Study on the deterioration and damage evolution characteristics of mechanical properties of siltstone after supercritical CO2 treatment

To quantitatively represent the deterioration law of the mechanical properties of siltstone after exposure to supercritical CO2(SC-CO2), experiments were conducted to test the mechanical parameters of rocks under the coupling effect of CO2–water–rock interactions. The purpose of this study is to determine the evolution law and damage characteristics of each siltstone mechanical parameter under the action of SC-CO2. In combination with numerical simulation results, this study revealed the damage evolution characteristics of rocks in a CO2 geological storage reservoir. The research findings indicate that SC-CO2 exhibits a certain time effect and causes non-uniform damage to siltstone. Both the deterioration law of the mechanical parameters and soaking time exhibited an exponential relationship. With increasing soaking time, both the uniaxial compressive strength and elastic modulus showed a decreasing tendency, whereas the Poisson's ratio showed a varying degree of increase. The total degree of deterioration gradually increased as the soaking time increased and approached a stable value. After exposure to SC-CO2, the damage deformation and failure mode transitioned from brittle to plastic in the siltstone samples. In addition, there was a shift from a single axial crack failure pattern to a complex failure mode with multiple cracks. This study also derived a uniaxial compressive strength prediction model for siltstone based on soaking time, constructed an elastic modulus damage model, and established a fluid–solid coupling model considering damage in the CO2 geological storage process. The experimental and numerical simulation results revealed that microstructural alterations occurred in the siltstone after exposure to SC-CO2. Furthermore, it was discovered that damage accumulation from the micro- to macroscale ultimately led to a gradual deterioration in mechanical properties. The research results provide a theoretical basis for mechanical stability evaluation in CO2 geological storage.