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Low-permeability sandstone reservoirs have been widely used as a gas storage medium worldwide. Compared with the high porosity and high permeability of sandstone, low-permeability sandstone may present different mechanical (deformation, damage or failure) and acoustic responses under cyclic loading-unloading processes caused by the high-rate injection–production of underground gas storage. In this paper, multistage triaxial loading–unloading tests with a continuously increased upper limit of stress were carried out on low-permeability sandstone under six different confining pressures. The results showed that the superposition of stress–strain curves become much denser in the process of each level of stress. Based on the variation of the elastic modulus of low-permeability sandstone under alternating loads, the mechanical behavior of low-permeability sandstone under cyclic loading is divided into three stages: cyclic hardening, stability and cyclic softening. According to the evolution of acoustic emission (AE) signal parameters, AE counts appear intensively at the initial stage of each level of stress and then gradually stabilize. The peak frequency presents the zonal distribution, which is divided into low-frequency, intermediate-frequency and high-frequency zones. Low confining pressure leads to a small b-value. The RA–AF distribution implies that the mixed tensile–shear cracks are continuously generated in low-permeability sandstone during the cyclic loading process, and the shear cracks are more obviously developed. [ABSTRACT FROM AUTHOR]

With the increasing scale and depth of underground engineering, the geological environment that engineering is faced with is becoming more complex. As the weak position of rock mass, the structural surface has a particularly great influence on the mechanical characteristics of the rock mass. In order to obtain the shear strength characteristic of the structural plane and analyze the influence of morphological parameters such as the undulating angle and bulge degree on shearing, taking medium-low permeability tight sandstone as the research object, four kinds of structural plane samples with different undulating angles (10, 20, 30 and 40°) were prepared with a Python and high-precision engraving machine. Direct shear tests under different normal stresses (2, 4, 6 and 8 MPa) and shear rates (0.6, 1.2 and 2.4 mm/min) were performed, and the shear mechanical properties were analyzed. The structural surfaces before and after shearing were scanned using a high-precision three-dimensional scanner, so as to evaluate the roughness of the structural surface and determine the influence from various factors on the shear characteristics. The test results showed that for the structural plane with the same undulating angle, the peak shear stress increased approximately linearly with an increase in normal stress at a 0.6 mm/min shear rate and an increment speed of approximately 0.82, while the peak shear stress negatively correlated with the shear rate at a value of 4 MPa for normal stress. The larger the undulating angle was, the greater the influence of the shear rate (the shear stress decreased by 2.31 MPa at a 40° angle). When the normal stress and the shear rate were fixed, the peak shear stress corresponding to the structural surface gradually increased with the increase in the undulating angle, and the maximum increment was 5.04 MPa at 4 MPa normal stress and a 0.6 mm/min shear rate. An analysis of the morphological characteristics of the structural plane showed that when the undulating angle (40°) and the normal stress (6 and 8 MPa) were larger, the damage of the structural plane became more obvious, the shear point was closer to the tooth valley position, and the mechanical bite force and friction force of the structural plane were better utilized. When the shear rate was lower (0.6 mm/min), the friction characteristics of the shear surface were more visible, the shear was increasingly sufficient, and the corresponding shear strength was also greater. [ABSTRACT FROM AUTHOR]

In a casing-cement sheath-formation system, the cement–formation interface is usually weakly cemented for the residual of drilling mud, in which a leakage path would easily form, threatening the safe operation of underground energy exploitation and storage. To evaluate the compressive and shear mechanical behavior of the cement–formation interface, cement–rock composite cylindrical specimens were prepared. Uniaxial and triaxial compression and direct shear tests were implemented. The flushing efficiency of the rock surface, compressive strength, interface incompatible deformation, parameters of shear strength, and morphology of shear failure surface were acquired and analyzed. Results show that the flushing efficiency of shale surface decreases from 76.7% to 64.2% with the surface roughness increasing from 0 to 2 mm. The flushing efficiency of sandstone is only 44.7%, remarkably lower than that of shale. With the stress condition transforming from uniaxial to triaxial compression, the feature of the stress–strain curves changes from elastic-brittle to elastoplastic, and the compressive strength increases from 20.6~60.1 MPa to 110~120 MPa. The cement part presents noteworthy plastic deformation and several micro shear fractures develop. There is incompatible deformation between cement and rock, which induces interface debonding for almost all the composite specimens. The internal friction angle and cohesive strength both decrease with the increase in pollution degree of drilling mud, and increase with the rise in surface roughness. The shear facture surface is not exactly the rock–cement interface, but usually manifests as a shear zone, in which the rock, cement, and interface all contribute to the final shear failure. The above findings would be valuable for the revealing of cement–formation interface failure mechanism. [ABSTRACT FROM AUTHOR]

