Experimental Investigation of Foam-Assisted CO2 Huff-n-Puff for Enhanced Oil Recovery in Fractured Tight Reservoirs

Tight oil reservoirs developed by volume fracturing commonly suffer from insufficient energy replenishment and rapid production decline. Although CO2 huff-n-puff can enhance oil recovery, it is prone to early gas channeling through fracture-dominated high-permeability channels, and its effectiveness decreases with successive cycles. To clarify the coupled effects of fracture morphology and foam on CO2 huff-n-puff performance, comparative experiments of multi-cycle CO2 huff-n-puff and foam-assisted CO2 huff-n-puff were conducted on fractured tight cores from the Xinjiang Mahu reservoir, combined with offline low-field NMR T2 analysis. The results show a clear first-cycle dominant effect, and better reservoir properties lead to higher initial recovery and slower decline in subsequent cycles. Cross fractures increase the final oil recovery by 81.1%, 83.4%, and 73.2% for the three reservoir types, respectively, whereas excessively large fracture apertures reduce recovery because of intensified gas channeling. Foam further improves oil recovery, with 0.6% giving the optimum performance and increasing final recovery by 20.11%, 14.79%, and 8.36% in Type-I, Type-II, and Type-III reservoirs, respectively. NMR results indicate that foam mainly enhances the mobilization of remaining oil in medium and large pore–throat systems by blocking preferential flow channels and enlarging the effective swept volume. This study provides an experimental basis for parameter optimization and mechanistic understanding of foam-assisted CO2 huff-n-puff in fractured tight reservoirs.