Understanding the synergistic effects of inorganic minerals and pore structure on coal pore wettability remains a critical knowledge gap. This study investigates the pore-scale wettability mechanism of five high-rank coals by integrating quantitative inorganic mineral analysis, pore morphological characterization, and low-field nuclear magnetic resonance (LF-NMR) T 2 relaxation spectroscopy. A series of water-droplet-coal wetting experiments, organized into three groups, was conducted to establish a cross-scale framework linking physical pore structure, mineral composition and wetting response. The results indicate that the dual mechanism of strong adsorption on hydrophilic mineral surfaces and capillary forces in micropores drives the wetting process. In terms of mineral aspect, clay minerals exhibit significantly higher wettability than non-clay minerals, and clay content shows a significant negative correlation with wetting parameters R g,24h measured by LF-NMR. Notable, when clay content ranges from 2% to 10%, its regulatory effect on pore wettability is particularly sensitive. In terms of pore structure, an exponential saturation model was employed to fit the pore wetting kinetics. The derived kinetic parameters, including the rate constant k and equilibrium pore diameter R eq are consistent with the trends observed in R g,24h . And the 24 h observation is confirmed to represent quasi-steady state for all samples. Additionaly, Pore wettability is governed by absolute pore capacity when pore volume differences exceed approximately two-fold, whereas at smaller differences, micropore complexity and connectivity dominate. Micropores play a critical role due to capillary force-driven physical contributions. Furthermore, a critical pore diameter of 42.16 nm is identified, above which wetting transitions from capillary-driven to surface-spreading or gravity-dominated regimes. A two-dimensional pore-wet coupling model is proposed, and five representative wettability types were defined. Among them, samples C4 and C5 exhibit synergistic enhancement (type A), while C1 shows mutual inhibition (type B). These results provide a new multidimensional coupling paradigm for understanding wettability in heterogeneous porous media.

Tectonically deformed coal (TDC) is a typical engineering geological medium, and its mineral cementation fracture network (MCFN) dominates the structural heterogeneity, seepage characteristics and hydraulic fracturing performance of coal reservoirs. Existing image-based fractal characterization methods struggle to quantitatively describe the spatial distribution heterogeneity of MCFN. In this study, a binarization-free multifractal analysis method based on scanning electron microscope (SEM) grayscale images was adopted to quantitatively characterize the MCFN structure of 25 groups of TDC samples from four mining areas in China. The generalized dimension spectrum was decomposed into left and right branches, and a four-index joint characterization system including ΔD, ΔD−, ΔD+ and ΔR was established. The results show that the pixel grayscale distribution of coal mass exhibits significant multifractal characteristics, and the development degree and spatial uniformity of MCFN in soft coal or soft layers with intense tectonic deformation are significantly better than those in hard coal or hard layers. The development of MCFN is jointly controlled by tectonic deformation degree, mineral content and filling dispersion. Based on multifractal parameters, coal samples can be divided into four typical structural types: well-developed uniform fracture network, uniform fracture network with single-phase differentiation, dual heterogeneity, and underdeveloped fracture network, corresponding to the progressive evolution process from strong to weak tectonic deformation, from dispersed to localized mineral filling, and from fully saturated to underdeveloped MCFN. The four structural types provide clear engineering guidance for hydraulic fracturing construction strategies. This study breaks through the limitations of traditional structural characterization methods and provides a new quantitative research approach for engineering geological evaluation and fracturing optimization of TDC reservoirs.

