Microscopic Visualization Study of Nitrogen Flooding Mechanisms in Fault-Controlled Carbonate Reservoirs

Fault-controlled carbonate reservoirs exhibit extreme fault-induced heterogeneity, causing poor sweep and low displacement efficiency under conventional recovery. Nitrogen flooding is an important enhanced oil recovery option, but its pore-scale behavior in these reservoirs is not fully clarified. This study uses a high-temperature, high-pressure microfluidic visualization platform to examine miscible and immiscible N 2 flooding. The main findings are as follows. (1) Under immiscible conditions, a sharp oil–gas interface with abrupt grayscale contrast is observed. Limited gas sweep and high interfacial tension hinder mobilization of oil in blind-end pores, yielding 45–50% recovery. (2) Under conditions of 130 °C and 55 MPa, the system exhibits pronounced dynamic miscible behavior. Nitrogen dissolves into the crude oil. Sweep becomes more homogeneous, and recovery rises to 65%, a 15–20% increase over immiscible flooding. (3) Strong heterogeneity drives N 2 to preferentially flow through dominant fracture channels (∼70% of total pathways), while vugs act as main residual oil traps, with film-type and column-type oil contributing 35–40% and 25–30%, respectively. (4) Volatile oil shows a much higher gas–oil ratio (458 vs 235 m 3 /m 3 ) and larger light-component fraction (64.74% vs 55.66%) than conventional oil. (5) In a single-channel vug system, miscible flooding primarily mobilizes oil adjacent to the main channel, whereas immiscible flooding mainly displaces oil in structurally high positions. These results elucidate the microscopic mechanisms of N 2 flooding and support optimization of enhanced oil recovery in fault-controlled carbonate reservoirs.