Rock Mechanics Bulletin最新文献

In order to investigate the spatial distribution of early warning threshold for landslide induced by rainfall in China, the literatures about rainfall thresholds of landslides in China in recent 20 years are selected. Statistical analysis and visualization methods were employed to systematically analyze the research progress of rainfall early warning thresholds at various scales. Taking the typical rainfall intensity-duration (I-D) threshold model as the research object, combined with the geographical characteristics of China and the average annual rainfall of 20 years, the spatial distribution of early warning thresholds for rainfall-induced landslide in China is depicted. The results show that the inspired rain intensity coefficient α of the rainfall threshold (I-D curve) in China roughly increases gradually with the decrease of topography. Moreover, under consistent annual rainfall conditions, the scalar index β exhibits regular changes corresponding to variations in terrain. Topography and rainfall are the two main factors strongly associated with the rainfall threshold. This research establishes a clear framework for studying the early warning thresholds for rainfall-induced landslides in China and holds significant scientific implications for developing more effective rainfall threshold models.

Hyper-gravity experiment enable the acceleration of the long-term transport of contaminants through fractured geological barriers. However, the hyper-gravity effect of the solute transport in fractures are not well understood. In this study, the sealed control apparatus and the 3D printed fracture models were used to carry out 1 ​g and N g hyper-gravity experiments. The results show that the breakthrough curves for the 1 ​g and N g experiments were almost the same. The differences in the flow velocity and the fitted hydrodynamic dispersion coefficient were 0.97–3.12% and 9.09–20.4%, indicating that the internal fractures of the 3D printed fracture models remained stable under hyper-gravity, and the differences in the flow and solute transport characteristics were acceptable. A method for evaluating the long-term barrier performance of low-permeability fractured rocks was proposed based on the hyper-gravity experiment. The solute transport processes in the 1 ​g prototype, 1 ​g scaled model, and N g scaled model were simulated by the OpenGeoSys (OGS) software. The results show that the N g scaled model can reproduce the flow and solute transport processes in the 1 ​g prototype without considering the micro-scale heterogeneity if the Reynolds number (Re) ​≤ ​critical Reynolds number (Re_cr) and the Peclet number (Pe) ​≤ ​the critical Peclet number (Pe_cr). This insight is valuable for carrying out hyper-gravity experiments to evaluate the long-term barrier performance of low-permeability fractured porous rock.

Subsurface excavation results in the formation of a zone called excavation damaged zone (EDZ) around the tunnel wall. An EDZ is a major concern in the field of high-level radioactive waste disposal because it may act as a flow path after the closure of a repository. In this study, first-arrival traveltime tomography was repeatedly conducted on the EDZ at a depth of 350 ​m in the Horonobe Underground Research Laboratory. However, the acquired data was highly affected by the support structure on the drift wall. For proper visualization of the EDZ, information about the structure was incorporated into the inversion by modifying the model constraint. The synthetic study showed that the approach reproduced the EDZ in the model without the artifacts. The method was applied to field data, and the EDZ around the drift was detected. The inversion was extended to a time-lapse inversion to trace the changes in P-wave velocity in the EDZ. The synthetic study demonstrated that temporal changes in the P-wave velocity distribution could be detected. Data obtained from 12 surveys under open-drift conditions were analyzed by time-lapse inversion. The results indicated that the EDZ did not undergo sealing or evolution at the site for approximately seven years.

Fluid injection into rock masses is involved during various subsurface engineering applications. However, elevated fluid pressure, induced by injection, can trigger shear slip(s) of pre-existing natural fractures, resulting in changes of the rock mass permeability and thus injectivity. However, the mechanism of slip-induced permeability variation, particularly when subjected to multiple slips, is still not fully understood. In this study, we performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces. Our experiments show that compared to saw-cut fractures, natural fractures show much small effective stress when the slips induced by triggering fluid pressures, likely due to the much rougher surface of the natural fractures. For natural fractures, we observed that a critical shear displacement value in the relationship between permeability and accumulative shear displacement: the permeability of natural fractures initially increases, followed by a permeability decrease after the accumulative shear displacement reaches a critical shear displacement value. For the saw-cut fractures, there is no consistent change in the measured permeability versus the accumulative shear displacement, but the first slip event often induces the largest shear displacement and associated permeability changes. The produced gouge material suggests that rock surface damage occurs during multiple slips, although, unfortunately, our experiments did not allow quantitatively continuous monitoring of fracture surface property changes. Thus, we attribute the slip-induced permeability evolution to the interplay between permeability reductions, due to damages of fracture asperities, and permeability enhancements, caused by shear dilation, depending on the scale of the shear displacement.

