Study on effectiveness of supercritical CO2 on pore enlargement and permeability enhancement in deep ultra-low-permeability volcanic reservoirs

The deep volcanic reservoirs of the Huoshiling Formation in the Songliao Basin face severe challenges for economically efficient development due to ultra-low permeability and extreme compactness, while also presenting potential target reservoirs for CO2 utilization and storage under CCUS scenarios. To address this challenge, this study explored and verified a water-rock interaction modification method based on supercritical carbon dioxide (SC-CO2) synergized with formation water. Through SC-CO2 saturation dissolution reaction experiments, combined with X-ray diffraction (XRD) mineral quantitative analysis, field emission scanning electron microscopy (FE-SEM) microstructural characterization, and rock mechanical property testing, the modification effects of SC-CO2 synergized with formation water on the reservoir were systematically investigated. The experimental results showed that SC-CO2 preferentially dissolved minerals such as plagioclase and calcite, leading to a significant reduction in clay mineral content and the formation of microscopic fractures and pore throats. Three-dimensional digital core models constructed from CT scans further revealed that SC-CO2 treatment significantly improved reservoir pore structure: the proportion of dominant flow channels with coordination numbers (CN) >3 increased by approximately 11%, while pore volumes with throat radii >6 μm expanded by over 16.5%. The trends of simulated permeability were consistent with the changes in actual gas permeability measurements, both showing year-on-year increases exceeding 90%. Meanwhile, rock mechanical tests indicated that after SC-CO2 treatment, the compressive strength of rock samples decreased by 19.6%, the elastic modulus decreased by 13.2%, and the Poisson’s ratio increased by 8.7%. Combined with scanning electron microscopy (SEM) observations, these results confirmed that mechanical weakening effectively induced a secondary fracture network. The study indicated that SC-CO2, owing to its nanoscale molecular diffusion capability and zero interfacial tension, could effectively penetrate micro- and nano-scale pores and react with pore-bound water to form carbonates. Through water-rock interactions, it deeply dissolved the interior of the reservoir, effectively overcoming the limitation of traditional acid fluids in accessing micro- and nano-scale pores. This method provides new theoretical foundations and technical pathways for the cost-effective development of deep volcanic reservoirs and for CO2 co-storage and enhanced recovery modification in CCUS technology.