To further raise the gas extraction efficiency of the tight sandstone, the liquid nitrogen (LN₂) freezing-thawing cycles method can be employed to improve the permeability of the low-permeability reservoirs. Permeability is generally regarded as a macroscopic description of the pore structure and usually has functional relationship to pore structure properties. The permeability of the rock is closely related to the change of microscopic pore structure. The permeability of rock depends on how the subzero temperatures changed the microscopic pore structure of rock, but it has not yet been confirmed obviously. In this study, the nuclear magnetic resonance (NMR) technique was adopted to investigate the pore structure evolution law and permeability of the tight sandstone with different LN₂ freezing-thawing cycles. The results demonstrate that the LN₂ freezing-thawing cycles promotes pore development and micro-fracture connection, and enhances the pore connectivity. The proportion of meso-pores, macro-pores and micro-fractures in the sandstone samples increases significantly, which provides a channel for the sandstone gas flow and extraction. Total porosity and effective porosity of the samples present a trend of rapid increase as the number of LN₂ freezing-thawing cycles increasing, while the residual porosity decreases as the number of LN₂ freezing-thawing cycles increasing. Coates model, SDR model (mean T₂ model) and PP model were used to calculate and evaluate the permeability of the samples subjected to different LN₂ freezing-thawing cycles. Furthermore, PP model can provide a better permeability estimate than the classical Coates and SDR model.

The acoustic emission (AE) characteristics of rock during loading can reflect the law of crack propagation and evolution in the rock. In order to study the fracture mode in the process of rock fracture, the AE characteristics and crack types of red sandstone during fracture were investigated by conducting Brazilian indirect tensile tests (BITT), direct shear tests (DST), and uniaxial compression tests (UCT). The evolution law of AE event rate, RA and AF values, and the distribution law of RA–AF data of red sandstone samples in three test types were analyzed. Based on the kernel density estimation (KDE) function and the coupling AE parameters (RA–AF values) in DST and BITT, the relatively objective dividing line for classifying tensile and shear cracks was discussed, and the dividing line was applied to the analysis of fracture source evolution and the failure precursor of red sandstone. The results show that the dividing line for classifying tensile and shear cracks of red sandstone is AF = 93RA + 75. Under uniaxial compression loading, the fracture source of red sandstone is primarily shear source in the initial phase of loading and tensile source in the critical failure phase, and the number is far greater than shear source. K = AF/(93RA + 75) can be defined as the AE parameter index, and its coefficient of variation CV (k) can be used as the failure judgment index of red sandstone. When CV (k) < 1, it can be considered that red sandstone enters the instability failure phase.

Experimental research on the growth of internal flaws has rarely been reported due to the fact that it is difficult to cut internal flaws in specimens and cannot capture the initiation and propagation processes of internal flaws through direct observations. This paper proposed a method for creating internal flaws in specimens by utilizing the volatilization of camphor. A series of compression tests were performed on rock-like specimens including two embedded circular flaws, and CT techniques were used to investigate the internal damage behavior of flawed specimens. Experimental results illustrate that the strength and deformation properties of flawed specimens increase nonlinearly with the confining pressure as well as flaw inclination angle. Crack coalescence patterns and failure modes of flawed specimens depend on not only the confining pressure but also the flaw inclination angle. The crack coalescence pattern varies from wing crack coalescence to mixed tension-shear crack coalescence and then to the shear crack coalescence as the crack inclination angle increases. Confining pressure contributes to shear crack growth and has an inhibiting effect on the propagation of tension cracks. For specimens with the same flaw inclination angle, the failure mode changed from tension failure to mixed shear-tension failure or from mixed shear-tension failure to pure shear failure with the increase of confining pressure.

The evolution of rock permeability has been studied exhaustively, and a broad array of permeability models has been proposed. These models are normally derived under the assumption of elastic deformation when subjected to external stress. Under this assumption, these models define fracture permeability as a function of either gas pressure or effective stress. However, experimental observations indicate that rock fracture may experience unrecoverable deformation during the loading process. The goal of this study is to resolve this contradiction. In this study, derivation of fracture permeability correlation for elastoplastic contact of rough surfaces is presented. The proposed method for describing permeability evolution not only considers the topography of fracture surfaces but also and, more importantly, integrates the plastic deformation of rock fracture. Subsequently, the deformation and permeability change of shale sample containing a single rough‐walled fracture is experimentally investigated. The results show that the permeabilities obtained during the loading process are larger than the permeabilities obtained during the unloading process under the same stress conditions and that the fracture deformation cannot be fully recovered during the unloading process. At last, the proposed permeability model is applied to the experimental results and it is shown that the proposed model can predict the laboratory permeability data.