The linear elastic hydraulic fracture criterion is not applicable to deep reservoirs when nonlinear behavior is present over an extensive zone at the fracture tip. This study aims to develop a criterion for nonlinear hydraulic fracture considering the fracture process zone (FPZ) and seeks to reveal the causes of nonlinearity during fracture propagation in deep reservoirs. A closing stress profile considering the in-situ stress was established by using the cohesive zone model (CZM) to describe the FPZ at the fracture tip. An analytical model for the FPZ length was derived, while the criterion for nonlinear fracture propagation was proposed. The FPZ fully developed and the fracture began to propagate when the apparent stress intensity at the fracture tip reached the apparent fracture toughness or when the in-situ stress intensity reached the in-situ fracture toughness. The proposed criterion can clearly determine the length of the FPZ, accurately predict the breakdown pressure during fracturing operations, and establish a relationship between these two parameters. It addresses the inherent limitations of conventional linear elastic fracture mechanics (LEFM), which often underestimates fracture toughness and neglects the effects of the FPZ. This research is expected to enhance the fracturing design in deep reservoirs.

Laser-induced electro-response (LIER), as a new method that complements conventional rock physics testing techniques, is expected to address issues such as of unclear mechanisms, model deficiency, inconsistent evaluation parameters, and difficulty in separating multiple coupling factors in shale anisotropy evaluation, and establish a more complete and reliable shale physical property evaluation system. A testing strategy for out of plane anisotropy (OPA) was proposed for characterising anisotropy by LIER, where the near infrared (NIR) continuous laser (CL) and nanosecond pulsed laser (PL) were used to irradiate the surface of oblique cut shale, and the transverse LIER of the surface was measured. A LIER detection model is constructed from the laser-thermal effect, residual transverse polarization electric field and thermionic emission transport mechanism, which is strongly relying on laser power, bias voltage, and inclination angle of the measurement direction relative to the bedding plane of shale. For OPA test on the slice of oblique cut shale under CL irradiation, the relationship between the product of LIER simulation parameters and the tilting angle can be described by a cubic function and an impulse function with a maximum value at the threshold angle. In addition, the thermal accumulation and transient thermal effects are induced in the shale under a high-energy short laser pulse irradiation, and the simulation results indicate that there is an exponential relationship between the product of parameters in the LIER model and the tilt angle. Thus, for OPA test under CL and PL irradiations, it is recommended to use the product of parameters as an evaluation index for shale anisotropy. Furthermore, to solve the problem of multiple influencing factors entangled in the exponential term of the LIER model, the tangential LIER measurement was performed on the side of cylindrical shale core, where the provided LIER model effectively presented the anisotropy of tight shale plug, especially the effects of bias voltage and laser power on LIER were relatively separated as independent variables. Finally, the LIER at the end of laser drilling is presented well using the optimized model under a focused ns NIR PL irradiation, indicating that LIER is expected to be a real-time means for characterizing shale anisotropy during laser drilling processes. These results show that the present work is fundamental for the precise evaluation and effective development of anisotropic shale reservoirs, and will drive the advances of LIER in the exploration for shale oil and gas.

Gas explosions in coal mine goafs are associated with the roof rock fracturing. An experimental system was established to investigate the potential for electrical ignition induced by sandstone fracturing. The electrical responses, luminescent emissions, and ignition characteristics during tensile and compressive failure of sandstones were analyzed in methane/air premixed gas environments. Results indicate that the application of mechanical loading induces the emergence of electrical signals on rock surfaces and in the surrounding atmosphere. This phenomenon is attributed to the generation, accumulation, and subsequent release of free charges during the deformation and fracture within the sandstone. Compressive failure proved to be more conducive to free charge generation than tensile failure, owing to more crack connections. Furthermore, a precipitous increase in surface and external voltages was observed during complete fracturing, a consequence of electron emission from crack tips within the rock structure. Moreover, the ionization induces luminous emissions owing to the collision of energetic electrons released from gas molecules in methane/air mixtures. A strong positive correlation (R²=0.9429) was identified between luminescence intensity and the magnitude of electrical discharge resulting from rock fracture. Notably, such discharge by rock fracturing can be capable of igniting the premixed gas, particularly when the quartz content exceeds 61%. Piezoelectric effects and crack propagation are crucial mechanisms in the causal chain of the charge generation, discharge, and ionization triggered by rock fractures. Based on the above laboratory results, electric ignition of the transient roof fracturing caused by stress mutations can serve as a new potential ignition source for gas explosions in the goaf. These results offer new insights into the prevention and control of gas explosions.

