Rock Mechanics Bulletin最新文献

Understanding the effects of mineral composition on geomechanical characteristics is critical in order to design and optimize the hydraulic fracturing necessary for shale gas reservoir production. Fundamental information is still missing in effects of mineral content and the experimental methodologies used. This paper provided an in-depth assessment of the various experimental methodologies and their applications in the relationship between the mineralogical and geomechanical features of the shale formation. The results revealed that more brittle minerals increase their strength, but chemical reaction that creats pores decrease their strength. High content of carbonate or quartz increases a rock's brittleness, while a high content of clay increases a rock's plasticity and decreases its brittleness. As phyllosilicate content increases, the uniaxial compressive strength decreases, and this could be because phyllosilicate minerals have a weakening effect on the mineral bond. Young's modulus often climb as clay minerals decline and as silica with carbonate concentration rises, however Poisson's ratio increases in relation to an increase in clay minerals, which also increases the ductility of the reservoir shale rock. However, compared to minerals and matrix, does not significantly impact the strength of shale rock. Besides, the benefits and drawbacks of using uniaxial and triaxial compression, ultrasonic testing, and nano-indentation techniques in unconventional reservoirs were described. The findings suggest that, because of the possibility for experimental testing repeatability for increased accuracy, ultrasonic testing is the most appropriate experimental approach in the scenes of assessing static and dynamic geomechanical properties of reservoir shale rock. We suggested that numerically-based simulation of experimental techniques used for shale geomechanical evaluations and numerical modeling of heterogeneous shale rock samples will be necessary in light of the limitations faced in the applications of experimental techniques for shale geomechanical evaluation.

This paper presents an investigation of brittle rock failure by the quaternion-based bonded-particle model in discrete element method (DEM). Unlike traditional approaches that utilize Euler angles or rotation matrices, this model employs unit quaternions to represent the spatial rotations of particles. This method simplifies the representation of 3D rotations, providing a more intuitive framework for modelling complex interactions in granular materials. The numerical model was validated by the uniaxial compression tests on rock, with good agreement with well-documented experimental data in terms of the rock uniaxial compression strength (UCS) and failure mode. During loading, the rock sample demonstrated a linear-elastic response at an axial strain of smaller than 0.45%. However, as internal bond breakage accumulated, this linear relationship weakened, and the stress-strain curve began to deviate from its initial linear trajectory. The bond breakage and the overall deformation of the rock were primarily controlled by the shear bonding force. The UCS was achieved at an axial strain of 0.625%, at which point the internal shear bonding force chains were predominantly aligned vertically. The brittle failure occurred when the internal damage of solids nucleated to form an interconnected failure plane, accompanied by a sharp rise in the internal damage ratio. The area of failure plane increased with the loading strain rate, gradually transforming the failure pattern from the local damage to a complete fragmentation.

The efficient exploitation of geothermal energy through enhanced geothermal systems (EGS) has been a relevant topic for hot dry rock (HDR) geothermal resources. When cryogenic fluid is injected into a thermal reservoir, improving heat exchange efficiency is key to achieving the optimal exploitation of HDR. In this paper, granite outcrops from Gonghe Basin were used as the testing sample. The natural fractures in the granite samples were relatively well developed. To simulate long-term injection and production from multi-wells in situ, physical experiments were performed in a newly-developed, in-house large-scale true triaxial experimental system. Geothermal extraction performance of an HDR was simulated for long-term injection and production operations. Simultaneously, the mode of one-injection and multiple-production wells was represented. In the paper, the effects of the production-injection well spacing, the number of production wells and the injection rate on the production temperature and flow rate are discussed. The results show that, during long-term injection and production, there are two stages of production temperature variation, namely stabilization and attenuation. When the number of the production wells is increased, the heat extraction efficiency is accelerated. Moreover, competitive diversion of fluid among fractures occurred due to different conductivities. Furthermore, under different production modes, the production flow rate contributed differently to the heat extraction. Finally, the effect of the production-injection wells spacing on the heat exchange performance was analyzed; this is mainly reflected in the change of the effective heat exchange area between the rock and the injected fluid. The results emphasize the importance of designing an appropriate production mode and optimizing the injection-production parameters to ensure efficient HDR exploitation.