Analyzing the energy evolution of coal under load is fundamental to understanding its failure mechanisms. Direct monitoring of the energy state during the loading process remains a significant challenge. This study performed uniaxial compression tests on 9 coal samples with closely matched P-wave velocities and masses under varying loading rates. The experimental setup enabled synchronous monitoring of stress, strain, resistivity, and acoustic emission (AE). A coupled analysis of dissipated energy, elastic energy, and a resistivity fluctuation index characterized energy accumulation and release throughout the complete failure process. Results indicate that increasing the loading rate enhances both the rate and the extent of energy accumulation. A transition in failure mode was observed, with the proportion of shear cracks increasing by over 10%. The energy release mechanism shifts from progressive dissipation dominated by tensile fractures under low loading rates to abrupt release governed by shear failure under high rates, corresponding macroscopically to a transition from shear slip to brittle fragmentation. Cumulative AE energy shows a correlation coefficient of 0.82 with stored elastic energy at peak stress. Resistivity decreases gradually during compaction and elastic stages, increases during yielding, and surges at stress drops, reaching four times the initial value post-peak. The resistivity fluctuation index correlates with dissipated energy proportion at a coefficient of -0.83, confirming its utility in characterizing cumulative structural damage and energy dissipation. The loading rate governs the energy evolution process by altering the fracture mechanism. AE parameters quantify elastic energy release, while resistivity variations reflect damagedriven dissipation.

This study develops a finite element-based numerical model that integrates elastic damage mechanics, porous media flow, and a viscous plugging mechanism to investigate the interaction between hydraulic fractures (HF) and natural fractures (NF) during temporary plugging fracturing (TPF). The model quantitatively analyzes the effects of conventional and temporary plugging fracturing on fluid pressure, fracture geometry (length, area, fractal dimension, damage degree, offset), and propagation patterns. A systematic parametric study examines the influence of NF approach angle, length, Young’s modulus (strength), and permeability on HF behavior before and after temporary plugging. The results indicate that a natural fracture approach angle of 30 45° yields the optimal temporary plugging performance, achieving the best balance between pre-plug capture and post-plug breakthrough. When the NF length exceeds 35 m, conventional fracturing leads to inward fracture retreat; however, TPF mitigates this effect and stabilizes fracture dimensions. For formations where NF strength significantly exceeds the matrix strength, TPF provides limited improvement in fracture distribution and is therefore not recommended. High NF permeability enhances post-plug fracture development, while low permeability results in fractures propagating primarily from both ends of the NF, leaving the middle section underdeveloped—suggesting that future designs should consider simultaneous plugging at both ends. These findings provide theoretical insights and practical guidance for optimizing temporary plugging strategies to achieve more uniform and radially extended fracture networks in naturally fractured reservoirs.

SiO2 nanofluids exhibit significant potential for preventing coal mine gas disasters through the technology of coal seam water injection by enhancing coal wettability and weakening its mechanical properties. Identifying the optimal nanofluid concentration is crucial for maximizing its modification effectiveness. Grounded in fluid dynamics, this study investigates the mechanisms of progressive failure and strength weakening in coal treated with SiO2 nanofluids at varying concentrations through triaxial mechanical testing, acoustic emission (AE) monitoring, computed tomography (CT) scanning, and scanning electron microscopy testing. Results indicate that, unlike the instantaneous failure of untreated coal, nanofluid-modified coal samples exhibit progressive failure. While overall strength is weakened, mechanical parameters, specifically elastic modulus and Poisson's ratio, exhibit the non-monotonic trend, decreasing initially and then increasing with nanofluid concentration. The plastic failure gradually intensifies, with elastic energy released progressively rather than instantaneously during failure. Furthermore, the AE signals shift from the post-peak concentrated release to a pre-peak sustained, low-amplitude distribution. CT analysis reveals that nanofluid modification increases crack quantity, crack volume ratio, and fractal dimension. Among all coal samples, the 0.5 wt. % SiO2 nanofluid modification results in the most significant crack propagation and distinct crushed fissures, correlating with the most significant progressive failure and strength weakening. However, higher concentrations cause excessive nanoparticles agglomeration, restricting further improvement. The observed non-monotonic trends are attributed to fluid dynamics mechanisms. The optimal concentration (0.5 wt. %) achieves a balance between enhanced capillary imbibition and interfacial lubrication facilitated by nanoparticle adsorption. Conversely, higher concentrations lead to dominant particle agglomeration, which restricts fluid penetration and thereby partially restores mechanical resistance. These findings not only provide theoretical guidance for optimizing nanofluid concentration in coal seam water injection but also offer a fundamental fluid physics framework for understanding how nanoparticle suspensions regulate the mechanical behavior of porous media.

