Acid-Induced fracture and pore structure evolution in basalt

Basalt has emerged as a promising candidate for in-situ mineral carbonation in CCUS applications. Developing fracture and pore networks in basalt formations is crucial for facilitating CO₂-rock interactions by increasing the reactive surface area. This study investigates the mechanistic effects of acid-induced weakening on fracture propagation and pore structure evolution in basalt. Notched semi-circular bend (NSCB) tests were conducted under various pH conditions, combined with multi-scale characterization techniques including acoustic emission (AE) monitoring, nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results show that with decreasing pH, the elastic modulus, peak load, and fracture toughness of basalt significantly decline, while the length of the fracture process zone (FPZ) increases. Meanwhile, the number of AE events and associated energy release intensify, suggesting earlier crack initiation and more rapid propagation. AE responses exhibit earlier onset, higher cumulative energy, and a marked increase in mid-to-low frequency, high-amplitude signals. Additionally, the proportion of tensile mode fractures increases with increasing acidity. NMR results indicate substantial increases in both micro- and macropore volumes following acid treatment. As acidity increases, the spatial and temporal fractal dimensions of AE events, along with the full-scale pore fractal dimension, generally decrease, reflecting enhanced internal connectivity of the pore network. Under stronger acidic conditions, these dimensions increase slightly, implying the development of more complex and dispersed microcracks and pores. The full-scale pore fractal dimension is positively correlated with the spatial and temporal fractal dimensions of AE events. From an engineering perspective, the integration of downhole logging with geophysical monitoring may offer a useful reference for real-time reservoir evaluation, contributing to improved injectivity and CO₂ sequestration efficiency in basalt formations.