In March 2022, construction was started at Yunlong Lake Laboratory of Deep Underground Science and Engineering, China, on an underground gas storage experimental facility with the capacity to achieve composite structure design and material development. Underground gas storage can provide a solution to address the intermittency of renewable energy supply. Currently, lined rock caverns (LRCs) are regarded as the best option for compressed air and hydrogen storage, since they have excellent sealing properties and minimum environmental impacts. However, the load transfer, damage, and failure mechanisms of LRCs are not clear. This prevents the design and selection of mechanical structures. Particularly, the gas sealing capacity in specific gas conditions (e.g., stored hydrogen-induced chemical reaction) remains poorly understood, and advanced materials to adapt the storage conditions of different gases should be developed. This experimental facility aims at providing a solution to these technical issues. This facility has several different types of LRCs, and study of the mechanical behavior of various structures and evaluation of the gas-tight performance of the sealing material can be carried out using a distributed fiberoptic sensing approach. The focus of this study is on the challenges in sealing material development and structure design. This facility facilitates large-scale and long-term energy storage for stable and continuous energy supply, and enables repurposing of underground space and acceleration of the realization of green energy ambitions in the context of Paris Agreement and China's carbon neutralization plan.
The 10 000-m ultradeep dolomite reservoir holds significant potential as a successor field for future oil and gas exploration in China's marine craton basin. However, major challenges such as the genesis of dolomite, the formation time of high-quality reservoirs, and the preservation mechanism of reservoirs have always limited exploration decision-making. This research systematically elaborates on the genesis and reservoir-forming mechanisms of Sinian–Cambrian dolomite, discussing the ancient marine environment where microorganisms and dolomite develop, which controls the formation of large-scale Precambrian–Cambrian dolomite. The periodic changes in Mg isotopes and sedimentary cycles show that the thick-layered dolomite is the result of different dolomitization processes superimposed on a spatiotemporal scale. Lattice defects and dolomite embryos can promote dolomitization. By simulating the dissolution of typical calcite and dolomite crystal faces in different solution systems and calculating their molecular weights, the essence of heterogeneous dissolution and pore formation on typical calcite and dolomite crystal faces was revealed, and the mechanism of dolomitization was also demonstrated. The properties of calcite and dolomite (104)/(110) grain boundaries and their dissolution mechanism in carbonate solution were revealed, showing the limiting factors of the dolomitization process and the preservation mechanism of deep buried dolomite reservoirs. The in situ laser U-Pb isotope dating technique has demonstrated the timing of dolomitization and pore formation in ancient carbonate rocks. This research also proposed that dolomitization occurred during the quasi-contemporaneous or shallow-burial periods within 50 Ma after deposition and pores formed during the quasi-contemporaneous to the early diagenetic periods. And it was clear that the quasi-contemporaneous dolomitization was the key period for reservoir formation. The systematic characterization of the spatial distribution of the deepest dolomite reservoirs in multiple sets of the Sinian and the Cambrian in the Chinese craton basins provides an important basis for the distribution prediction of large-scale dolomite reservoirs. It clarifies the targets for oil and gas exploration at depths over 10 000 m. The research on dolomite in this study will greatly promote China's ultradeep oil and gas exploration and lead the Chinese petroleum industry into a new era of 10 000-m deep oil exploration.
Rock is geometrically and mechanically multiscale in nature, and the traditional phenomenological laws at the macroscale cannot render a quantitative relationship between microscopic damage of rocks and overall rock structural degradation. This may lead to problems in the evaluation of rock structure stability and safe life. Multiscale numerical modeling is regarded as an effective way to gain insight into factors affecting rock properties from a cross-scale view. This study compiles the history of theoretical developments and numerical techniques related to rock multiscale issues according to different modeling architectures, that is, the homogenization theory, the hierarchical approach, and the concurrent approach. For these approaches, their benefits, drawbacks, and application scope are underlined. Despite the considerable attempts that have been made, some key issues still result in multiple challenges. Therefore, this study points out the perspectives of rock multiscale issues so as to provide a research direction for the future. The review results show that, in addition to numerical techniques, for example, high-performance computing, more attention should be paid to the development of an advanced constitutive model with consideration of fine geometrical descriptions of rock to facilitate solutions to multiscale problems in rock mechanics and rock engineering.
