Pore–throat activation characteristics of different displacement fluids in tight oil reservoirs based on nuclear magnetic resonance (NMR) experiments and microscopic simulation

Tight oil reservoirs have fine pore–throats and insufficient formation energy. Water flooding often faces the problem of “difficult injection and poor production,” thus urgently requiring the replacement of displacement media in the field. Based on nuclear magnetic resonance principles and the level set method, this study conducted core displacement experiments and microscale numerical simulations for water, N2, and CO2 flooding. It aims to reveal the microscopic pore–throat activation characteristics of different injected media. Results show that the minimum activated pore–throat sizes were 0.1 μm (water flooding), 0.06 μm (N2 flooding), and 0.07 μm (CO2 flooding). Displacement front velocity followed the order of N2 flooding > CO2 flooding > water flooding, while microscale oil displacement efficiency followed the order of CO2 flooding (79.1%) > N2 flooding (68.03%) > water flooding (48.97%). Water flooding failed to form an oil–water transition zone, and water had limited access to fine pores, thus leaving significant amounts of columnar and blind-end residual oil. N2 (small molecules) entered small pores but exhibited weak dissolution and diffusion effects, causing severe gas channeling and clustered residual oil. CO2 flooding formed a wide oil–gas transition zone, where CO2 dissolved heavily in crude oil and diffused into small-to-medium pores. It completely displaced columnar residual oil, improved oil properties in these pores, and thereby enhanced microscale oil displacement efficiency. This study provides theoretical guidance for the replacement of displacement media in tight oil reservoirs.