Pore structure-controlled CO₂ huff-n-puff efficiency in Jimusar shale oil reservoirs: Insights from classified oil reservoirs

CO₂ huff-n-puff is a promising enhanced oil recovery technique for shale oil reservoirs, but its efficiency in relation to pore structure across classified oil reservoirs remains unclear. This study investigates three reservoir classes (Types Ⅰ–Ⅲ) in the Jimusar Sag using high pressure mercury intrusion, nitrogen adsorption, and NMR to characterize pore architectures. Results show that the shale cores from the Jimusar shale oil reservoir are overall dominated by medium pores, with generally small pore radii. Among them, the Type Ⅰ oil reservoir class has a higher proportion of large pores (> 300 nm), whereas the Type Ⅲ oil reservoir class has a higher proportion of small pores (< 50 nm) than the other two classes. Online NMR monitored CO₂ huff-n-puff experiments under reservoir conditions (363.15 K, injection pressure > 24 MPa) reveal significant cumulative oil recovery differences: 56.36% (Type Ⅰ), 46.81% (Type Ⅱ), and 28.30% (Type Ⅲ) after four cycles. Recovery correlates with pore size: The Type Ⅰ oil reservoir class, with a higher proportion of large pores, exhibits stronger CO₂ flow capacity, whereas the Type Ⅲ oil reservoir class, with a larger proportion of small pores, significantly restricts oil mobilization. A second derivative analysis of the recovery–pore radius curve quantifies mobilization thresholds, indicating a lower limit effective pore radius of 20–35 nm. Sensitivity analysis shows that increasing injection pressure more effectively improves recovery and lowers the mobilization threshold than extending soaking time.