The Edikan Mine, which consists of Fobinso and Esuajah gold deposits, lies within the Asankrangwa Gold Belt of the Birimian Supergroup in the Kumasi Basin. The metasedimentary rocks in the Basins and the faulted metavolcanic rocks in the Belts that make up the Birimian Supergroup were intruded by granitoids during the Eburnean Orogeny. This research aims to classify granitoids in the Edikan Mine and ascertain the petrogenetic and geochemical characteristics of some auriferous granitoids in the wider Kumasi Basin, Ghana, to understand the implications for geodynamic settings. A multi-methods approach involving field studies, petrographic studies, and whole-rock geochemical analysis was used to achieve the goal of the study. Petrographic studies revealed a relatively high abundance of plagioclase and a low percentage of K-feldspars (anorthoclase and orthoclase) in the Fobinso samples, suggesting that the samples are granodioritic in nature, while the Esuajah samples showed relatively low plagioclase abundance and a high percentage in K-feldspars, indicating that they are granitic. The granitoids from the study areas are co-magmatic. The granitoids in Esuajah and Fobinso are generally enriched in large ion lithophile elements and light rare earth elements than high field strength elements, middle rare earth elements, and heavy rare earth elements, indicating mixing with crustal sources during the evolution of the granitoids. The granitoids were tectonically formed in a syn-collisional+VAG setting, which implies that they were formed in the subduction zone setting. Fobinso granodiorites showed S-type signatures with evidence of extensive crustal contamination, while the Esuajah granites showed I-type signatures with little or no crustal contamination and are peraluminous. Gold mineralization in the study area is structurally and lithologically controlled with shear zones, faulting, and veining as the principal structures controlling the mineralization. The late-stage vein, V3, in the Edikan Mine is characterized by a low vein angle and is mineralized.
The corrosion of waste canisters in the deep geological disposal facilities (GDFs) for high-level radioactive waste (HLRW) can generate gas, which escapes from the engineered barrier system through the interfaces between the bentonite buffer blocks and the host rock and those between the bentonite blocks. In this study, a series of water infiltration and gas breakthrough experiments were conducted on granite and on granite–bentonite specimens with smooth and grooved interfaces. On this basis, this study presents new insights and a quantitative assessment of the impact of the interface between clay and host rock on gas transport. As the results show, the water permeability values from water infiltration tests on granite and granite–bentonite samples (10−19–10−20 m2) are found to be slightly higher than that of bentonite. The gas permeability of the mock-up samples with smooth interfaces is one order of magnitude larger than that of the mock-up with grooved interfaces. The gas results of breakthrough pressures for the granite and the granite–bentonite mock-up samples are significantly lower than that of bentonite. The results highlight the potential existence of preferential gas migration channels between the rock and bentonite buffer that require further considerations in safety assessment.
The flow characteristics of coalbed methane (CBM) are influenced by the coal rock fracture network, which serves as the primary gas transport channel. This has a significant effect on the permeability performance of coal reservoirs. In any case, the traditional techniques of coal rock fracture observation are unable to precisely define the flow of CBM. In this study, coal samples were subjected to an in situ loading scanning test in order to create a pore network model (PNM) and determine the pore and fracture dynamic evolution law of the samples in the loading path. On this basis, the structural characteristic parameters of the samples were extracted from the PNM and the impact on the permeability performance of CBM was assessed. The findings demonstrate that the coal samples' internal porosity increases by 2.039% under uniaxial loading, the average throat pore radius increases by 205.5 to 36.1 μm, and the loading has an impact on the distribution and morphology of the pores in the coal rock. The PNM was loaded into the finite element program COMSOL for seepage modeling, and the M3 stage showed isolated pore connectivity to produce microscopic fissures, which could serve as seepage channels. In order to confirm the viability of the PNM and COMSOL docking technology, the streamline distribution law of pressure and velocity fields during the coal sample loading process was examined. The absolute permeability of the coal samples was also obtained in order for comparison with the measured results. The macroscopic CBM flow mechanism in complex low-permeability coal rocks can be revealed through three-dimensional reconstruction of the microscopic fracture structure and seepage simulation. This study lays the groundwork for the fine description and evaluation of coal reservoirs as well as the precise prediction of gas production in CBM wells.
