The fluid–shale interaction during carbon dioxide (CO₂) enhanced shale gas recovery processes may have significant influences on the mechanical behaviour of shale, and it is also of great importance for the long-term safety of CO₂ geological sequestration in shale formations. In this work, shale samples from the Longmaxi formation in the Sichuan basin were used to investigate the mechanical properties and the acoustic emission (AE) behaviour of shale upon saturation with different fluids (water, brine, and CO₂–brine mixture with different CO₂ phase states) at 45 °C. X-ray fluorescence (XRF) analysis results show that major element alterations occur after shale is treated with supercritical CO₂–brine and subcritical CO₂–brine, the element content of Ca, K and Al in supercritical CO₂–brine saturated shale was decreased by 16.7%, 5.8% and 6.3%, respectively. Field emission scanning electron microscopy (FESEM) analysis results indicate that significant surface structural changes occur in shale after saturated with CO₂ and brine. Furthermore, the uniaxial compressive strength (UCS) and elastic modulus (E) of shale with different fluid saturation were decreased. Supercritical CO₂ cause a greater reduction of UCS and E compared to subcritical CO₂ due to its higher adsorption capability and larger CO₂ adsorption induced swelling. The maximum reduction in UCS and E of shale was observed in the case of CO₂–brine saturated samples, and the influence of the CO₂ phase on the mechanical behaviour of shale under brine conditions is not negligible. The fracture propagation pattern changes in shale after fluid saturation are also explained by the AE analysis.

Moisture is one of the most important factors that influences coal seepage and coal-bed methane (CBM) extraction.

The interaction between shale and various fluids is crucial as it modifies pore structures, which govern the effective development of shale gas and the geological storage of carbon dioxide in shale formations. In this study, samples from the Longmaxi Formation shale in Sichuan Basin of China were exposed to different fluids, including 6 MPa CO₂, 12 MPa CO₂, 6 MPa CO₂+brine, and 12 MPa CO₂+brine, at 45 °C for 100 days. Various methods, including X-ray diffraction (XRD), X-ray fluorescence (XRF), field-emission scanning electron microscopy (FESEM), and the low-pressure gas adsorption (N₂) test, were adopted to evaluate chemical and structural changes during the exposure process. After being treated with supercritical CO₂+brine and subcritical CO₂+brine, the shale underwent significant changes in its major element composition. The content of Ca, Al, and K in shale saturated with supercritical CO₂+brine decreased from 13.00% to 10.34%, from 3.65% to 3.36%, and from 1.56% to 1.37%, respectively. Meanwhile, the content of Si and Na in the same shale increased slightly after saturation. The amount of quartz and dolomite increased, while the levels of clay and calcite slightly decreased. The surface of the shale sample became rougher and small bumps and cracks appeared after saturation with different fluids, as shown by the FESEM analysis results. Furthermore, the changes in both the total pore volume and pore size followed a similar pattern to the alterations in the specific surface areas. The highest level of variation occurred with the shale that was saturated with 12 MPa of CO₂, indicating that gas pressure and CO₂ phase state have a significant influence on the shale’s pore structure. In addition, the distribution of pore sizes showed a bias towards larger sizes across all diameters; this suggests that the reaction resulted in a decrease in the number of micropores. This also highlights that the impact of varying fluid saturation was primarily focused on micropores and macropores. The results of this study provided experimental evidence to further test the mechanisms and permeability of geological storage of CO₂ in organic-rich self-sourced shale.

A new model is established to analyze mining stope stability, using variable weight theory to calculate the index weight for each factor in different stopes and unascertained measure evaluation technique to predict the risk grade of stope stability. In this model, an evaluation index system by virtue of the 7 most important factors is established, including rock saturated uniaxial compressive strength, rock quality designation, rock joint and fissure, stope span, condition of pillar, groundwater seepage volume, and rate of supporting pit roof. And each index is divided into 5 grades by assignment value and the classification method of standardization. Accordingly, the analysis result is also classified into 5 risk grades. This model is used for the 6 main stopes from the -270 m section in Xin-Qiao Mine, China. The results, giving risk grade for each stope and guiding the use of corresponding measures, avoided the problem of state out of balance caused by conventional invariable weight theory models and have ensured no accident occurred in mining production in recent years. This model can be used in other mines widely, by assigning values for the 7 factors on basis of current in situ cases.

To study the influence of slenderness ratio effect on the mechanical behavior, acoustic emission properties, and energy evolution of sandstone, the uniaxial compression tests coupled with acoustic emission technology are carried out at different slenderness ratios D (0.5, 1.0, 1.5, 2.0, 3.0). The results show that a logarithmic function relationship is observed between the peak strength, the peak strain, and the elastic modulus with slenderness ratio. The failure patterns of the tested sandstone varied significantly with the increasing slenderness ratio. When the slenderness ratio, D, is lower than 1.5, complex failures and multiple shear planes are formed, while simple failures and single shear planes are generated at D larger than 1.5. Besides, the AE ringing counts are more obvious with a higher slenderness ratio, D, at the initial compression stage due to the greater body volume and more defects in the sandstone. The energy evolution curves and energy ratio distribution curves can be divided into four stages, corresponding to the stress‐strain curves.