Production mechanism of enhanced oil recovery by CO₂ huff and puff and storage prospect in interbedded shale oil reservoirs

To address the challenges of low formation pressure, rapid decline in horizontal wells, and ineffective water injection in the Triassic Chang 7 shale oil reservoir of Ordos Basin, this study investigates the microscopic mechanisms of gas injection for enhanced oil recovery (EOR) and develops efficient development methods for continental shale oil. The study integrates CO₂ huff and puff experiments with nuclear magnetic resonance (NMR) technology to quantitatively characterize oil recovery efficiency and residual oil distribution under varying injection durations. Physical simulation experiments replicate in-situ CO₂ injection processes to analyze hydrocarbon fractionation and the impact of prior water injection on gas injection efficacy. Field trials are conducted in horizontal wells within the X233 well block of Longdong area, supported by numerical simulations to optimize gas injection parameters. NMR results indicate that CO₂ initially mobilizes oil from large pores, with small-pore oil gradually activated over time, showing weak correlation with porosity and permeability. Physical simulation demonstrates that with increasing huff and puff cycles, crude oil components exhibit distinct fractionation characteristics, where light hydrocarbons are preferentially extracted and heavy hydrocarbons gradually accumulate. Field applications demonstrate that optimized synchronous huff and puff enhances single-well estimated ultimate recovery(EUR) by 50.4%, elevates formation pressure by 25%~29%, confirming the effectiveness of gas injection in low-pressure shale oil reservoirs. CO₂ huff and puff enhances shale oil recovery through selective extraction of light hydrocarbons and pore-pressure synergy. The synchronous injection mode is economically viable but requires avoiding pre-water injection interference. This study provides theoretical and technical foundations for the efficient development of continental low-pressure shale oil reservoirs.