Understanding the mechanical behavior of a reservoir rock within the CO₂-injection zone is essential for the safety of CO₂ geological storage (CGS). The water saturation of the reservoir rock could be changed with the injection of huge amounts of supercritical CO₂ (scCO₂). Triaxial compression tests were performed on sandstone under dry, H₂O-injected, scCO₂-injected, and CO₂–H₂O biphasic conditions at effective confining pressures ranging from 10 to 50 MPa to investigate the mechanical behavior of the sandstone within the CO₂-injection zone comprehensively. The results indicated that the strength of the sandstone was significantly reduced by H₂O, scCO₂, and CO₂­–H₂O biphasic fluids. The CO₂–H₂O biphasic fluid reduced the strength most significantly, followed by H₂O, and finally scCO₂. Water/scCO₂ monophasic and CO₂–H₂O biphasic fluids significantly affected the cohesion of the sandstone, but they did not affect the internal friction angle. Compared with the strength of the H₂O-injected sample, the strength of the CO₂–H₂O biphasic sample was reduced by −6.94%, 16.94%, and 3.28% at 10, 30, and 50 MPa, respectively. The CO₂–H₂O biphasic fluid significantly deteriorated the deformation modulus and stiffness and enhanced the ductility and compaction of the sandstone. A higher confining pressure could inhibit the enhanced cracking effect of scCO₂. The CO₂–H₂O biphasic zone within the reservoir was the most dangerous because faulting, surface subsidence, and permeability reduction of formation might occur in this zone. The results provided new information on the evolution of mechanical changes in saline aquifers injected with scCO₂ in CGS.

Currently, p-y models have been broadly adopted for estimating the lateral bearing capacity of large-diameter monopiles in offshore engineering. However, the existing p-y curves cannot reflect the effect of pile diameter D on the lateral response of monopiles under local scouring conditions. In order to extend the existing p-y model to a large-diameter pile, a well-calibrated three-dimensional pile-soil model performed by ABAQUS is used to study the effect of the pile diameter D on the ultimate soil resistance P and the initial stiffness k of the Lin’s p-y model. Based on the numerical simulation results, two diameter-related parameters A and B, which represent the relationships between the ultimate soil resistance and the initial stiffness obtained by the numerical model and the Lin’s p-y model, respectively, are proposed and introduced into the Lin’s p-y model to present a modified p-y model. The comparison results show that the proposed modified p-y model is capable of providing a better estimation of the lateral response of large-diameter monopiles than the existing p-y model under a scouring condition.

The influence of pore fluid pressure on the normal deformation behaviors of joints is vital for understanding the interaction between hydraulic and mechanical processes of joints. The effect of pore fluid pressure on the normal deformation of a granite matched joint was investigated by laboratory experiments. Experimental results indicate pore fluid pressure significantly affects the normal deformation of jointed sample, and the relative normal deformation of jointed sample during fluid injection consists of the opening of the joint and the dilation of host rock. The action of pore fluid pressure on the joint follows the Terzaghi’s effective stress law. The normal deformation of the joint can be well quantitated by the generalized exponential model. The relative normal deformation of host rock during fluid injection would have a linear relationship with pore fluid pressure, and if affected by gas is more pronounced than water.

Understanding the mechanical behavior of a reservoir is essential for securing the safety of CO₂ storage projects. In this study, the effect of CO₂-H₂O biphasic fluid on the mechanical behavior and failure characteristics of sandstone was revealed. The preliminary results indicate that CO₂-H₂O biphasic fluid could significantly affect the mechanical behavior of sandstone, including reducing the strength, elastic modulus, Poisson’s ratio, and bulk modulus of sandstone; enhancing the bulk compaction; and postponing the onset dilatancy of sandstone. However, the strength of water-bearing sandstone is weakly increased when CO₂ is injected into the sample due to the capillary forces that develop under the unsaturated conditions. In addition, CO₂-H₂O biphasic fluid notably affects the failure characteristics of sandstone at macro- and microscales. A complex network of cracks is generated at macrostructure scale. The bonds between particles are damaged by CO₂-H₂O biphasic fluid at the microstructure scale.