditions and fracture surface roughness. The employed modeling framework effectively captured the experimental observations. Our results indicate that the maximum permeability increases twofold.

Bearing capacity of energy piles may be affected by the Radial Interaction between Energy Piles and Soils (RIEPS) such as energy pile expansion and transient radial heat conduction. This paper proposes a cavity-expansion-based solution to investigate the thermo-elastic RIEPS. Transient temperature distributions are shown by assuming heat conduction in the radial direction and constant temperature at the pile-soil interface. With the temperature distributions, a thermo-elastic solution is obtained to capture the changes in stresses and displacements around energy piles. It is found that the solution under the combined thermal-mechanical loading pattern is the linear superposition of those under the thermal loading and mechanical loading patterns. Hence, the stresses, strains and displacements in soils are determined by the competitive relationships between thermal and mechanical loading patterns. The expression for radial stress change at the pile-soil interface is discussed by the cavity expansion analysis and comparison with field data. For typical soil and pile parameters, the expression could be quite general considering transient temperature distributions and soil/pile moduli. This paper can benefit to the capacity design of energy piles by taking the RIEPS into account.

This paper introduces innovative practical methodologies for evaluating the thermal performance of thermo-active pile groups. First, a streamlined approach for determining G-functions within such groups, based on the G-function of a single thermo-active pile is introduced. This is accomplished through a newly introduced thermal interaction factor for G-functions quantifying the increase in temperature when a pile is subjected to thermal interference from another pile. Subsequently, the paper proposes a method for calculating the power of piles within thermo-active pile groups when subjected to transient inlet temperatures. A thermal interaction factor for power is derived, quantifying the power reduction resulting from thermal interference due to another pile operating in the vicinity. These simplified methodologies are shown to reproduce the thermal performance of pile groups simulated using three-dimensional thermo-hydraulic analyses with excellent levels of accuracy without the associated computational cost. Finally, the proposed design process is applied to a 3 × 3 thermo-active pile group subjected to transient thermal loads, yielding accurate estimations of power, G-functions, and temperature changes of the thermo-active pile group. Overall, these simplified methodologies offer a robust framework for evaluating and optimising the thermal performance of thermo-active pile systems.