The karst cave serves as the primary storage space in carbonate reservoirs. Simultaneously connecting multiple karst caves through hydraulic fracturing is key to the efficient development of carbonate reservoirs. However, there is lack of systematic research on the mechanisms and influencing factors of fracture propagation in carbonate rocks. This paper established models including karst cave models, single natural fracture-cave models, and multiple natural fracture-cave models based on the discrete lattice method. It thoroughly studied how geological and operational factors affect the fracture propagation and the connectivity of karst caves. The final step involved establishing a prototype well model and optimizing operation parameters. The research indicates that an increase in the Young's modulus and pore pressure of karst cave could facilitate hydraulic fracture connecting with caves. When the pore pressure is lower than that in the matrix, it will generate a repulsive effect on hydraulic fractures. The natural fracture along the hydraulic fracture path significantly facilitates the connection with caves. When the wellbore azimuth is less than 60°, the fracture's diversion radius is small, and hydraulic fractures primarily connect with karst cave through natural fractures. When the wellbore azimuth exceeds 60°, the fracture's diversion radius increases. Under the combined action of hydraulic fractures and natural fractures, the stimulated volume of the karst cave noticeably increases. Under the same liquid volume, increasing the injection rate could enhance the cave stimulated volume. Combining the findings from numerical simulation studies resulted in the development of a diagram that depicts the connectivity of karst caves, providing valuable insight for hydraulic fracturing operations in carbonate reservoirs.

This study investigates the impact of intermediate (σ₂) and minimum (σ₃) principal stress unloading on damage behavior and the confining pressure influence on crack initiation stress (σ_ci) in true triaxial stress conditions, utilizing large-scale cubic samples. Two distinct true triaxial tests were executed, examining the effects of confining stress (σ₂ and σ₃) unloading on porous sandstone damage and the correlation between confining stress and σ_ci. Acoustic emission (AE) parameters, signal characteristics, and wave velocity variations were utilized to elucidate cracking mechanisms and damage development in the samples. Unloading tests reveal consistent velocities in three orthogonal directions (V₁₁, V₂₂, and V₃₃) during the initial two unloading stages. In the subsequent three stages, confining stress unloading leads to a decrease in wave velocity in the corresponding direction, while velocities in the other two directions remain nearly constant. Notably, σ₂ unloading generates higher amplitude AE signals compared to σ₃ unloading, with over 70% of the micro-cracks categorized as tensile. In the incremental loading tests, σ_ci is found to be contingent on confining pressure, with σ₂ playing a crucial role. During σ₁ loading, V₃₃ decreases, indicating additional crack formation; conversely, σ₃ loading results in V₃₃ increase, signifying the continuous closure of existing cracks. Limitations of the experiments are summarized and prospects in this domain are outlined.

Broken coal and rock (BCR) are an important component medium of the caving zone in the goaf (or gob), as well as the main filling material of fault fracture zone and collapse column. The compaction seepage characteristics of BCR directly affect the safe and efficient mining of coal mines. Thus, numerous laboratory studies have focused on the compaction seepage characteristics of BCR. This paper first outlines the engineering problems involved in the BCR during coal mining including the air leakage, the spontaneous combustion, the gas drainage, and the underground reservoirs in the goaf. Water inrush related to tectonics such as faults and collapse columns and surface subsidence related to coal gangue filling and mining also involve the compaction seepage characteristics of BCR. Based on the field problems of BCR, many attempts have been made to mimic field environments in laboratory tests. The experimental equipment (cavity size and shape, acoustic emission, CT, etc.) and experimental design for the BCR were firstly reviewed. The main objects of laboratory analysis can be divided into compression tests and seepage test. During the compaction test, the main research focuses on the bearing deformation characteristics (stress-strain curve), pore evolution characteristics, and re-crushing characteristics of BCR. The seepage test mainly uses gas or water as the main medium to study the evolution characteristics of permeability under different compaction stress conditions. In the laboratory tests, factors such as the type of coal and rock mass, particle size, particle shape, water pressure, temperature, and stress path are usually considered. The lateral compression test of BCR can be divided into three stages, including the self-adjustment stage, the broken stage, and the elastic stage or stable stage. At each stage, stress, deformation, porosity, energy, particle size and breakage rate all have their own characteristics. Seepage test regarding the water permeability experiment of BCR is actually belong to variable mass seepage. While the experimental test still focuses on the influence of stress on the pore structure of BCR in terms of gas permeability. Finally, future laboratory tests focus on the BCR related coal mining including scaling up, long term loading and water immersion, mining stress path matching were discussed.