To investigate the sensitivity and underlying mechanisms of key parameters for gas extraction, a three-dimensional numerical model was established. Using the control variable method, the effects of critical extraction parameters on gas pressure evolution, the effective extraction radius of drilling, and cumulative gas production were systematically analyzed. The results indicate the following: Extraction time is the primary factor controlling the expansion of the pressure disturbance zone, and gas extraction exhibits a significant characteristic of diminishing marginal returns. Increasing the extraction negative pressure and drilling diameter mainly improves near-drilling flow conditions and contributes only marginally to the overall extraction effectiveness. Drilling length determines the gas resource volume controlled by a single drilling operation and its sustained extraction capacity while exerting only a limited influence on the effective extraction radius. The initial porosity of the coal seam is the dominant factor controlling both the effective extraction radius and extraction efficiency. Field extraction data verified the model’s reliable representation of extraction patterns and parameter influence characteristics. A synergistic gas control strategy integrating long drilling coverage, enhanced permeability, reasonable negative pressure, and continuous extraction was proposed. These research results can provide a theoretical basis and technical support for the optimization of gas extraction parameters.

To characterize the governing effect of loading rate on the fracture mechanism of granite, this study combines experimental and numerical approaches to analyze the induced mechanical-acoustic-electrical multi-physical responses. Experiments were conducted on granite specimens with closely matched mass and P-wave velocity under five loading rates, with synchronous monitoring of mechanical data, resistivity, and acoustic emission (AE). A heterogeneous numerical model of the rock was established based on the Weibull distribution characteristics of resistivity, enabling simulation of the electric potential distribution. Results show that increasing loading rate leads to a nonlinear enhancement in fracture toughness. The peak load rose significantly from 2.39 kN to 2.80 kN, an increase of 17.15%. Resistivity exhibited high sensitivity to macroscopic fracture, surging abruptly upon unstable main-crack propagation, with the jump amplitude reaching up to eight times the preceding gradual change. Crack propagation substantially reconfigured the electric field distribution in granite, resulting in a sharp rise in the electric potential gradient at the crack tip. AE activity shifted from a continuous, distributed pattern to one concentrated near the peak load as the loading rate increased. The cumulative number of AE events decreased, while the proportion of high-energy events grew. AE energy followed a power-law distribution with a decreasing slope as loading rates increased, and dominant-frequency events clustered in the 200-280 kHz band, together indicating that faster energy input leads to more localized and unstable fracture. The rate effect systematically governs the evolution of mechanical strength, electrical parameters, and AE signals during fracture by controlling the damage accumulation process and crack propagation path.

This study presents an integrated approach combining metre-scale physical experiments, numerical simulations and theoretical modelling to systematically investigate the mechanisms and controlling factors of fault slip induced by hydraulic fracturing, with the ultimate goal of establishing fundamental design principles for fault slip mitigation. First, we quantify the influence of fault geometry and mechanical properties on stress concentration by deriving a quantitative stress concentration equation through multivariate regression analysis. Second, a novel three-dimensional (3D) Coulomb failure stress (CFS) expression incorporating stress concentration coefficients is proposed to overcome the limitations of conventional regional stress analysis. Third, the theoretical stress transfer model is validated against experimental data, showing strong agreement between predicted and measured CFS changes, with a relative error of less than 10 %. Our results demonstrate that hydraulically isolated faults are primarily controlled by regional stress states during slip initiation. Critically stressed faults exhibit significant slip near injection points, while non-critical faults remain stable. The proposed 3D CFS expression successfully discriminates between stress-transfer induced PNR-1z and pore-pressure driven PNR-2 seismic events. Finally, faults are classified into four distinct types, each associated with tailored hydraulic fracturing design protocols: type I faults require mandatory avoidance; type II-III faults require controlled injection parameters; and type IV faults permit cost-optimised operations. These findings provide theoretical advances and practical guidelines for mitigating induced seismicity in hydraulic fracturing.