Estimating formation permeability is crucial for production estimation, hydraulic fracturing design optimization and rate transient analysis. Laboratory experiments can be used to measure the permeability of rock samples, but the results may not be representative at a field scale because of reservoir heterogeneity and pre-existing natural fracture systems. Diagnostic Fracture Injection Tests (DFIT) have now become standard practice to estimate formation pore pressure and formation permeability. However, in low permeability reservoirs, after-closure radial flow is often absent, which can cast significant uncertainties in interpreting DFIT data. Without knowing the fracture dimension prior, open fracture stiffness/compliance can't be determined, which is required for formation permeability estimation. Previous work has to assume a fracture radius or fracture height in order to estimate formation permeability, thus dent the confidence in the interpretation results. In the study, we present a new approach to determine fracture dimension, leak-off coefficient and formation permeability uniquely based on material balance and basic fracture mechanics, using data from shut-in to after-closure linear flow. Field examples are also presented to demonstrate the simplicity and effectiveness of this new approach.

The modeling of heat recovery from an enhanced geothermal system (EGS) requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity. These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery. In the present study, we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs. Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model. The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid, and thus improves thermal performance. The decrease of specific heat capacity due to temperature reduction, on the contrary, impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity. Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease, the overall effect of temperature-dependent thermal parameters is relatively small. Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.

Underground energy storage is a promising option for the global ambition of moving towards carbon neutrality. To achieve safe and reliable energy storage in underground caverns, it is essential to understand the contributions of thermal and mechanical loads to the deformation of containment materials (e.g., concrete and geomaterials) and to forecast potential risks related to unexpected failure of these materials. A temperature gradient test system is developed to investigate the thermo-mechanical responses of containment materials under simulated temperature gradient and earth pressure conditions. The test system has advantages of establishing a temperature gradient of over 400 ​°C/m across a large-scale specimen and examining the resulting strain based on the digital image correlation analysis. This study sheds light on 3 typical applications of the test system to examine the thermal and mechanical responses of intact limestone, flawed limestone, and fractured concrete. The results demonstrate that the mechanical load mainly controls the strain evolution of the intact limestone, while the thermal load strongly affects the strain evolution around the circular hole. The failure pattern of concrete primarily influences the mechanically induced strain, and the thermally induced strain is insensitive to the concrete failure. This test system can be modified and upgraded to study various research topics related to underground energy storage.

A large number of mines are closed or abandoned every year in China. Geothermal utilization is one of the important ways to efficiently reuse underground resources in abandoned mines. How to calculate the volume and distribution of underground water storage space is the key to accurately evaluate the sustainable geothermal production in abandoned mines. In this paper, according to the multi-scale characteristics of the underground space in abandoned mine, the flow and heat transfer equations in the multi-scale space are sorted out systematically, and the calculation methods of different secondary space volumes are derived in detail. Taking Jiahe abandoned mine as the background, the volume and distribution of underground secondary space are calculated, and three heat storage evaluation models considering different water storage spaces are established by using COMSOL. The simulation results show that there are great differences among different models, and the results of the equivalent porous media model considering the multi-scale space are most consistent with the reality. Sensitivity analyses of key parameters model results indicated that the heat production is closely related to not only the recharge flow rate but also the recharge temperature and operating time. Furthermore, the energy saving and emission reduction benefits of geothermal utilization in abandoned mines are calculated, the results show that geothermal utilization of abandoned mines can effectively reduce energy consumption and CO₂ emissions, and it has great economic benefits.

The fault is potentially vulnerability's geological structure in the working face and its vicinity, and it is also a crucial geological factor affecting coal mine safety exploitation. To investigate the unstable failure of surrounding rock induced by fault activation under the influence of adoption, which was studied utilizing field case and numerical analysis for the deformation and failure process of surrounding rock near the fault-affected zone. Combined with field cases, this paper analyzes disturbance stress and roof abscission layer monitoring in effecting zones of fault activation. Using the discrete element 3DEC numerical analysis method, the model of surrounding rock unstable fracture induced by fault activation under adoption is established. The unstable fracture and stress variation characteristics of surrounding rock induced by fault activation during the excavation of the upper side wall and lower side wall of the faults are simulated and analyzed. Field analysis shows that as the coal working face continues to advance, the mining stress gradually increases. There is a zigzag wave on the relationship curve between coal mining and roof displacement near the fault, which reveals that the surrounding rock of the fault activation affected zone is in the superposition state of static load and dynamic load. Furthermore, the simulation results show that the stress and displacement of surrounding rock near the fault increase with the advance of coal mining face. The closer to the fault plane, the displacement gradually returns to zero, and the stress is also in a lower state.

Geological formations containing laumontite-rich rock are usually treated as problematic for geo-energy production projects because the presence of laumontite mineral can promote complex mechanical behaviors. Previous laboratory results indicate that rock formations with a higher laumontite content display severe stress sensitivity in poromechanical responses. With an increase in confining pressure, there is a transition from dilation to compression regime and the resulting localization styles range from shear dilation to compaction bands. In this study, we conduct finite element modeling of constitutive behaviors of rocks retrieved from the tight glutenite reservoir formation using a thermodynamic-consistent plasticity model. The shear dilation to compaction transition is well characterized. Poromechanical analysis is also conducted to analyze the plastic zone development around a borehole drilled in an over pressured reservoir. The simulated stress-paths of key points around the borehole are used to demonstrate the plastic strain development processes. The impact of in-situ stress on the wellbore stability is highlighted, and a comparison with the results from using the traditional plastic constitutive model is conducted.