• A thermal-hydro-mechanical-damage coupling model is established to investigate the evolution process of hydraulic fractures. • The investigation was conducted on the effects of natural fracture aperture change after fluid injection. • The study focuses on the impact of the initial stress field on the evolution of formation damage after fluid injection. • The study examined the influence of the temperature of the injected fluid on damage evolution. • The impact of well location on damage progression was examined in the presence of natural fractures. Enhanced geothermal systems (EGSs), particularly hydraulic stimulation techniques, show promise in extracting high-enthalpy geothermal energy from hot dry rocks. However, creating an efficient volumetric fracture network in hot dry rocks remains challenging due to issues such as inadequate connectivity, significant water injection loss, and early thermal breakthrough. In this study, a damage variable is introduced into the improved thermal-hydro-mechanical coupled model to describe fracture propagation using damage evolution. Numerical simulations consider the effects offracture aperture, injection temperature, stress state, well position, etc. The results indicate that the damage zone tends to propagate parallel to the maximum primary stress in hot dry rocks. Additionally, the extent of rock damage due to thermal stress is directly proportional to the temperature difference between the fluid and the rock. The placement of double wells and the orientation of the maximum principal stress greatly affects the spatial distribution of the damaged zone under specific natural fracture network conditions. [ABSTRACT FROM AUTHOR]

Impure salt rock strata are extensively distributed in China, giving them great significance in the study of the physical properties of impure salt rock for the construction of underground gas storage in salt mines. To investigate the confining pressure and impurity effects on the mechanical properties and gas permeability characteristics of salt rock, permeability tests under hydrostatic confining pressure and conventional triaxial compression (CTC), on salt rock samples with different impurity contents, were carried out. The results demonstrate that the confining pressure effects cause an increase in triaxial compression strength, but a decrease in permeability. However, impurity enhances the bearing capacity and permeability of the salt rock; both rock strength and permeability increase with an increase in impurity content. Moreover, the broken salt rock specimens were analyzed after the CTC test using Computed Tomography (CT) equipment. To understand the relationships between pore volume and permeability, considering the confining pressure and impurity effects, the cracks were divided into four groups according to different crack diameter ranges: ~0.05 mm, 0.05–1 mm, 1–10 mm and ~10 mm. The CT analysis results show that while the pore volume of smaller cracks shows an "increasing–decreasing" trend by increasing confining pressure, the pore volume of large cracks gradually decreases, indicating that the rock permeability is highly related to macro-cracks. However, impurity has more complicated implications on rock permeability and cracks, and needs further investigation. [ABSTRACT FROM AUTHOR]

Mechanical stratigraphy can provide valuable knowledge for evaluating and predicting the distribution of structural fractures and in situ stress. The fracture stratigraphy and mechanical stratigraphy of the Triassic Yanchang Formation sandstone reservoirs in the Huaqing area of the Ordos Basin were investigated. In this study, borehole images were used to identify and characterize the orientation and properties of natural fractures, and array acoustic logs were used to analyze the rock mechanical properties. By considering the positional and scale cycles, the mechanical stratigraphy and areal fracture density at multiple scales were calculated, and the statistical correlation between them was analyzed. The results show that when the dynamic Young's modulus is less than 45 GPa, the areal fracture density generally increases with increasing Young's modulus; when the dynamic Young's modulus is greater than 48.5 GPa, the areal fracture density decreases with increasing dynamic Young's modulus. Additionally, the areal fracture density peaks when the dynamic Young's modulus is in the range of 45–48.5 GPa. In general, the dynamic Poisson's ratio is negatively correlated with the areal fracture density at multiple scales; when the dynamic Poisson's ratio is less than 0.24, the areal fracture density rapidly decreases as the dynamic Poisson's ratio increases; when the dynamic Poisson's ratio is greater than 0.24, the dynamic Poisson's ratio is only weakly correlated with the areal fracture density. The fitting results between the mechanical stratigraphy and areal fracture density show that the mathematical model relating these two parameters is related to the statistical scale. • A multiscale quantitative analysis of the fracture and mechanical stratigraphy is performed. • As Young's modulus increases, the fracture density first increases and then decreases. • With the increase in Poisson's ratio, the fracture density decreases, and the correlation between the two weakens. [ABSTRACT FROM AUTHOR]