The complex fault structures at varying depths significantly impact the engineering stability of coal mining. This study focuses on intersecting faults as a representative complex geological feature by integrating large-scale true triaxial physical experiments, multi-software coupled numerical simulations, and engineering case studies from the Huainan Coalfield. The mechanical behavior and response mechanisms of intersecting fault zones under depth-driven effects are systematically elucidated. Furthermore, it provides engineering guidance for optimizing the stability of gas extraction boreholes in complex faulted zones. The results indicate that intersecting faults substantially disrupt the stress transmission paths in coal seams, thereby increasing the spatial heterogeneity of stress distribution. With increasing depth, the relative reduction in the maximum principal stress is monitored within specimen—compared to the applied loading stress—decreases from 19.1 to 13.5

The utilization of low-concentration coalbed gas as an energy source in coal mines has garnered significant attention due to the intensifying global warming trend. Achieving a uniform gas mixture is essential for the effectiveness of various gas utilization technologies. In this study, we conducted a series of numerical studies on the pressure loss and mixing uniformity of a swirl-type static low-concentration coalbed methane (SLCCM) mixer with spiral blades as turbulence elements. The results indicate that installing spiral blades at the low-concentration gas inlet can significantly improve gas mixing uniformity, while installing spiral blades at the air inlet has a relatively small impact on gas mixing uniformity. When spiral blades are installed at both the low-concentration gas and air inlets, and the spiral direction of the two sets of blades is the same, the pressure loss of the mixer is smaller than that of the mixer with opposite spiral directions. Moreover, the mixing uniformity of the former is better than that of the latter when the length of the mixing zone is shorter. However, as the length of the mixing zone increases, the mixing uniformity of the mixer with opposite spiral directions of the two sets of blades will be significantly improved and better than that of the mixer with the same spiral direction. When a set of spirals with opposite directions to the blades at the low-concentration gas inlet is added at the forefront of the mixing zone, the mixing uniformity at the outlet of the mixer can be greatly improved, but it will increase the mixing pressure loss of the system. When the two spirals are in the same direction, the impact on the system pressure loss and mixing uniformity is relatively small. The numerical simulation results show that by setting a set of spiral blades with the same rotation directions at the inlet of low-concentration gas and at the forefront of the mixing zone (i.e., b2-type structure), the spatial non-uniformity of the mixing is about 5.24%, and the system pressure loss is less than 400 Pa, which is significantly better than the requirement of less than 15% spatial non-uniformity for engineering requirements. Moreover, with the increase in the length of the mixing zone, the spatial non-uniformity still tends to decrease.

To address the challenges of low signal-to-noise ratio and difficulty in phase picking of microseismic signals in-duced by coal fracturing during hydraulic fracturing in coal seams,a deep neural network model based on a masking strategy is proposed(Mask Residual Attention Denoiser,termed MRAD).The model is built upon the classical U-Net ar-chitecture,incorporating a mask-guided strategy to enable the network to learn the feature distributions of effective micro-seismic signals and noise separately.It outputs corresponding signal and noise masks,and applies the effective signal mask to the time-frequency matrix of the raw microseismic signal via Hadamard product to achieve mask-weighted filter-ing for denoising.A total of 18 670 microseismic training samples were constructed by iterative superposition of manu-ally labeled clean signals and random noise.Structurally,residual blocks replace standard convolution and downsampling layers in U-Net to mitigate gradient vanishing and prevent local minima;spatial attention mechanisms are further intro-duced in skip connections to enhance the network's focus on valid signal regions.Experimental results show that,after de-noising with MRAD,the average SSNR of the test signals improves to 18.22 dB,the root mean square error is reduced to 0.042 4,the normalized cross-correlation reaches 0.969 9,and the energy ratio is 1.028 6.The denoising performance is particularly significant for signals with original SNR in the range of 0-10 dB.Moreover,the model processes a single mi-croseismic signal in under 30 ms,with low computational demands,meeting the requirements for real-time microseismic monitoring and data processing in hydraulic fracturing operations.To validate the effectiveness of MRAD,30 synthetic mixed signals were denoised using both MRAD and traditional methods.Comparative results demonstrate that MRAD outperforms in terms of signal quality improvement and distortion reduction.Field application in a coal mine in Ningxia,China,shows that after denoising the microseismic signals from three fractured boreholes,the SNR was concentrated in the range of 10-25 dB,with an average increase of 6.90 dB.Noise suppression was effective,leading to improved P-wave ar-rival picking accuracy,and the number of detected microseismic events increased from 487 to 653,a growth of approxim-ately 1.34 times.Source localization analysis of the denoised signals indicates that the unidirectional extent of induced fractures ranges from 12 to 37 meters,aligning well with the construction parameters of the borehole fracturing sections.These results confirm that the proposed method offers strong technical support for real-time monitoring and effectiveness evaluation of microseismic signals in coal seam hydraulic fracturing.

