Assessment of CO₂-energized fracturing methods: Their impacts on fracturing fluid flowback and CO₂ geological storage in deep tight gas reservoirs

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.