The identification of rock mass discontinuities is critical for rock mass characterization. While high-resolution digital outcrop models (DOMs) are widely used, current digital methods struggle to generalize across diverse geological settings. Large-scale models (LSMs), with vast parameter spaces and extensive training datasets, excel in solving complex visual problems. This study explores the potential of using one such LSM, Segment anything model (SAM), to identify facet-type discontinuities across several outcrops via interactive prompting. The findings demonstrate that SAM effectively segments two-dimensional (2D) discontinuities, with its generalization capability validated on a dataset of 2426 identified discontinuities across 170 outcrops. The model achieves 0.78 mean IoU and 0.86 average precision using 11-point prompts. To extend to three dimensions (3D), a framework integrating SAM with Structure-from-Motion (SfM) was proposed. By utilizing the inherent but often overlooked relationship between image pixels and point clouds in SfM, the identification process was simplified and generalized across photogrammetric devices. Benchmark studies showed that the framework achieved 0.91 average precision, identifying 87 discontinuities in Dataset-3D. The results confirm its high precision and efficiency, making it a valuable tool for data annotation. The proposed method offers a practical solution for geological investigations.

Reliable forecasting of coal seam gas production and gas injectivity (e.g., CO₂ or air) requires an accurate understanding of coal’s anisotropic permeability, which governs the directional flow of gas. Although the anisotropic nature of coal permeability is well recognized, little attention has been paid to how this ratio evolves with changes in effective stress or with the injection of gases that have different affinities to coal. In this work, more than 600 permeability tests were conducted on eight cubic Australian coal samples using He, N₂ and CO₂ gases under varying effective stresses, providing a comprehensive dataset that allows the combined effects of effective stress and gas adsorption on permeability anisotropy to be robustly assessed on the same samples. The results demonstrated that all coal samples exhibited evident permeability anisotropy, with ratios ranging from 1.11 to 6.55. For the first time, quantitative relationships between the anisotropy ratio, effective stress, and initial permeability were established for each of the three injection gases, highlighting how gas adsorption and effective stress changes both anisotropic permeability magnitude and ratio. These findings provide new insights into the directional flow behavior of gases in coal seams, with implications for underground compressed air energy storage and CO₂ sequestration.

Joints are widely distributed structural defects in rock masses, and their geometric characteristics play a decisive role in the overall stability of rocks under complex stress conditions. To clarify the influence of joint geometry on the mechanical behavior of jointed rock under such conditions, this study investigated the mechanical properties and failure mechanisms of composite jointed rock specimens with varying joint roughness and joint dip angles. Three typical failure modes under triaxial loading were identified, and a mechanical analysis model incorporating joint roughness and dip angle was established. The failure mechanism was revealed, and a discrete element model was developed to analyze the micro-damage evolution process of the specimens. The results show that the mechanical parameters of the specimens exhibit pronounced anisotropy. Both the elastic modulus and peak strength reach their minimum values at a joint dip angle of 60 °. Increasing joint roughness significantly reduces the degree of anisotropy and enhances the energy storage capacity of the specimens. A strong linear relationship is observed between the elastic strain energy and the peak deviatoric stress, confirming the applicability of the linear energy storage law in composite jointed rocks. Discrete element simulations revealed the evolution path and dominant types of microcracks between the joint and matrix. The joint dip angle governs the transition of dominant crack types from tensile to shear and then back to tensile. Increased joint roughness significantly suppresses damage localization along the joint and results in an approximately 20% increase in the proportion of shear microcracks within the matrix. These findings clarify the regulatory role of joint geometrical parameters in the damage evolution process.

Fractures in rock strata serve as flow pathways for gas flow. The undulation of fracture channels can influence the guidance of gas flow. In this context, four-point bending experiments on prefabricated fractured rocks at different angles under stable stepped loading stress. The experiment results clarified the evolutionary law that the undulation degree of the rock tensile fracture surface is separated by an initial fracture angle of 45°. The high undulation intervals were less than 45°, whereas the low undulation intervals were more than 45°. Furthermore, the relative undulation degree, undulation frequency, and matching degree of the fracture surface were quantified. The relationship between the change in fracture surface undulation and gas flow guidance was established. Based on this, the stability, tortuosity, and uniformity of the gas flow in the fracture channel were quantitatively characterized. Subsequently, numerical models of the fracture channels were constructed to validate the indices proposed in this study. The results of the study clarified the influence of different initial fracture angles on the undulation changes of fracture surfaces, and established the relationship between these changes and gas flow, which is conducive to understanding the role of internal fracture channels in rocks in guiding the gas flow process.