To optimize the performance of the RC-DTH air hammer, a mathematical model detailing each phase of the piston’s movement has been constructed in the present work. Simultaneously, a novel piston structure of the RC-DTH air hammer (Type B) with diverse internal flow has been proposed. The impact performance of the structurally modified RC-DTH hammer is analyzed using Computational Fluid Dynamics (CFD). Additionally, an impact energy testing system for the RC-DTH air hammer is developed to confirm the validity of the numerical simulation results. Research results have shown that enhancing both the intake stroke of the upper chamber (F1) and the outlet stroke of the lower chamber (R2) of the RC-DTH air hammer piston can effectively improve the piston’s impact performance. Conversely, increasing the inlet stroke of the lower chamber (R1) and the outlet stroke of the upper chamber (F2) tends to diminish the piston’s impact performance. Moreover, the quality of the piston influences its striking frequency while having a minimal impact on single-impact energy. As the piston quality increases, the power of the impact diminishes. Once the piston valve stroke parameters are optimized, its impact performance is enhanced by 20.32%. Compared to the GQ89 hammer, the Type B hammer exhibits an 84% increase in impact energy and a 74% increase in impact power.

Understanding the deformation evolution in the borehole area is beneficial in grasping the regional gas desorption and high-stress release, thereby preventing methane disasters. This study is aimed at investigating the mechanical response and instability characteristics around the gas extraction borehole in composite stratum under true triaxial loading, with the aid of internal strain-brick monitoring and acoustic emission (AE) monitoring. First, the internal stress evolution surrounding the borehole was elucidated. Furthermore, the frequency distribution characteristics, primary failure localization, and crack extension modes were investigated. Finally, the structural damage and indicative instability features around the borehole were evaluated. The results reveal the internal cracks in different strata exhibit a synergistic temporal relationship, while the potential failure point is identified. Additionally, the crack mode for specimens underwent a progression from "tension cracks dominated" to "tension-shear cracks mixed", where the proportion of shear cracks (mode II) increased from 31.5 % to 45.9 % with the increasing borehole depth. Moreover, the AE signal frequency of low-frequency and highamplitude increases with the borehole depth, and the dominant frequency bands during structural damage to the specimen are 150-180 kHz and 260-285 kHz. Meanwhile, the location distribution of AE events reveals that coal strata on the weaker side of the coal-rock interface are more prone to high-energy damage events as drilling depth increases. The results of this study can provide new theoretical insights into the quantitative characterization of the in-situ behavior around the gas extraction borehole, which is significant for disaster prevention in the composite stratum around boreholes.

Accurate identification of lithological variations and inversion of geological structures based on drilling parameters are essential for enhancing intelligent sensing capabilities in complex strata. In this study, a digital drilling test platform was independently developed to replicate field conditions. The system integrates multiple control modes and enables high-precision, real-time acquisition of key drilling parameters, including weight on bit (WOB), rate of penetration (ROP), cutting torque, rotational speed, and drilling depth. Using this platform, a comprehensive experimental program was conducted, encompassing comparative analysis of drilling control modes, evaluation of lithology-responsive indicators, characterization of transient responses at lithological interfaces, and field validation in a coal mine. The results demonstrate that the constant WOB control mode offers superior stability, clearer response characteristics, and better alignment with actual drilling conditions compared to the constant ROP mode, making it more suitable as a baseline for laboratory investigations. Under the constant WOB condition, ROP exhibits higher sensitivity and consistent response trends than the more variable cutting torque, establishing it as a more robust indicator for lithology identification. Transitions across lithological interfaces are characterized by a distinctive transient pattern—namely, a “V-shaped” torque fluctuation coupled with a sharp ROP shift—where torque changes consistently precede ROP responses by 1.3 to 1.9 s on average. Moreover, the amplitude of these responses reveals a pronounced direction-dependent asymmetry: when transitioning from hard to soft rock, torque decreases to 0.47 times its initial value, while ROP increases by a factor of 5.75; in contrast, the reverse transition causes torque to increase by a factor of 1.83 and ROP to decrease to 0.24 times its original value. Field experiments confirm the stability and applicability of these features under complex geological conditions, providing both theoretical support and practical guidance for intelligent drilling operations and forward-looking geological sensing in stratified formations.

