I. Introduction The energy demand has increased significantly, and unconventional oil and gas resources such as shale oil and shale gas have become important alternatives. The complex geological conditions and stress environments of unconventional reservoirs deeply affect the behavior and characteristics of shale. Deviatoric stress is the main cause of rock deformation and damage, but its influence on the seepage characteristics of shale has not been fully studied. Comsol, as a powerful multi - physics simulation software, provides a powerful tool for studying shale micro - damage and seepage. Combining CT image processing and three - dimensional digital reconstruction techniques can accurately simulate the shale microstructure and pore distribution, and then study the damage evolution and fluid - flow behavior under deviatoric stress.
II. Research Methods CT Image Processing and Three - Dimensional Digital Reconstruction Advanced CT scanning technology is used to image reservoir shale to obtain high - precision two - dimensional images, and then they are transformed into three - dimensional digital core models through specific image processing algorithms. When reconstructing, the bedding structure and pore distribution characteristics of shale are fully considered to ensure the accuracy of the model. Water - Force - Damage Coupling Control Equation The water - force - damage coupling control equation is constructed for the digital core composed of pores and matrix. This equation couples the fluid - flow, rock mechanics and damage evolution processes, and can accurately describe the shale micro - damage and seepage characteristics under deviatoric stress. Comsol Simulation Settings In the Comsol software, physical fields such as stress and fluid - flow and boundary conditions are set, and the water - force - damage coupling control equation is solved by numerical simulation to obtain the stress distribution, damage distribution, fluid velocity distribution and permeability evolution of digital cores under different deviatoric stresses.
III. Results and Analysis Micro - damage Evolution The simulation results show that when the deviatoric stress increases from 0 to 50 MPa, the deformation and damage degrees of the digital core increase. The damage and deformation in the pore area are much larger than those in the matrix area, which is in line with the shale microstructure characteristics. Deviatoric stress causes the expansion of internal cracks in the rock and the increase of pore connectivity, intensifying the micro - damage evolution. Fluid Velocity Distribution The fluid velocity distribution differences between the matrix area and the pore area are significant. In digital core D1, the fluid velocity in the pore area is 10 - 13 times that in the matrix area; in digital core D2, the fluid velocity in the pore area is 100 - 250 times that in the matrix area. Sheet - like connected cracks significantly deflect the fluid flow lines, promote the fluid - flow in the pore area, and lead to the non - uniform fluid velocity distribution. Pressure Distribution The internal pressure distribution of the digital core changes under the action of deviatoric stress. The pressure gradient in the pore area is large, and the fluid is more likely to flow; the pressure gradient in the matrix area is small, and the fluid - flow is relatively difficult. This non - uniform pressure distribution further affects the fluid seepage behavior. Permeability Evolution The permeabilities of digital cores D1 and D2 first decrease due to rock deformation and then increase due to the increase of damage as the deviatoric stress increases. It shows that the influence of deviatoric stress on shale permeability is complex and is closely related to rock deformation and damage evolution.
IV. Conclusions Through the Comsol damage model research, the micro - damage and seepage characteristics of reservoir shale under deviatoric stress are deeply understood. CT image processing and three - dimensional digital reconstruction techniques lay the foundation for accurately simulating the shale microstructure. The water - force - damage coupling control equation can comprehensively consider the interactions of multiple physical fields. The research shows that deviatoric stress has a great influence on shale micro - damage and seepage behavior. The damage and deformation in the pore area are more serious, the fluid velocity in the pore area is significantly higher than that in the matrix area, the promoting effect of sheet - like connected cracks on fluid - flow cannot be ignored, and the permeability evolution first decreases and then increases, providing an important reference basis for the development of shale oil and gas resources. Future research can expand the application scope of the Comsol model, consider the influence of more factors such as temperature and chemical substances on shale micro - damage and seepage, and combine experimental research to verify and improve the simulation results, so as to provide stronger support for the efficient development of shale oil and gas resources.