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.

The underground gas storage (UGS) in depleted sandstone reservoirs forms the largest proportion of the UGS market in China. Multiple cycles of natural gas injection and production in the sandstone cause the rapid increase and drawdown of pore pressure, which may induce damage to the rock skeleton structure, and cause complex fluid flow paths in the sandstone reservoir. In this paper, transverse relaxation time (T2), nuclear magnetism resonance imaging, and high-pressure mercury intrusion analysis are combined to evaluate the variation in pore structure of medium-grained sandstone. The results show that cyclic injection and production of fluid leads to a slight increase in total pore volume, indicating that weak damage to rocks occurs. The T2 spectrum at the low pore pressure (10 MPa) and high pore pressure (25 MPa) both show that the shrinkage of the medium-size pores occurs after multiple cycles of injection and production. The pore volume of large-size pores was not highly correlated with the number of cycles. With the increase in pore pressure, the pore volume ratio under high pore pressure increased with the number of cycles, while it fluctuated strongly under low pore pressure.

There are many inter-salt rhythmic shale reservoirs in Qianjiang sag, and the mineral composition content with different rhythms is different. The thin interbedding characteristics of inter-salt shale oil reservoirs bring technical challenges to hydraulic fracturing. Taking one shale oil well in Qianjiang depression as an example, the mechanical properties and interface characteristics of rock under temperature and confining pressure are analyzed. The physical simulation test of fracture propagation under different fracturing fluid is completed, and the effects of four different factors on fracture propagation are analyzed by numerical analysis method. The results show that the mechanical characteristic and failure modes with different rhythms are obvious differences. Under uniaxial and triaxial compression, glauberite mudstone and shale have high strength, and salt rock shows obvious plastic deformation characteristics. The interbedded rock has clear interface characteristics. The cohesion of glauberite mudstone and shale bedding surface obtained from direct shear test is 0.60 MPa and 0.99 MPa. The fracture morphology of inter-salt shale is mainly affected by the development degree of rock bedding. The mechanical parameters, in situ stress difference, and the displacement have an important impact on the longitudinal propagation of fracturing fractures. The width and height of fracture propagation decrease, with the increase in the minimum horizontal principal stress in the salt layer, and the width of fracture in shale increases. The crack height decreases with the increase in tensile strength of the interlayer. With the increase in fracturing fluid injection rate from 3.0 to 7.0 ml/min, the propagation height of hydraulic fractures and the width of fractures in shale increase significantly. The research results can apply to understanding the mechanism of hydraulic fracture propagation in inter-salt shale formation.