The key to achieving rockburst warning lies in the understanding of rockburst precursors. Considering the correlation characteristics of rockburst acoustic emission (AE) parameters, a self-organizing map neural network (SOMNN) based method for rockburst precursor inversion was proposed. The feature of this method lies in a cyclic data segmentation iteration process based on the thinking of “interference signal screening”, “key signal extraction”, and “precursor signal inversion”. The rationality of this method has been verified in three groups of rockburst experiments. The results revealed that rockburst AE precursor signals consist of a series of signals characterized by long duration, high energy, low average frequency, high energy amplitude, and low peak frequency. Subsequently, potential value in long term rockburst warning of the precursor obtained in this study was shown via the comparison of conventional precursors. Finally, a preliminary interpretation for rockburst precursor was proposed under the framework of AE parameters physical significance, and it is revealed that AE precursor signals are likely linked to the creation of large-scale tensile cracks before rockburst.

Rocks can deform at varying scales (nano-, micro- and macro-scale) under different temperatures, pressures, stresses, and time conditions. Sub-core scale (nano-to micro-scale) changes in rock properties can influence local (fine-scale) and bulk scale (macro-scale) rock deformation. However, there is a lack of comprehensive knowledge on how rock deformation at sub-core scale (i.e., nano-to micro-scale) is assessed and its potential to accurately predicte and estimate the macro-scale mechanical behavior of rocks. This study presents a comprehensive and forward-leaning review of the assessment of nano-scale and micro-scale rock mechanical parameters, their time-dependent behavior, and potential applications in rock engineering. Also, we highlighted the key findings based on experimental and numerical methods for evaluating rock mechanical parameters, and presented the limitations of these approaches. Further, we discussed the reliability of sub-core scale mechanical assessments in predicting macromechanical (larger-scale) properties and the behavior of rocks in geo-engineering. Finally, we offer recommendations to advance investigations focused on rock mechanical assessments at these smaller scales and provide a more accurate characterization at the sub-core scale.

Horizontal boreholes have been widely used to extract natural gas from coal seams. However, these boreholes can encounter severe instability issues leading to production interruption. Optimizing drilling azimuth is a potential solution for enhancing borehole stability while considering gas production. In this work, we improved and implemented a dual-porosity, fully coupled geomechanical-hydraulic numerical model into COMSOL Multiphysics to investigate into this factor. The sophisticated numerical model incorporates various critical factors, including desorption-induced matrix shrinkage, stress-dependent anisotropic fracture permeability, and the interactions of gas flow and reservoir deformation in matrices and fractures.

A suite of simulation scenarios (e.g., varying coal strength) was carried out to quantify the impact of drilling azimuth on coal permeability evolution, cumulative gas production, and the borehole break-out width for Goonyella Middle Seam of Bowen Basin, Australia. The model was calibrated against both theoretical permeability values and field gas production data. Due to the lack of directly measured matrix permeability data, the actual gas production was used to back calculate the best-matched matrix permeability, which is 0.65 μD for this particular work. Moreover, based on the breakout shape and induced volumetric strains around the borehole, drilling along the maximum horizontal stress does not necessarily lead to the best stability of the borehole, as generally believed. A drilling azimuth between 0° and 60° results in similar breakout width, whereas a drilling azimuth between 60° and 90° achieves the most efficient gas production. By considering both gas production efficiency and borehole stability, for this particular reservoir condition, the optimum drilling azimuth is determined to be between 45° and 60°.

This study presents a practical approach for determining the optimum drilling azimuth in coal seam gas extraction through in seam boreholes.

The unloading effect by excavation may cause irreversible and severe damage to the surrounding rock masses in underground engineering. In this paper, both conventional triaxial compression (CTC) tests and triaxial unloading confining pressure (TUCP) tests were conducted on fine-grained granite to study its triaxial compression failure processes due to unloading. Based on the crack volumetric strain (CVS) method, the crack axial strain (CAS) method and crack radial area strain (CRAS) method were proposed to identify the failure precursor information (including stress thresholds and axial strain at the initiation point of crack connectivity stage) during the rock failure processes. The results of the CTC tests show that the stable crack development stress ${\sigma }_{sd}$, unstable crack development stress ${\sigma }_{usd}$, and crack connectivity stress ${\sigma }_{ct}$ identified by the CAS method are 6%, 74%–84%, and 86%–97% of the peak stress, respectively. For the TUCP cases, as the confining pressure increases, the stress thresholds, axial pressure at failure and axial strain at the start of the crack connectivity stage increase, while the time ratio of the crack connectivity stage to the entire unloading stage decreases. This indicates that fine-grained granite is prone to generate more cracks and leads to fail suddenly under high confining pressure. Furthermore, this new method demonstrates that the point at which the derivative of the radial crack area strain transitions from stable to a sudden increase or decrease is defined as the precursor point of rock failure. The results of axial strain at the starting point of the crack connectivity stage are very close to those predicted by the AE method, with β₁ no more than 11%.