Geomechanics and Geophysics for Geo-Energy and Geo-Resources最新文献

Rockburst is a common geological hazard in deep underground engineering, and it often occurs in strata consisting of brittle rocks. In this study, the moisture content effect on the rockburst intensity of sandstones is systematically studied. A series of triaxial unloading compression tests along with the acoustic emission monitoring are performed for sandstone specimens with different moisture content levels. The mechanical properties, failure characteristics, and dilatancy behaviors of sandstone specimens are then properly compared. Comparative results reveal that the triaxial compressive strength and total strain energy of the saturated specimen decrease by about 30% and 35%, respectively, as compared to those of the dry specimen. Moreover, the magnitude of elastic strain energy tends to decrease with the increasing water content. The effect of moisture content on the rockburst intensity of sandstones is, therefore, significant. Besides, it is also found that the onset of dilatancy is generally unaffected by the water content, whereas the extent of dilatancy significantly decreases with the increasing water content. Numerical simulations for a tunnel excavation model confirm that injecting water into the surrounding rock is an effective way of reducing the rockburst intensity during tunnel excavations. These results have a guiding significance for the prevention and control of rockbursts in underground engineering.

Geothermal energy is a sustainable energy source that meets the needs of the climate crisis and global warming caused by fossil fuel burning. Geothermal resources are found in complex geological settings, with faults and interconnected networks of fractures acting as pathways for fluid circulation. Identifying faults and fractures is an essential component of exploiting geothermal resources. However, accurately predicting fractures without high-resolution geophysical logs (e.g., image logs) and well-core samples is challenging. Soft computing techniques, such as machine learning, make it possible to map fracture networks at a finer resolution. This study employed four supervised machine learning techniques (multilayer perceptron (MLP), random forests (RF), extreme gradient boosting (XGBoost), and support vector regression (SVR)) to identify fractures in geothermal carbonate reservoirs in the sub-basins of East China. The models were trained and tested on a diverse well-logging dataset collected at the field scale. A comparison of the predicted results revealed that XGBoost with optimized hyperparameters and data division achieved the best performance than RF, MLP, and SVR with RMSE = 0.02 and R² = 0.92. The Q-learning algorithm outperformed grid search, Bayesian, and ant colony optimizations. The blind well test demonstrates that it is possible to accurately identify fractures by applying machine learning algorithms to standard well logs. In addition, the comparative analysis indicates that XGBoost was able to handle the complex relationship between input parameters (e.g., DTP > RD > DEN > GR > CAL > RS > U > CNL) and fracture in geologically complex geothermal carbonate reservoirs. Furthermore, comparing the XGBoost model with previous studies proved superior in training and testing. This study suggests that XGBoost with Q-learning-based optimized hyperparameters and data division is a suitable algorithm for identifying fractures using well-log data to explore complex geothermal systems in carbonate rocks.

Graphical abstract

A novel highly stable aqueous foam was synthesized using CO₂, sodium silicate (SS) and anionic surfactant of sodium dodecylbenzene sulfonate. The influence of CO₂ foam on the mechanical properties and its underlying mechanisms of foamed backfill material was investigated. The experimental results revealed that the addition of CO₂ and SS effectively reduced the drainage of the foam while strengthening the liquid film of the Plateau borders, which stabilizes the foam. The excellent stability is attributable to the gel network developed after SS exposed to CO₂, that adhere to the foam surface. Furthermore, due to the interaction between encapsulated CO₂ and hydration products, micro CaCO₃ formed and filled the pore wall; thus, precast foam forms robust pore structures in the hardened foamed backfill.

The presence of random joints, cracks, and other defects significantly affects the meso-damage mechanism and macro-mechanical behavior of the rock. This study employed micro-CT scanning, digital image processing (DIP), and the rock failure process analysis system (RFPA3D) to reconstruct a genuine mesostructure, creating a three-dimensional (3D) numerical model of jointed sandstone. Under uniaxial stress, this model facilitated the meso-damage evolution process of prefabricated cracks in sandstone with varying dip angles. Additionally, the influence of jointed sandstone heterogeneity and prefabricated cracks with various dip angles on its failure mode and meso-damage mechanical properties were investigated. Utilizing the MATLAB platform, a 3D box dimension algorithm was developed to analyze the fractal characteristics of the mesodamage evolution in the sample. This algorithm enabled the quantitative characterization of the meso-damage evolution of sandstone. This study categorized three types of sandstone final failure modes: composite shear failure, shear failure along the joint surface, and tensile failure. Additionally, linear variations in the elastic modulus and compressive strength of the jointed sandstone were observed with increasing prefabricated fracture inclination, highlighting significant anisotropy. The presence of joints was found to induce and control the failure mode of sandstone. The meso-damage evolution process of sandstone was described in terms of the fractal dimension, indicating that more severe damage corresponded to a larger fractal dimension. This approach offers a novel statistical method for studying the progression of rock damage.

Mine slope stability and mining sustainability are related to the local geological structures, which could change the rock mass structure in deep mining. After 20 years mining in a mudstone mine, western China, the slope structure transforms from anti-dipping structure into a bedding structure by a recently discovered fault (F₁), further inducing the two landslides (Landslide #I and Landslide #II). Landslide investigation suggested the residual deposits in Landslide #I first slid over 100 m and overburdened the rear of Landslide #II. The bedding rock with weak interlayers at footwall is separated from the anti-dipping rock at the hanging wall by F₁. After excavation, a weak interlayer was exposed and softened by rainfall, resulting in the slip of footwall rock mass and further inducing large scale toppling deformation. The fragmented rock mass sliding along a weak interlayer triggers consequent deformation of adjacent slope, reducing safety reserve of the open mine. The discrete element analysis reveals that the bedding rock mass of footwall slid once the weak interlayer was exposed by mining. And retrogressive deformation transmitted to the hanging wall and induced bending and toppling deformation of anti-dipping rock mass. Mine feasibility assessment should recognize the potential deep geological structures as important in the future.

