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The different directions of joints in rock will lead to great differences in damage evolution characteristics. This study utilizes DIP (digital image processing) technology for characterizing the mesostructure of sandstone and combines DIP technology with RFPA2D. The mesoscale fracture mechanics behavior of 7 groups of jointed sandstones with various dip angles was numerically studied, and its reliability was verified through theoretical analysis. According to digital image storage principle and box dimension theory, the box dimension algorithm of rock mesoscale fracture is written in MATLAB, the calculation method of fractal dimension of mesoscale fracture was proposed, and the corresponding relationship between mesoscale fractal dimension and fracture damage degree was established. Studies have shown that compressive strength as well as elastic modulus of sandstone leads to a U-shaped change when joint dip increases. There are a total of six final failure modes of joint samples with different inclination angles. Failure mode and damage degree can be quantified by D (fractal dimension) and ω (mesoscale fracture damage degree), respectively. The larger the ω, the more serious the damage, and the greater the D, the more complex the failure mode. Accumulative AE energy increases exponentially with the increase of loading step, and the growth process can be divided into gentle period, acceleration period, and surge period. The mesoscale fracture damage calculation based on the fractal dimension can be utilized for quantitatively evaluating the spatial distribution characteristics of mesoscale fracture, which provides a new way to study the law of rock damage evolution.

Evaluation of the performance of existing support in underground tunnels is of great importance for production and interests. Based on field investigation, the shape and number of joints and fractures were investigated in the mining area. Then, the stability of each structural blocks is analyzed by 3D wedge stability analysis software (Unwedge). Moreover, a new analysis method based on critical block theory is applied to analyze the stability of excavated laneways in continuous and discontinuous rock and monitor the stress changes in a fractured tunnel rock mass. The test results indicate that the 3D wedge stability analysis software for underground excavation can evaluate deep tunnel support. Besides, there is no direct relation between the size of the block and the instability of the tunnel. The support method, on large and thick key blocks, needs to be improved. In a broken tunnel section, U-shaped steel support can effectively promote the stress state of the surrounding rock. By monitoring the surrounding rock, it is proven that the vibrating string anchor stress monitoring system is an efficient and real-time method for tunnel stability evaluation.

To explore the influence of brittle mineral composition and morphological characteristics within the shale matrix on the generation of complex fractures and reservoir reconstruction, the microstructure and fractal characteristics of minerals within the shale matrix of the Lower Cambrian Niutitang formation in northern Guizhou were investigated using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectrometer (SEM-EDS), and micro-computed tomography (Micro-CT). Based on image processing technology and digital core modelling, a three-dimensional shale digital core model was established. The distribution and morphological characteristics of brittle minerals within the shale matrix were studied from the fractal perspective. The model for evaluating the mineral particles and pores within the shale matrix was established. The results show that the siliceous mineral composition is high, and it is mainly derived from the interaction of continental detrital quartz particles and biogenetic effects; the density of the brittle minerals is greater than other mineral components mainly distributed in the range of equivalent diameters of 200–800 μm; the distribution complexity of the brittle minerals was quantitatively analysed from the fractal perspective; the fractal dimension partly reflects the generating ability of complex fractures; the distribution angle and complexity of brittle mineral particles within the shale matrix partly determine the fracture morphology characteristics. The physical properties of shale reservoirs were studied from three aspects: complex fracture generation ability, pore connectivity and heterogeneity, and the evaluation model of mineral particles and pores was established using the value of fractal dimension, ratio of organic matter pores, tortuosity of pores, relative deviation of pore size distribution, and relative deviation of specific surface area. The model provides a basis for further study on the relationship between reservoir reconstruction potential and physical properties of shale reservoirs.

The Lower Cambrian Niutitang Formation is one of the most important shale reservoirs in the Northern Guizhou. To study the characteristics of pore structures and the relationships between shale's physical properties and pore structures in the complex tectonic area, from the perspective of hydrocarbon generation and evolution, the mineral composition, pore structure characteristics of shale samples in northern Guizhou were analyzed using X-ray diffraction (XRD), optical microscopic composition analysis, focused ion beam-scanning electron microscopy (FIB-SEM), vitrinite reflectivity and nitrogen adsorption experiments. Results indicate that the pore structures can be categorized into conical, ink bottle, spherical, elliptical, beaded, irregular, and plate-like intersecting microfractures. Shale samples are organic-rich with an average total organic carbon content of 4.39%. Organic matter is consumed within the shale matrix during the hydrocarbon generation and evolution, which increases the organic pore size and the proportion of organic pores. The high TOC content is conducive to developing organic pores. In addition, the average content of brittle minerals is greater than 70%. High content of brittle minerals during diagenesis is conducive to developing the residual pores at the edges of mineral particles and dissolved pores within mineral particles. Thus, the high content of brittle minerals and TOC content is favorable for the development of meso-macropores. The content of clay minerals within the shale samples ranges from 6.3% to 42.2%, with an average of 20.02%. Micropores and mesopores mainly occur in the clay minerals. Pores in clay minerals are easily compacted under tectonic stress. Meanwhile, high thermal evolution degree and the vitrinite reflectance Ro greater than 3.5 is not conducive to developing pores. Hence, high TOC content, high brittle minerals content, and thermal evolution degree ranging from 0.6% to 3% make the Lower Cambrian Niutitang Formation shale reservoirs favorable for shale gas exploration.

The Cambrian Niutitang Formation shale reservoirs are widely distributed in the complex tectonic area in northern Guizhou. Fifteen organic-rich Niutitang Formation cores were selected from the tectonic deformation zones and tectonic stability zone in northern Guizhou, including the FC1 well related to fold, TM1 well related to faults, and TX1 well in the tectonic stability zone. Pore structures and reservoir physical properties of shale were tested via microphysical experiments and low-temperature nitrogen adsorption (LTNA) test. Results indicate that the thermal evolution maturity of shale is in the over mature stage, with RO distributed between 2.30 and 3.74% and the total organic carbon content (TOC) between 3.8 and 7.5%. Shale cores in the tectonic stability zone have higher TOC content (6.25%) than that of shale (5.16%) in the tectonic deformation zones. Shale cores in the tectonic deformation zones have higher thermal evolution maturity Ro (2.98%) than that of shale (2.76%) in the tectonic stability zone. The pore size of shale cores ranges from 4.15 to 15.62 nm. The total pore volume of shale cores ranges from 9.71 to 33.20 cm3/kg, and the meso-macropore volume ranges from 7.4 to 28.21 cm3/kg. Shale cores in the tectonic deformation zones have higher average pore size, total pore volume, and meso-macropore volume percentage than that of shale cores in the tectonic stability zone. Meanwhile, the specific surface area of shale cores (SBET) ranges from 7.34 to 30.37 m2/g, and the micropore volume of shale cores ranges from 2.29 to 5.82 m3/kg. Shale cores in the tectonic stability zone have higher micropore volume, specific surface area, and percentage of micropore volume than that of shale cores in the tectonic deformation zones. Shale cores in the tectonic stability zone mainly develop micropores and mesopores. Mesopores and macropores are more susceptible to geological tectonism than micropores. The pore structures of shale in the tectonic stability zone have been effectively preserved during tectonic movements. The research results have certain guiding significance for the exploration of shale gas in the complex tectonic regions.