Multiscale digital core fluid–solid damage coupling simulation for deep reservoirs based on free-flow and seepage coupling

Abstract

The efficient exploitation of deep energy resources and secure CO2 sequestration hinge on understanding fluid–solid damage coupling in reservoir rocks. However, existing pore-scale models often overlook the multiscale nature of pore systems and the evolution of rock damage. To address this, we develop a multiscale fluid–solid damage coupling simulation method based on digital core technology and continuum damage theory. By fusing high-resolution SEM images with large-field-of-view CT scans via machine learning, we construct a digital core that explicitly represents both micrometer-scale macropores (from CT) and four categories of sub-resolution matrix pores (from SEM). The Darcy-Brinkman-Stokes equation couples free flow and seepage, while the Span-Wagner equation captures the properties of supercritical CO2. Microstructural analysis shows total damage is most correlated with micropore fraction (R2 = 0.95), while apparent permeability strongly correlates with average micropore size (R2 = 0.89). The results reveal that confining pressure critically regulates the damage mechanism. When the confining pressure is below 60 MPa, total damage decreases and tensile failure dominates; when the confining pressure exceeds 60 MPa, total damage increases because shear failure rises significantly. Consequently, permeability loss can reach 63.7% under high confining stress. Micrometer-scale macropores dominate flow. Well-connected Category 1 matrix pores show flow velocities 298 times higher than the least connected matrix pore category, demonstrating the strong control of pore connectivity on microscale flow dynamics. Furthermore, increasing CO2 injection pressure reduces effective stress, enhancing tensile damage and apparent permeability by up to 37%, but elevated fluid viscosity creates a competing resistance that can suppress flow velocities. This work provides a quantitative framework for predicting flow capacity and damage evolution in deep reservoirs under multi-field coupling conditions.