Experimental Study on Enhancing Oil Recovery by CO2 Fracturing Fluid Compound Stimulation in Tight Conglomerate Reservoirs

The development of tight conglomerate oil reservoirs primarily relies on horizontal wells combined with staged fracturing. However, under natural energy depletion, oil well production declines rapidly, and reservoir pressure is difficult to maintain. To address the low primary recovery efficiency after fracturing in tight conglomerate reservoirs, an experimental method of CO2/fracturing fluid composite huff-and-puff was proposed to enhance oil recovery. Indoor physical simulation experiments were conducted to evaluate the oil mobilization effects and recovery potential of CO2, fracturing fluid (as single media), and CO2/fracturing fluid composite huff-and-puff. The study also investigated the influence of different injection sequences on recovery enhancement during the process. Nuclear magnetic resonance (NMR) and laser confocal microscopy were employed to characterize the mobilization degree of oil in different pore types by CO2/fracturing fluid mixtures and to analyze changes in oil occurrence states before and after huff-and-puff. The results indicate that a single cycle of CO2 huff-and-puff achieved 21.41% recovery efficiency, with effective mobilization of large pores. CO2 exhibited significant extraction of light hydrocarbon components during the process. The 0.2% mass fraction ACE surfactant fracturing fluid demonstrated low oil-water interfacial tension and strong wettability alteration capability, forming an effective composite slug with CO2. Among composite huff-and-puff methods, the sequence of CO2 followed by ACE surfactant outperformed ACE-first injection and continuous CO2-ACE co-injection. Staged injection (CO2 followed by surfactant) showed optimal performance. Composite huff and puff can simultaneously mobilize oil in both large and small pores, facilitating the repeated exploitation of the reservoir's production potential.