A large amount of gas rapidly released during coal and gas outburst is an important cause of dynamic effects such as outburst shock waves and coal-gas two-phase flow. To clarify the influence mechanism of desorbed gas on outburst dynamic effects, physical simulation tests of outburst dynamic effects under different gas pressures and adsorbed/non- adsorbed gas conditions were carried out. The influence of gas on the characteristic parameters of outburst shock waves and the migration of coal-gas two-phase flow was analyzed. The concept of “gas desorption equivalent particle size” was defined to reflect the gas desorption rate of the outburst crushed coal particle group. Combined with gas diffusion dynamics theory, a model of initial desorbed gas expansion energy of outburst coal was constructed, and the action mechanism of desorbed gas was discussed from the perspective of energy sources. The results show that due to the extremely short formation time of the outburst shock wave (several milliseconds), the desorbed gas in coal has a limited influence on the shock wave. The gas pressure has a significant influence on the outburst shock wave; as the gas pressure increases, the peak overpressure gradually increases, and its attenuation along the roadway tends to increase. The outburst intensity increases with the increase of gas pressure and gas adsorption capacity; under CO2 test conditions, the mass of outburst coal is 1.4 times that under helium test conditions. The energy dissipation of outburst dynamic effects is divided into crushing work, throwing work, and energy dissipated by the outburst shock wave. Under low gas pressure conditions, energy dissipation is dominated by crushing work (accounting for 73.9% at 0.3 MPa). With the increase of gas pressure and adsorption capacity, the proportion of throwing work gradually increases, while the proportion of energy dissipated by the outburst shock wave is basically less than 10%. The energy contribution rate of desorbed gas from outburst coal to the work done by outburst dynamic effects ranges from 33% to 90%, increasing with the increase of gas pressure and gas adsorption performance. A model of initial desorbed gas expansion energy from crushed coal based on the concept of gas desorption equivalent particle size was established. The calculation results of the model show that the desorbed gas expansion energy of outburst coal during the outburst process is fully involved in the work done by outburst dynamic effects. The influence of different factors on the desorbed gas expansion energy of outburst coal was discussed: the gas desorption equivalent particle size has the most significant influence on the desorbed gas expansion energy of outburst coal. The desorbed gas expansion energy of outburst coal increases slowly with the decrease of equivalent particle size, and increases significantly when the equivalent particle size decreases to 0.5 mm. Thus, stress can also affect the release of gas expansion energy by influencing the degree of coal and rock fragmentation. The desorbed gas expansion energy of outburst coal increases with the increase of adsorbed gas pressure and initial diffusion coefficient, with the increasing amplitude gradually decreasing; it shows an almost linear growth relationship with the increase of gas content.

Before effectively analyzing the stimulation of coalbed methane (CBM) reservoirs using microseismic (MS) monitoring, it is necessary to accurately distinguish signals caused by hydraulic fracturing (HF) from interference signals. In this study, the Mel-frequency cepstral coefficient-fuzzy decision tree (MFCC-FDT) signal classification method was used. To minimize the loss of crucial details during preprocessing, feature extraction is accomplished by computing the MFCC values. This is followed by a decrease in the dimensionality and fuzzification of the dataset. Finally, the preprocessed data are entered into the FDT classifier that has been trained, thereby completing the automated identification of the induced signals. The proposed technique was applied to examine MS signals during the staged HF stimulation of a CBM reservoir. These findings suggest that the MFCC-FDT method outperforms the other combinations in terms of Accuracy, Precision, Recall, and F1-Score. Thirty-five interference MS events, including signals from tunneling blasts and machine operation during reservoir stimulation, were eliminated. The total stimulated reservoir volume under the MFCC-FDT technique validation was 22 966.29 m3, 1954.34 m3 less than the volume prior to the interference signals being removed. The proposed method reveals the nonlinear frequency characteristics of the induced MS signals and can be utilized to render more accurate MS signals for quantifying CBM reservoir stimulation by subsequent source inversion.

