Fractal Characteristics of Microscale CO2 Sequestration Damage in Glutenite Reservoirs Based on NMR

Abstract To reveal the impact of CO2 dissolution and storage on the reservoir of gravel rock, core samples from the Junggar Basin in China were subjected to CO2-brine saturation treatment. Nuclear Magnetic Resonance (NMR), CT scanning, and Scanning Electron Microscopy (SEM) were used to analyze changes in mineral composition and fluid storage after CO2 reaction. Additionally, fractal theory was employed to characterize the heterogeneity and complexity of the pore system. The results indicate that, in heterogeneous gravel rock, the destructive effect of mineral dissolution due to chemical reactions is more significant than the mineral swelling effect. Under CO2 exposure, the fractal dimensions and porosity at different scales tend to increase, which is mainly related to the content of minerals such as sodium feldspar, illite, and quartz. NMR T2 results show that the change in porosity after CO2-brine saturation is primarily due to the increase in the number of relatively large pores (characterized by fluid relaxation time T2 > 10 ms). Two-dimensional NMR T1-T2 further reveals an increase in the content and range of movable water within large pores. CO2 immersion reduced the fractal dimensions of gravel rock by nearly 2%, indicating a decrease in pore surface roughness and structural complexity. This study contributes to understanding the evolution of gravel rock microstructures under geological conditions during long-term CO2 storage.