Geomechanics for Energy and the Environment最新文献

In this paper, the one-dimensional rheological consolidation characteristics of multilayered saturated soil foundations under time-dependent loading and heating are investigated by considering the semi-permeability and the interface thermal resistance. By introducing the fractional derivative model and the thermos-elastic theory, a thermo-mechanical coupling model is established to describe the rheological properties of saturated soils. Semi-analytical solutions for strain, temperature increment, pore water pressure and settlement were derived through the Laplace transform and its inverse. The accuracy of the solutions proposed in this paper has been verified by comparing with existing solutions. The effects of different thermal contact models of the interface on the rheological properties of saturated soils under semi-permeable boundary are discussed, and the effects of fractional derivative order, constitutive material parameters, and thermal conductivity of soil on the thermal consolidation process are investigated. The results show that: neglecting the thermal resistance effect can result in an overestimates of the impact of rheological properties on the thermal consolidation process of saturated soils under semi-permeable boundaries; As the thermal resistance coefficient increases, the influence of soil thermal conductivity on settlement decreases.

This study focuses on the modelling of a gas injection experiment to assess the effects of incorporating heterogeneous material properties. The numerical model considers a two-phase flow coupled hydro-mechanical problem, and includes embedded fractures that open with deformation, thereby enhancing permeability. The approach used is integrated in the CODE_BRIGHT software, which allows for the consideration of geomaterials with a spatially correlated heterogeneous field of porosity that follows a normal distribution. This spatial correlation can be either isotropic or anisotropic. A key aspect of this approach is that material properties such as intrinsic permeability, diffusivity or cohesion are defined as a function of porosity. Consequently, these properties also exhibit heterogeneity with spatial correlation and, eventually, anisotropy. The results derived from the numerical model align well with in-situ measurements. The study also includes sensitivity analyses to the variation of critical variables. The calibration of the model has been validated through a similar experiment. The findings indicate that the consideration of heterogeneous material properties can have a significant influence on gas injection problems, particularly when a hydraulic fracture is formed.

During the long-term operational lifespan of nuclear waste repositories in crystalline rock formations, large earthquakes along nearby seismogenic fault zones may occur, coseismically triggering shear displacements of secondary fractures within the respository site. In addition, these secondary fractures that may be associated with slip-weakening friction could accommodate significant slip instabilities and large shear displacements. A cumulative shear displacement exceeding 50 mm could affect the integrity of waste canisters, potentially resulting in the escape of hazardous radionuclides into the groundwater system. To investigate this problem, we develop a novel 3D seismo-mechanical model to simulate the transient rupture of a primary seismogenic fault zone and coseismic slips in a network of secondary fractures located around the primary fault. A plausible postglacial earthquake scenario is studied, where the rupture along the seismogenic fault propagates outward from a predefined hypocenter, with the resulting static stress changes and dynamic ground vibrations captured. We explore different cases with secondary fractures having different degrees of slip-weakening friction, which is found to strongly control the spatial decay of coseismic fracture displacements in the system. The findings derived from our study have significant implications for assessing the long-term safety of nuclear waste repositories in faulted and fractured crystalline rocks.

Produced Water Re-Injection is a standard practice in oil and gas operations. When performed at sufficiently high pressures, it can trigger the fracturing of the reservoir, which should be controlled to stimulate the formation without compromising its safety. While the mechanisms of the hydraulic fracturing of brittle rocks are well-understood, this understanding is more challenging in the case of unconsolidated sand reservoirs. Recent experimental studies indicate that pseudofractures in such formations primarily result from shear banding and flow channelization. There is a need for further interpretation of the experimental findings through numerical models that account for the proper physical phenomena. For that, a coupled finite element model of water percolation, particle transport and strain localization is developed to replicate the experiments of radial injection of water in mixtures of sand and fines by Nguyen et al., 2022: An experimental setup with radial injection cell for investigation of fracturing in unconsolidated sand reservoirs under fluid injection. Journal of Petroleum Science and Engineering 213:11036. To simulate shear banding, the model accounts for strain-softening behavior and introduces material imperfections in a few weak elements. Particle mobilization starts once a critical fluid velocity is reached and permeability is assumed to be a function of particle concentration. The numerical models can replicate the geometry of the observed radial pseudofractures, as well as the measured fracturing pressures. They also qualitatively capture the effects of the initial stress state on the fracturing pressure, the fracture length and on the permeability increases during the fracturing regime as observed in the laboratory tests. Notably, the numerical results gave hints on the role of the coupling between strain localization and particle transport on the formation of the pseudofractures. These findings are used to propose an updated conceptual model for the hydraulic fracturing of sand packs.

Ensuring project safety for maritime geotechnical structures primarily composed of calcareous sand is crucial. Cement-based reinforcement is a promising strategy to enhance integrity and deformability, especially for overlying infrastructures in high-pressure ocean engineering. Firstly, the specimens are created by blending Portland cement and Gypsum into calcareous sand at contents of 16% and 22% and then subjected to curing periods of 7 and 3 days to explore their resistance to loading. Secondly, a triaxial consolidated drained test is conducted, applying different confining pressures ranging from 100 to 1200 kPa. This test aims to assess the mechanical behavior, strength parameters, failure criteria, and stress dilatancy behaviors of treated specimen. The results illustrating the peak shear strength points of all treated specimens display a concave nonlinear shape sloping downwards. An equation is presented to modify the mean effective stress and accommodate the influence of bonding strength. Finally, A revised criterion is formulated by integrating this equation into the failure criterion. Significantly, this refined failure criterion accurately defines the failure envelopes. An equation was established to reveal the relationship between the brittleness index and confining pressure. Additionally, two stress ratio parameters are defined based on the brittleness index to describe bonding degradation comprehensively.