Microcrack growth during progressive compressive failure in brittle rocks strongly influences the safety of deep underground engineering. The external shear stress τxy on brittle rocks greatly affects microcrack growth and progressive failure. However, the theoretical mechanism of the growth direction evolution of the newly generated wing crack during progressive failure has rarely been studied. A novel analytical method is proposed to evaluate the shear stress effect on the progressive compressive failure and microcrack growth direction in brittle rocks. This model consists of the wing crack growth model under the principal compressive stresses, the direction correlation of the general stress, the principal stress and the initial microcrack inclination, and the relationship between the wing crack length and strain. The shear stress effect on the relationship between y-direction stress and wing crack growth and the relationship between y-direction stress and y-direction strain are analyzed. The shear stress effect on the wing crack growth direction during the progressive compressive failure is determined. The initial crack angle effect on the y-direction peak stress and the wing crack growth direction during the progressive compressive failure considering shear stress is also discussed. A crucial conclusion is that the direction of wing crack growth has a U-shaped variation with the growth of the wing crack. The rationality of the analytical results is verified by an experiment and from numerical results. The study results provide theoretical support for the evaluation of the safety and stability of surrounding rocks in deep underground engineering.
This research presents the square root sum of squares response of displacements and tunnel moments under the Kobe and Loma Prieta seismic excitations with a peak ground acceleration of 0.05 g for various dry relative densities of local sand in Bangladesh. For this reason, a one-dimensional gravitational shake table test was performed after calibration to determine the seismic performance of the concrete tunnel–sand–pile interaction model. A vertical 40 kg load was applied on each pile cap along with the seismic excitations. The experimental results obtained were compared with the previous numerical study conducted by using field data so as to better interpret the variations of results. In the case of vertical sand displacement, the ratio between the previous field data obtained through numerical study and the present study is found to be 0.96. Moreover, the experimental results were compared with the 3D full-scale numerical analysis results of Plaxis considering the Mohr–Coulomb constitutive model of sand. Variations of experimental and numerical results show a satisfactory level of alignment with the previously published work. According to the shake table test results, the lateral displacement of the tunnel is greater than the vertical displacement because of the transverse directional seismic excitation on the tunnel body. The minimum difference between lateral and vertical displacements of the tunnel is found to be 31% for a relative density of 27% under the Loma Prieta earthquake. However, this research may be advanced in the future by considering various peak ground accelerations, tunnel–pile clearance, and geometric properties.
Attributed to its superior water-to-solid ratio and quick setting time, the high-water material is widely adopted in underground spaces as a cost-effective and environmentally friendly backfill material. To elucidate the bleeding mechanism of high-water material under the high confining pressure, a total of 57 tubular specimens were prepared and tested, critical parameters of which included the water-to-solid ratio, curing time, and lateral confinement pressure. Test results showed that no obvious cracks were observed from the surface of confined high-water material, which is different from that of unconfined high-water material, which featured shear cracks. Moreover, the volume of these confined high-water materials under compaction exhibited a continuous shrinkage associated with the water bleeding. The threshold values of the water bleeding are significantly affected by the water-to-solid ratio, followed by the confining pressure and curing time. When other parameters are constant, the higher confinement is requested for these specimens with a small water-to-solid ratio. Meanwhile, the mass of bleeding water increased with the lateral confinement, showing a quick increase at the initial stage. During the bleeding process, the free water stored in the pores was compacted, the evidence of which is the transformation of the hydration products (calcium aluminate hydrate) from its natural fibrous structure into the rod-shaped or dense agglomerate structures. These research outcomes provide an in-depth insight into the fundamental mechanics of the high-water material under the high lateral confinement when it is used for underground spaces.
Squeezing phenomena can lead to severe loads in deep tunnels, especially in the presence of a low ratio of surrounding rock strength to overburden pressure. For this reason, it is highly imperative to analyze and identify a suitable methodology to estimate the squeezing potential and select a proper support system of rock mass. This study aims to reveal the causes of failure of Tishreen tunnel in the west of Syria and develop remediation measures accordingly so as to bring the tunnel back into service. The tunnel in question was subjected to successive failures such as buckling and spalling of side walls, floor heave, and extremely large convergence reaching the failure state of the tunnel lining. In this study, an effective way was demonstrated to evaluate the squeezing potential of the tunnel lining and appropriate modeling of the long-term response of a tunnel excavated in weak rock. Specifically, the causes of failure of Tishreen tunnel were first evaluated by empirical approaches. Then, a numerical model was developed using a time-dependent constitutive model to investigate the time-dependent response of the tunnel lining. On this basis, this study proposed an effective reinforcement schemes including steel ribs, grout injection, ground anchors, and new lining of reinforced concrete. The results show that the Burger viscoplastic model simulates effectively the resulting deformation and creep behavior of squeezing ground. It is also observed that using a combined heavy support system can provide efficient control over squeezing deformation and maintain the serviceability of the tunnel under study.