CO₂-energized fracturing holds great potential for enhancing fracturing fluid flowback and enabling effective CO₂ sequestration, while the effect of the choice of CO₂ energization strategy on these processes is not yet fully understood. This study experimentally investigated three representative CO₂ energization methods: Pre-fracturing injection, foam injection, and co-injection. Nuclear magnetic resonance techniques were applied to systematically analyze the influence of various CO₂ injection parameters on fracturing fluid flowback behavior and CO₂ storage in tight formations. The results showed that CO₂ pre-fracturing increases displacement pressure and significantly improves flowback efficiency, with optimal performance achieved at a moderate injection volume. Reducing the injection rate and increasing the volume further enhanced the CO₂ storage ratio. Foam injection facilitated flowback by improving foam quality, particularly in macropores. Co-injection achieved a favorable balance between high flowback efficiency and substantial CO₂ retention. Furthermore, the three energization strategies were shown to lead to distinct fluid redistribution patterns within porous media: Pre-fracturing promoted CO₂ retention in micropores and mesopores, foam injection reduced retention in macropores, and co-injection provided the most balanced performance in mesopores. These findings provide new insights into CO₂-energized fracturing and sequestration mechanisms and offer technical guidance for optimizing CO₂-based stimulation strategies in deep unconventional tight gas reservoirs.

Complex physical and chemical reactions during CO₂ sequestration alter the microscopic pore structure of geological formations, impacting sequestration stability. To investigate CO₂ sequestration dynamics, comprehensive physical simulation experiments were conducted under varied pressures, coupled with assessments of changes in mineral composition, ion concentrations, pore morphology, permeability, and sequestration capacity before and after experimentation. Simultaneously, a method using NMR T₂ spectra changes to measure pore volume shift and estimate CO₂ sequestration is introduced. It quantifies CO₂ needed for mineralization of soluble minerals. However, when CO₂ dissolves in crude oil, the precipitation of asphaltene compounds impairs both seepage and storage capacities. Notably, the impact of dissolution and precipitation is closely associated with storage pressure, with a particularly pronounced influence on smaller pores. As pressure levels rise, the magnitude of pore alterations progressively increases. At a pressure threshold of 25 MPa, the rate of change in small pores due to dissolution reaches a maximum of 39.14%, while precipitation results in a change rate of −58.05% for small pores. The observed formation of dissolution pores and micro-cracks during dissolution, coupled with asphaltene precipitation, provides crucial insights for establishing CO₂ sequestration parameters and optimizing strategies in low permeability reservoirs.