Fiber Bragg grating (FBG) measuring bolts, as a useful tool to evaluate the behaviors of steel bolts in underground engineering, can be manufactured by gluing the FBG sensors inside the grooves, which are usually symmetrical cuts along the steel bolt rod. The selection of the cut shape and the glue types could perceivably affect the final supporting strength of the bolts. Unfortunately, the impact of cut shape and glue type on bolting strength is not yet clear. In this study, based on direct tension tests, full tensile load–displacement curves of rock bolts with different groove shapes were obtained and analyzed. The effects of groove shape on the bolt strength were discussed, and the stress redistribution in the cross-section of a rock bolt with different grooves was simulated using ANSYS. The results indicated that the trapezoidal groove is best for manufacturing the FBG bolt due to its low reduction of supporting strength. Four types of glues commonly used for the FBG sensors were assessed by conducting tensile tests on the mechanical testing and simulation system and the static and dynamic optical interrogators system. Using linear regression analysis, the relationship between the reflected wavelength of FBG sensors and tensile load was obtained. Practical recommendations for glue selection in engineering practice are also provided.

The large spatial variability of in-situ stress and initial reservoir pressure in steeply-dipping ultra-thick coalbed methane (UTCBM) reservoirs exert strong control on the initial distribution of stress-sensitive permeability. This results in significant differences in the propagation of reservoir depressurization, gas production characteristics, distribution of fluid saturation, and evolution of permeability relative to flat-lying and thin counterpart coalbed methane (CBM) reservoirs. We contrast these responses using the Fukang mining area of the Junggar Basin, Xinjiang, China, as a type-example using coupled hydro-mechanical modeling. Production response indicates: (1) Dual peaks in CBM production rate, due to the asynchronous changes in the gas production rate in each the upper and lower sections of the reservoir; (2) higher depressurization and water saturation levels in the lower section of the reservoir relative to the upper at any given distance from the production well that ameliorate with time to be similar to those of standard horizontal reservoirs; (3) the heterogeneity in effective stress is further amplified by the asymmetry of the initial pressure drawdown distribution of the reservoir to exert extreme control on the down-dip evolution of absolute permeability—with implications for production. Field drainage data and simulation results obtained in this study more accurately reflect the drainage characteristics of the steeply-dipping UTCBM reservoirs. For ultra-thick low-rank coal seams, permeability anisotropy plays an important role in determining the utility of horizontal wells and hydraulic fracturing to maximize rates and yields CBM production, and requiring further study.

Hydraulic fracturing is a key technology to stimulate oil and gas wells to increase production in shale reservoirs with low permeability. Generally, the stimulated reservoir volume is performed based on pre-existing natural fractures (NF). Hydraulic fracturing in shale reservoirs with large natural fractures (i.e., faults) often results in fault activation and seismicity. In this paper, a coupled hydro-mechanical model was employed to investigate the effects of injection site on fault activation and seismicity. A moment tensor method was used to evaluate the magnitude and affected areas of seismic events. The micro-parameters of the proposed model were calibrated through analytical solutions of the interaction between hydraulic fractures (HF) and the fault. The results indicated that the slip displacement and activation range of the fault first decreased, then remained stable with the increase in the distance between the injection hole and the fault (L_if). In the scenario of the shortest L_if (L_if = 10 m), the b-value—which represents the proportion of frequency of small events in comparison with large events—reached its maximum value, and the magnitude of concentrated seismic events were in the range of −3.5 to −1.5. The frequency of seismic events containing only one crack was the lowest, and that of seismic events containing more than ten cracks was the highest. The interaction between the injection-induced stress disturbance and fault slip was gentle when L_if was longer than the critical distance (L_if = 40–50 m). The results may help optimize the fracturing treatment designs during hydraulic fracturing.

Comprehending the water absorption process inherent to coal, including the associated spatial distribution patterns of water, proves indispensable in the design and evaluation of coal pillar dams in underground water reservoirs. To better understand this process, a series of NMR (nuclear magnetic resonance) tests were carried out on cylindrically shaped coal samples immersed in water for varying durations, with the upper and lower surfaces of the samples sealed. A method involving image digital processing and finite element simulation was used to quantitatively characterise the water absorption process, as well as the spatial distribution of water in the samples. The results showed that NMR imaging colour brightness differences were positively correlated with water content and that the wetted ring gradually increased in width as the water immersion time increased. The expectation and sum of squared deviations of the pixel greyscale values of the NMR images, which were used to characterise the water saturation and spatial distribution of the coal samples, represented positive and negative exponential functions of the water immersion time, respectively. This indicated that the water saturation gradually increased and became more uniformly distributed. Furthermore, based on the set threshold value of the target variable rate of change, the limiting expectation of the pixel greyscale values was obtained, and the limiting water absorption time of the coal sample was predicted. The water diffusion equation was then used to characterise the water absorption process of the coal samples, and a water diffusion model was developed to accurately obtain the wet ring boundary data. A reasonable value of the diffusion coefficient was determined by comparing and correcting the results of the numerical simulation and physical experiments with full consideration of the non-homogeneity of the numerical model. This water diffusion model can better characterise the water transport phenomena in the macroscopic barrier zone of coal pillar dams. Finally, the application prospects in terms of practical engineering were investigated.

The authors wish to correct the affiliation of co-author Guangyao Si, due to name changes of which he was unaware during his leave of absence

As the key load-bearing structure of a coal mine underground reservoir, the coal pillar dam body is subjected to the long-term coupling effect of overburden stress and mine water erosion. To investigate the coupled mechanical-seepage evolution mechanism of coal-rock under mining-induced stress, triaxial compression-seepage tests were conducted on coal samples taken from the coal pillar dam body of the underground reservoir in Chahasu Coal Mine. The effects of confining pressure and stress loading path on the mechanical deformation and permeability evolution of coal-rock were analyzed, a damage-permeability model incorporating fracture compaction, damage-induced cracking, and matrix deformation was established, and sensitivity analysis of key parameters was performed. Additionally, the model’s predictive accuracy was verified against experimental data, and its overall characterization capability was further assessed. The results show that: (1) The peak axial stress, peak deviatoric stress and elastic modulus of coal samples increase exponentially with increasing confining pressure. A distinct inflection point is observed in the stress–strain curve under the unloading confining pressure-axial loading (UCP-AL) and constant confining pressure-axial loading (CCP-AL) paths, with the stress increasing at a markedly higher rate before the inflection point than after it. (2) Under different stress loading paths, the permeability of the coal samples first decreases, then slowly increases, and then rapidly increases with increasing axial strain, and it shows higher sensitivity to deviatoric stress. The minimum permeability exhibits an exponentially decreasing trend with increasing confining pressure. (3) A damage-permeability model incorporating a Logistic-type damage variable was developed to continuously characterize the permeability evolution of coal specimens over the entire loading process from elastic deformation to macroscopic failure. Furthermore, the key model parameters (ω, ζ, and τ) are strongly correlated with confining pressure. (4) The single-stress-parameter model exhibits limitations in characterizing permeability evolution under triaxial stress conditions. To address this deficiency, a multi-parameter modified damage-permeability model was established, in which the effective total stress is used to describe the fracture compaction effect, while the effective deviatoric stress accounts for the damage-induced cracking effect. These laboratory-scale findings provide an experimental and theoretical basis for understanding the stress–damage–seepage evolution of coal. Further upscaling and calibration using field monitoring data (piezometric and drainage measurements from underground reservoir) is required before its application in seepage prediction and stability evaluation of in situ coal pillar dams.