Small-scale faults associated with major faults in geological structure are difficult to measure accurately by existing geological exploration techniques. Identifying key parameters, such as fault strike and shape, is critical for optimizing mining operations. This study introduces a novel method for the precise detection of small-scale faults within coal seams based on data from coalbed gas drainage boreholes. The borehole data were preprocessed by considering the elevation phase difference of the faults and the similarity in prior coal burial depths. The particle swarm optimization (PSO) algorithm enhanced the nearest neighbor propagation clustering, achieving 80.56

Nanofluid-surfactant compound liquid exhibits significant application potential in improving the mechanical properties of wetted coal and enhancing the effectiveness of coal seam water injection. Determining its optimal ratio is the key to enhancing its performance. Through stability analysis, contact angle test, nanoindentation mechanical experiments, and scanning electron microscopy tests, the influences of different ratios of compound liquid on the micromechanical parameters of the coal are investigated. The results indicate that the liquid stability is significantly improved by adding cationic surfactant. In addition, the contact angle of coal remarkably reduces after the modification of the nanofluid-surfactant compound liquid. Maximum indentation depth of the coal first increases and then decreases as the compound ratio rises. When the compound ratio reaches 2, the mechanical parameters such as hardness, fracture toughness, and elastic modulus of the coal decrease significantly, and the proportion of elastic energy is minimized. Appropriately increasing the ratio of SiO2 nanofluid can boost the effectiveness of coal modification. This facilitates greater water molecule penetration into the coal, the binding forces between mineral particles are weakened, and the mechanical properties are ultimately degraded. The research findings are expected to provide a basis for optimizing the wetting agent ratio in coal seam water injection.

Hydraulic fracturing (HF) has achieved significant commercial success in unconventional oil and gas development. However, it has the potential to induce fault slip. This study investigates the physical mechanisms underlying potential fault slip triggered by HF operations under varying geological and operational constraints. First, we elucidate the relationship between the critical stress state and the elastic modulus of the fault, and refine a formula for the maximum crustal stress difference on critically stressed faults, including stress concentration, friction, and dip. Second, we compare the role of injected fluid in permeable faults with that in impermeable faults, and demonstrate that fault slips can be triggered by a combination of friction decrease and pore pressure increase, even after ceasing injection. Specifically, we reveal that friction decline dominates induced fault slip on high permeable and hydraulically connected fault. Third, based on experimental results and theoretical analysis, we quantify the influence region of stress transfer under different conditions of well location and injection pressure. The results reveal that the elastic modulus of the fault controls the stress concentration on the fault plane. The dip of the fault and the stress concentration jointly determine the maximum crustal stress difference required for failure in critically stressed reverse faults. Thus, our study is more accurate in estimating the proximity of the in-situ stress to the critical state, compared with traditional methods. For critical reverse faults, the risk of induced slip is positively correlated with both injection pressure and friction of fault plane. When the injection pressure (PI) is 100 MPa and the friction (μ) is 0.8, the safe distance from injection point to critically stressed faults along the direction of maximum principal stress and maximum principal stress (dH and dv) should exceed 25 and 18 times as the hydraulic fracture half-length. When PI is 100 MPa and μ is 0.6, dH and dv are 23 and 17 times as the hydraulic fracture half-length, respectively. When PI is 60 MPa and μ is 0.6, dH and dv are 18 and 13 times as the hydraulic fracture half-length, respectively. The works enhance our understanding of HF-induced fault slip and potentially guide designs of the shale gas well location and trajectory for safer production.