Imbibition-Enhanced Oil Recovery with 1D and 2D Nuclear Magnetic Resonance Methods in Lacustrine Shales

Abstract Imbibition is an important mechanism governing oil and water displacement in shale oil reservoirs. However, accurate characterization of the oil displacement process and fluid occurrence states during imbibition remains difficult because the nuclear magnetic resonance (NMR) signals from pore oil, pore water, oil film, water film, crystal water, and kerogen overlap. In this study, eight shale samples were selected from different formations in the Songliao and Bohai Bay Basins. The oil displacement process was characterized from the initial oil-saturated state through three sequential imbibition stages involving a 4% KCl solution, deionized water (DI water), and DI water at 25 MPa. Changes in the occurrence of oil and water in different pores were characterized by combining NMR T2 (1D) spectra with NMR T1–T2 (2D) maps. The results indicate that the NMR signals of shale can be divided into short relaxation signals (SRS), the pore water signal, and the pore oil signal. The SRS mainly comprises signals from kerogen, oil film, water film, and crystal water. During imbibition, water preferentially entered small pores driven by capillary forces and displaced oil into large pores. With continued water entry, oil in the large pores was expelled, while water films gradually replaced oil films on the pore surfaces. Because the interaction between pore oil and water films was weaker than that between pore oil and oil films, the movable oil signal shifted toward longer relaxation times. On this basis, a fluid identification chart based on NMR data was established to distinguish pore oil, pore water, and the SRS. This chart provides an effective approach for characterizing the migration and redistribution of oil and water in different pores during shale imbibition.