The response of fine-grained soils to the combined effects of stress and temperature is a problem of growing concern in geoenvironmental engineering. Unlike most materials, fine-grained soils subjected to heating under drained conditions can exhibit either reversible expansion or irreversible contraction, depending on their loading history. This clay complex thermo-mechanical behaviour is widely reported in the literature, but its origin is still unknown. This paper explores the particle-scale origin of clay thermo-mechanical behaviour and helps to inform constitutive thermo-mechanical models. Clay particle interactions include non-contact forces, which are electrochemical in nature and prevail in face-to-face configuration and contact forces, which are mechanical forces transferred from one particle to another through a contact surface, typical of edge-to-face configuration. Non-contact forces include electrostatic Coulombic forces and van der Waals attractive forces. This paper proposes a combined numerical and analytical approach to quantify the elementary interactions between clay particles. The results are used to interpret typical stress-thermal paths, such as compression tests at different temperatures and heating-cooling cycles at constant mechanical stress. It is concluded that the electrochemical interactions governing the face-to-face particle configuration can only explain the elastic volumetric response of over-consolidated clays subjected to heating. The thermo-plastic behaviour associated with the reduction of the pre-consolidation pressure with temperature and the volumetric plastic compressive strain in response to heating in normally-consolidated clays is attributed to the edge-to-face particle configurations.

To ensure the safety of geological disposal of high-level radioactive waste, in-situ experiments have been carried out to examine the behavior of rocks in underground research laboratories (URLs). At the Meuse/Haute-Marne URL in France, the French National Radioactive Waste Management Agency (Andra) has been assessing the Callovo-Oxfordian claystone (COx) as potential host rock of geological disposal by subjecting the COx to in-situ heating mimicking exothermic radioactive waste. Results of the in-situ experiments are used to validate and bolster the numerical simulators for predicting the thermo-hydromechanically (THM) coupled behavior of the COx. The numerical simulators are, however, yet to be tested for predicting the failure and fracture development of the COx during heating, which is of paramount importance to the safety of the geological disposal. In this research, we modelled a recently carried out in-situ experiment at the Meuse/Haute-Marne URL using the TOUGH-FLAC simulator to predict the failure and fracture development of the COx during heating. The objectives are to examine the effects of (i) the weak bedding planes, (ii) the softening rate of matrix/weak plane strengths, and (iii) the stiffness anisotropy of the COx on the development of shear and tensile fractures during heating. Results show that considering failure along the weak planes enabled accurate predictions of fracture development. Also, fracture development intensified at a softening rate beyond a threshold level and the geometry of fractures was significantly affected by the stiffness anisotropy. These results will help boost the reliability of the safety and performance assessment of geological disposal in claystone.

Underground water reservoirs (UWR) of coal mine plays a significant role in enhancing the ecological environment and safeguarding water resources. The water stored in UWR, known as brine solution, seriously damages the mechanical properties of the coal pillar. However, the mechanical degradation characteristics of coal under the action of different concentration solutions are still unclear. In this paper, we propose a novel coal corrosion device to simulate the real environment, designed to ensure the effective dissolution of the solution to the samples. Here we show the results of brine solution corrosion tests, considering ion composition changes, microscopic CT imaging, and mechanical properties. The solution ion composition experiences significant changes after the corrosion test. The variation in $M{n}^{2+}$ ion content can reflect the reaction degree and trend of water–rock interaction; CT test visually demonstrates the microstructural changes, allowing for direct observation of dissolution, generation, and attachment phenomena. The specimen’s overall porosity increases after corrosion; Triaxial compression tests were conducted, and both solution concentration and corrosion time can cause varying degrees of mechanical parameters degradation. Our results demonstrate how solution concentration affects coal and the extent of its impact. We anticipate our research will contribute to the construction and long-term safety of UWR.

Tension failure is a unique phenomenon in solid Earth that occurs on scales ranging from large plate rift valleys to small laboratory rocks. On a slope scale, deep unloading tension fractures are distinct from conventional unloading fractures and are a unique geological phenomenon in valleys with high in-situ stress. To accurately reproduce the development and evolution of deep unloading tension fractures and to support major excavation projects, a series of works including geological investigation, laboratory tests, intrinsic model establishment, and numerical simulation were carried out in this study. The unloading rheological tests, accounting for time-dependent effects, uncover the heterogeneity in the rheological attributes of rock-mass strength parameters during valley downcutting. This heterogeneity manifests as a transition in rock-mass strength parameters that cohesion weakening-friction strengthening (CWFS). Drawing on the results of laboratory tests, a novel viscoelastic-plastic model, termed the WSR model, was proposed. This model takes into account both CWFS and rheological considerations. It has been applied to simulate deep unloading tension fractures in the Jinping I hydropower station (JP-I) and compared with conventional models. The results show that WSR model accurately reproduced the development and evolution of deep unloading tension fractures and the heterogeneity of rock-mass deformation during age evolution leads to the formation of deep unloading tension fractures. In this study, the rock-mass heterogeneous rheological properties were summarized as the heterogeneity of the rock-mass strength parameters during age deterioration and the heterogeneity of rock-mass deformation during age evolution; the WSR model was proposed to characterize the heterogeneous rheological property of rock-mass strength parameters and to reproduce the development and evolution of deep unloading tension fractures in the JP-I. This novel contribution to deep unloading tension fractures emphasizes that the rock-mass heterogeneous rheological properties lead to the formation of deep unloading tension fractures.