Experimental investigation of CO₂ huff- n-puff in tight oil reservoirs: Effects of the fracture on the dynamic transport characteristics based on the nuclear magnetic resonance and fractal theory

CO2 huff-n-puff is a promising method for enhancing oil recovery in tight reservoirs while simultaneously contributing to CO2 geological storage. However, the performance of this method is significantly affected by fractures resulting from hydraulic fracturing and natural fractures. The impact of fracture on oil transport behavior in different pores remains unclear and warrants further investigation. In this study, CO2 huff-n-puff experiments were conducted under induced fracture conditions. A new method for quantifying the relationship between transverse relaxation time T-2 and pore radius was proposed by fitting the slopes of the T-2 spectrum and the pore size distribution (PSD) curve derived from Micro X-ray computed tomography (Micro-CT). The impact of the fracture on the effective utilization limit was explored, and the fractal dimensions of different cycles were evaluated during CO2 huff-n-puff. The results showed that the oil recoveries of the fracture-free and the fractured samples were 19.97% and 31.16%, corresponding to the effective utilization limits of 1.52 mu m and 0.96 mu m, respectively. Moreover, based on the fractal characteristics of the whole process of CO2 huff-n-puff, the pore system was divided into micropores (<0.20 mu m), small pores (0.20-0.63 mu m), intermediate pores (0.63-2.00 mu m), and large pores (>2.00 mu m). Furthermore, the incremental recovery for each cycle mainly comes from large pores and gradually decreases as the fractal dimension is less than 2.85. The fracture has a significant impact on the migration characteristics of crude oil. The incremental oil in the intermediate pores of the fractured sample, caused by negative convection, was 2.88 times greater than that of the fracture-free sample. Finally, it was found that the gravity effect was more prominent, significantly affecting the remaining oil distributions in fractured reservoirs.