Nuclear Magnetic Resonance-Based Comparative Studies of Centrifugation, N₂ flooding, CO₂ Huff-n-Puff, and Spontaneous Imbibition for Oil Recovery in Lacustrine Shale in Eastern China

The advancement of horizontal drilling and hydraulic fracturing techniques has revolutionized the production of oil from shales. Despite these technological leaps, oil recovery remains at low levels, leaving substantial volumes of oil within shale reservoirs; hence, it is extremely important to investigate the application of enhanced oil recovery (EOR) methods. In this study, centrifugation, N2 flooding, CO2 huff-n-puff, and spontaneous imbibition experiments were conducted to investigate the recovery and distribution of movable oil in different pore sizes of shales by using nuclear magnetic resonance (NMR) techniques and clarify the movable mechanism of shale oil from various displacement experiments. The results showed that the movable oil recovery (MOR) from centrifugation (14.34%), N2 flooding (11.90%), CO2 huff-n-puff (13.72% and 23.62% for gaseous and supercritical CO2), water spontaneous imbibition (35.76%), and oil spontaneous imbibition (21.98%) are significantly different. The MOR of fractures and matrix pores is variable across different displacement methods. Additionally, the sequence of oil recovered from fractures and matrix pores may vary, reflecting the discrepancies in the mechanisms of movable oil recovery. The natural fractures and artificial cracks created during the experiments significantly increase the MOR of shales. For fractured shales, the MORs of matrix pores from multiple displacement methods show a ranking sequence of N2 flooding < centrifugation < gaseous CO2 huff-n-puff < water imbibition without cracks < supercritical CO2 huff-n-puff < oil imbibition < water imbibition with cracks, suggesting that a synergistic application of liquid imbibition and CO2 huff-n-puff is the potent strategy for a possible bolstering shale oil recovery in the oilfield. The integrated displacement experiments and comprehensive analyses presented in this study not only enhance the understanding of shale oil mobility but also provide insights to the formulation of effective strategies for the development of shale oil reservoirs.