The post-mining pore-fracture structure significantly influences fluid migration and storage within the goaf, yet it remains inadequately represented in most existing models. This study proposes and field-validates a mechanics-based method for reconstructing the three-dimensional porosity-permeability field of a goaf by coupling its stress and fracture fields. The method integrates an FLAC3D stress-recovery field, obtained using an upscaled double-yield model, with a 3DEC discrete-fracture field. These fields are coupled under an orientationand aperture-weighted fracture-tensor reconstruction through a matrix-compaction pathway constrained by mass conservation and a fracture-flow pathway based on Oda's fracture-tensor formulation. The reconstructed field reproduces the O-shaped annular permeability structure generated by the abutment-pressure arching mechanism and its pronounced vertical permeability anisotropy. A relatively stable stress-recovery plateau developed in the central goaf, with a maximum recovery ratio of 60.1%. Transport simulations show that 65%- 72% of the peak CO2 mass remains within the caved zone after 100 years of storage, with retention increasing as the height of the fractured zone decreases. By providing a mechanics-based representation of the complex and heterogeneous three-dimensional porosity-permeability structure of goafs, this study offers new insights into the spatial resource utilisation of goafs in abandoned coal mines.
The basic friction angle (pb) is a crucial parameter for evaluating the shear strength of rock joints. To investigate its evolution under cold environmental conditions, this study analyzed the effects of water saturation, low temperature, and freeze-thaw (F-T) cycles on pb by using tilt and direct shear tests. Results reveal a dual competitive effect of water. In low-stress tilt tests, the water loss weakens the capillary suction, decreasing pb. In high-stress direct shear tests, the matrix softening dominates, making saturated pb lower than dry values. At -20 degrees C, low-temperature strengthening increases pb for no visible surface ice film rocks. However, increased ice coverage reduces pb due to the low strength of ice and sliding lubrication. Furthermore, F-T damaged rocks exhibit a normal stress "threshold effect". Under a low normal stress (<= 2 MPa), F-T induced micro-roughness enhances mechanical interlocking, and thus increases the shear strength. Conversely, under high normal stress (>= 3 MPa), frost-weakened asperities are easily sheared off, acting as solid lubricants that transition the interface from sliding to rolling friction, thereby sharply reducing the shear strength. Given that tilt tests are overly sensitive to water, while high-stress shear tests obscure intrinsic mechanical properties, this study recommends direct shear tests under low normal stress (<= 2 MPa) to accurately determine the true baseline frictional properties.
The inherently low permeability of coal seams remains a major obstacle to the economically viable development of coalbed methane resources. Liquid nitrogen fracturing has recently emerged as a promising waterless stimulation technology capable of inducing strong thermal shock and enhancing coal permeability. However, the role of multi-scale pore evolution in controlling permeability enhancement remains poorly understood. In this study, a series of low-field nuclear magnetic resonance experiments were conducted to characterize the pore system of coal before and after thermal shock. Saturation and centrifugation techniques were further employed to control free water and bound water within the pore system. The results indicate that permeability enhancement is not solely attributed to the formation of new pores that increase total porosity. Besides, thermal shock induces a synergistic evolution of pore characteristics, including the development of flow channels, the improvement of pore connectivity, and a reduction in structural heterogeneity. Particularly, the decrease in fractal dimension reflects a more homogeneous and better-connected pore system, while the increase in maximum pore diameter indicates the expansion of dominant flow pathways. These findings demonstrate that permeability improvement results from the combined effects of pore generation, structural optimization, and channel enlargement across multiple pore scales. This study offers valuable guidance for optimizing cryogenic stimulation strategies to improve gas migration and production efficiency in low-permeability coal reservoirs.
Reliable, interpretable early warning of coal and gas outbursts remains challenging because monitoring data are noisy, temporal indicators and non-temporal parameters are heterogeneous, outburst samples are sparse, and geological conditions vary substantially. Existing statistical and deep-learning methods struggle to capture coupled precursor relationships among acoustic emission (AE), electromagnetic radiation (EMR), and gas signals under noisy, heterogeneous cross-scenario conditions, and usually provide probability-based warnings without mechanism-consistent explanations. To address these limitations, an integrated framework is developed. First, an adaptive denoising method that combines the discrete Fourier transform, the whale optimization algorithm, and hierarchical density-based spatial clustering of applications with noise (HDBSCAN) is proposed to suppress interference while preserving AE and EMR dynamics. Statistical analyses identify precursor response characteristics preceding gas-dynamic events. Second, a TimeCMA-based multimodal forecasting model using crossmodality alignment predicts the short- and medium-term evolution of AE, EMR, and gas signals. Using field monitoring data, it achieves mean absolute error values of 0.0312, 0.3775, and 0.4152 and mean squared error values of 0.0035, 1.7002, and 0.7448 at horizons of 32, 64, and 128 steps, respectively. Third, a transferlearning-based early-warning model embeds multi-scale temporal statistical features into structured prompts. Forecasting representations are transferred to early-warning classification to mitigate the scarcity and imbalance of labeled outburst-risk samples. The model achieves an accuracy of 0.976, an F1 score of 0.972, and an area under the receiver operating characteristic curve of 0.968, and field validation in coal mines in Guizhou and Sichuan confirms generalization. Finally, a coal-gas-outburst knowledge graph is constructed from standards, literature, and case reports. By integrating Shapley additive explanations (SHAP)-derived temporal attributions with non-temporal seam parameters, a graph retrieval-augmented generation (GraphRAG)-enhanced lightweight large language model generates evidence-grounded and mechanism-aligned interpretations of risk evolution. Comparative evaluation shows that the knowledge-graph-enhanced model provides higher causal consistency, domain specificity, and scenario adaptability than the baseline model. Overall, the proposed framework provides a deployable paradigm for proactive coal and gas outburst warning under limited labeled samples, delivering accurate forecasting, robust early warning, and mechanism-consistent interpretation through precursor representation transfer, model attribution, and decision support informed by domain knowledge.
Predicting the initiation and propagation of cracks in heterogeneous wellbore systems under complex insitu conditions remains challenging. We present a hybrid phase-field fracture framework to model crack growth in heterogeneous wellbore systems with weak interfaces. The framework is first validated against benchmark problems with available analytical and numerical solutions. Subsequently, numerical experiments are conducted to isolate the effects of interface strength and casing eccentricity on crack growth. The results show that casing eccentricity strongly influences both the pressure at crack initiation and the resulting crack paths, reducing the crack initiation pressure by up to 30% relative to the concentric configuration. For the examples considered here, beyond a critical eccentricity threshold of 50%, localized shear stresses drive the nucleation of inclined cracks in the rock formation in addition to radial cracking-a failure mode absent in concentric configurations. For sufficiently weak interfaces (i.e., interfaces with 30% of the toughness of the surrounding bulk material), radially propagating cracks in the cement sheath are deflected along the interface rather than penetrating into the rock formation. This deflection delays stress relaxation within the sheath, promotes the nucleation of additional radial cracks, and increases the risk of sustained casing pressure and wellbore failure. Finally, a three-dimensional simulation reveals depth-dependent crack nucleation, stress-shadow effects that suppress full-depth crack growth, and crack coalescence along the cement-rock interface-phenomena that are fundamentally inaccessible under plane-strain assumptions-demonstrating the applicability of the framework to realistic heterogeneous wellbore systems.