Many experiments have been performed to study the heating properties of concrete under microwave irradiation. Microwave provides the non-uniform heating process, which cannot be reflected clearly through the experimental investigations. In this paper, a theoretical method is presented to investigate the electromagnetic-thermal coupling process of double-layer cylindrical concrete under microwave heating. The wave transmission and reflection were considered. An analytic solution is presented to predict transient heating process within a 3-dimensional double-layer concrete model induced by microwave heating. The inner aggregate is a microwave high loss material and the outer mortar was microwave low loss medium. Poynting theorem was employed to calculate the electric field distribution and microwave energy loss within concrete. Transient heat transfer process with an internal microwave heat source was investigated based on the classical heat transfer theory by employing integral transform technique. The results indicate that microwave heating effect depend on the concrete size, dielectric properties as well as microwave energy input. The temperature gradient was formed at the mortar-aggregate interface, which varied with the microwave heating parameters inputs. The analytical study will provide significant insight to promote the understanding of electric and temperature field in the two-layer composite concrete materials under microwave heating.

We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses (lambda ={sigma }_{3}/{sigma }_{1}) and relative inclination of the bedding. When the lateral pressure coefficient ((lambda)) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for (lambda ge 0.67), shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress (({sigma }_{3})) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations ((beta =60^circ)), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

The weakening of circular tunnels is a global problem that has not been resolved satisfactorily. In the tunnelling process, surrounding rock of circular-tunnel performs a process of “excavating → weakening → continuous excavating → weakening strengthens”. Different rates of excavation affect the stress adjustment of the surrounding rock, and also have an impact on the weakening of a circular-tunnel. An instability failure test was conducted on a circular-tunnel with varying vertical loading rates. The loading rate was utilized as a representative measure for the excavation rate on the site. The results showed that the weakening process of a circular-tunnel can be divided into four distinct phases, hydrostatic pressure (E1), particle ejection (E2), flake stripping (E3), and instability (E4). The ordering of these phases is E3 > E4 > E1 > E2. In the weakening process of a circular-tunnel, the root cause is the original stress level, while the essential factor is the engineering disturbance. A faster vertical loading rate leads to greater stress adjustment, higher strain energy accumulation, and an increased probability of circular-tunnel instability. The presence of a quiet period of AE events in the middle and later phases of flake stripping is a precursory characteristic of circular-tunnel instability. This study has both theoretical and practical significance in terms of revealing the mechanism of circular-tunnel instability and achieving a reasonable arrangement of the circular-tunnel support process.

In rock engineering, high-voltage pulse technology has attracted attention because it offers environmental protection, controllable energy, and repeatable discharge. It is necessary to study the fracture behavior of rock under high-voltage pulse discharge (HVPD) for the parametric design of rock breaking thereby. HVPD experiments were conducted in red sandstone samples with the plasma channel spacing ranging from 26 to 66 mm at intervals of 10 mm. The stress wave generated by HVPD was obtained from the current waveform measured by Rogowski coils. In combination with numerical simulations, the distribution characteristics, propagation process, and formation mechanism of fractures were analyzed. The results showed that after two applications of HVPD at different positions, the sample was both broken down and two plasma channels and radial fractures centered around them were formed within. The stress wave decays exponentially with the increase of the distance from the plasma channel. When the spacing between plasma channels is less than or equal to 46 mm, fracture coalescence occurs between the two plasma channels; thereafter, the fractures formed by the second HVPD face resistance to propagation towards the fracture area formed by the first HVPD. In addition, numerical simulation results indicate that the second HVPD will generate significant tensile stress in the middle region of the two plasma channels, leading to near-horizontal fracture coalescence. When the spacing between plasma channels increases to 56 mm and 66 mm, the tensile stress induced by the second HVPD in the middle region of the sample is small, and it is difficult to form fracture coalescence between the two channels.

Large-scale karst caves are the principal storage spaces for hydrocarbon resources in fracture–cavity carbonate reservoirs. Drilling directly into these caves is considered the ideal mode of development, but many wells do not effectively penetrate karst caves. Therefore, acid fracturing is employed to generate artificial fractures that can connect with these caves. However, there are no appropriate well test methods for fracturing wells in fracture–cavity reservoirs. This study establishes a novel pressure transient analysis model for such wells. A new mathematical model is proposed that couples linear flow in acid fracturing cracks with radial flow in the oil drainage area. The Laplace transform and Stehfest numerical inversion provided analytical solutions for the bottomhole pressure. Typical log–log well testing curves were plotted to analyze oil flow, which occurs in ten stages. During the flow stage in fracturing cracks, the pressure and pressure derivative curves are parallel lines with a slope of 0.5. In the stage of karst cave storage, the pressure derivative curve is a straight line with a slope of 1. A comparison with previous models confirmed the validity of the proposed model. The influence of key parameters on the behavior of typical curves is analyzed. A field case study of the proposed model was carried out. Parameters related to fracturing cracks and karst caves, such as the crack length and cave radius, were successfully estimated. The proposed model has great potential for determining formation parameters of fracture–cavity reservoirs.