As the main reservoir of oil and gas resources, the physical and mechanical properties of the rock are complicated. The strong heterogeneity of rock and the flow in porous media further influences the nature of stress propagation. Therefore, considering the heterogeneous particle model and fluid-structure coupling, it is very important to study the variation law of rock mechanical properties under a stress state, which lays a foundation for formation fracturing and mining. In this paper, the mechanical properties of the rock are studied by the ideal, heterogeneous, and flow models in porous media. The results show that the stress of the contact surface between particles of the same size is large. Still, the stress of the contact surface between different particles is larger, which is more likely to become the weak mechanical link of failure. When the displacement of the z-axis is the same, the x and y-axis increase the axial stress of the z-axis. The peak stress is high and more sensitive when the particle size difference is small. When water is in the pores, the rock strength obtained under consolidated undrained is greater than that under consolidated drained.

Complex fractures and pore structures in the rock strongly influence the mechanical properties, and the process from compression to failure is complicated. Under the action of rock stress, pore structure deformation and fractures close or propagate, easily leading to deterioration in the rock mechanical properties until rock failure. Thus, the effects of microscale factors are critical in mechanical properties such as rock strength, elastic modulus, and stress–strain state under the triaxial stress state. It is difficult for physical and mechanical experiments to obtain the qualitative rules of regular structures, but numerical simulation can make up for this defect. In this work, the accuracy of the model was proven through a comparison with previous experimental results. The true triaxial numerical simulation experiments were conducted on representative rocks and natural pore structures. These simulated results revealed that the pore and throat parameters will change abruptly when the particle model volumetric strain is between 0.0108 and 0.0157. When the fracture angle is between 45° and 75°, the fracture has a great influence on the peak stress. The angle between the natural fracture and the fracturing direction should be less than 45° as much as possible. Clay affects the rock strength by influencing the force chains formed by the rock skeleton. Fracturing is easier when the structural clay content is higher than 25%. It is easier to fracture in a direction parallel to the laminated clay when the clay content is below 27%. This work indicates the effects of rock particles, fractures, and clay on the mechanical parameters, providing key fundamental data for further quantifying the fracturing patterns.