Chalcopyrite is often intergrown with talc, which, after grinding, forms ultrafine particles (<10 μm) that readily coat chalcopyrite surfaces, hindering flotation and causing significant losses in tailings. This study evaluates polyvinyl acetate (PVAc), a thermoplastic polymer, as a selective flocculant to enhance reverse flot ation separation of chalcopyrite from ultrafine talc. Flotation tests showed that at a PVAc dosage of 40 mg/L, talc can be effectively and selectively removed, enabling efficient separation. Laser particle size analysis and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) confirmed that PVAc promotes selective talc aggregation without affecting chalcopyrite. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that hydrogen bonding between PVAc ester groups and surface hydroxyls on talc drives the flocculation, while chalcopyrite lacks suitable binding sites. PVAc adsorption also enhances talc hydrophobicity. Furthermore, particle-bubble coverage angle measurements and extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory theoretical calculations demonstrated that PVAc-induced flocculation increases attractive interactions between talc and bubbles, shifting the total interaction energy from repulsive to attractive and promoting bubble-particle attachment. This study clarifies the selective adsorption and flocculation mechanisms of PVAc and reveals the coupling of flocculation and flotation of ultrafine talc from a particle-bubble capture perspective, while expanding the potential of ester-based polymers for ultrafine mineral recovery.

Understanding the complex flow behavior along a rough rock fracture under high-temperature, high-stress, and high-seepage pressure (HTHM) coupling conditions is of great significance for optimizing deep resource extraction. This study investigates the complex flow behavior of a single rock fracture under coupled HTHM conditions using a self-developed multi-field coupling experimental system, considering real-time high temperatures (20-90 °C), confining pressures (30-120 MPa), and seepage pressures (5-60 MPa). Experimental results show that as confining pressure increases, two typical nonlinear flow behaviors are observed, which are Forchheimer flow and low-velocity nonlinear flow. The increase in temperature and decrease in roughness significantly promote the fluid flow and enhance the nonlinear relationship between the volumetric flow rate and the hydraulic gradient at lower confining pressures (30 MPa). However, the change in temperature and fracture surface roughness does not affect the nonlinear type of fluid flow. Under a given hydraulic gradient, the influence of temperature and fracture roughness on the volumetric flow rate varies with changes in confining pressure. Additionally, this study considers both the viscous and inertial terms, and a modified Forchheimer equation is proposed using two parameters: the contact area ratio and the thermal expansion coefficient of the rock. The proposed model can effectively predict the nonlinear flow behavior of fluid along rough fractured rocks under varying temperatures and surface roughness. The experimental results and the proposed model provide valuable data and theoretical guidance for deep oil and gas exploration as well as hydraulic fracturing design.

To investigate the influence of non-uniform water distribution on the mechanical properties and failure behavior of red sandstone, we designed five immersion heights and durations to achieve varying non-uniform water distribution states. Uniaxial compression tests were conducted on red sandstone under these conditions. The effects of non-uniform water distribution on deformation, failure, strength, and energy characteristics of red sandstone were analyzed. The impact of non-uniform water distribution on the intensity of rock failure was discussed, and the failure mechanism under non-uniform water distribution was revealed. The hazards of low immersion heights on underground rock structures were analyzed. The results demonstrate that peak strength and elastic modulus of red sandstone exhibit high sensitivity to immersion height, with reductions of 38% and 23% respectively even at L=1/50H. Water immersion reduces both energy storage capacity and energy dissipation capability of red sandstone. The immersion height and duration influence the failure mode of red sandstone by controlling the migration and separation of dry-wet interfaces. Low immersion height poses significant risks to underground rock structures (e.g., a 38% strength reduction when L=1/50H), and the concentration degree of water non-uniform distribution is the key factor in assessing the weakening effect of water on rocks.

For hard rock cracking induced by laser irradiation, the failure modes and fracture characteristics among rocks of different types and sizes are still unclear. Therefore, the experiments on laser-induced fracturing of limestone, sandstone, and various-sized granite specimens were conducted. Real-time acoustic emission monitoring and laser scanning were employed to capture acoustic emission signals inside rocks during laser irradiation and to reconstruct the fracture surfaces after laser irradiation. Results indicate that abundant melts in sandstone and granite dissipated laser energy, leading to lower acoustic emission peak energy compared to limestone. Larger-sized specimen delayed the occurrence of peak energy. Crystal thermal expansion and changes in pore pressure induced tensile-shear composite failure in limestone, whereas thermal expansion of minerals in sandstone and granite promoted tensile failure. Fracture surface morphology was influenced by sampling interval, anisotropy, and size effects. The joint roughness coefficient and fractal dimension of sandstone exceed granite and limestone. Asperity heights and slope angles ranged from 1-14 mm and 0-40 °, respectively, with the average aspect angles exceeding 110 °. Granite exhibited the highest proportion of macropores after laser irradiation, approximately 4.8%. These findings provide valuable insights for the application of laser-assisted fracturing in hard rock excavation.