NMR-Based Fractal Characterization of Capillary-Force-Regulated Shut-in Imbibition in Continental Shale Oil: Pore-Size-Dependent Recovery, Nanopore Mobilization Threshold, and Permeability Enhancement

Continental shale oil reservoirs contain multiscale pore–fracture systems with strong heterogeneity and fractal characteristics, which complicate oil mobilization during post-fracturing shut-in imbibition. In this study, shale cores from the LGS Formation (a lacustrine continental shale oil formation in China) were used to investigate capillary-force-regulated pressurized shut-in imbibition by integrating interfacial tension measurements, apparent contact angle tests, capillary pressure calculation, time-lapse nuclear magnetic resonance (NMR), NMR-based fractal characterization, visual observations, and pre-/post-imbibition permeability measurements. Two surfactant-based imbibition agents with different capillary-force regulation mechanisms were compared to represent different capillary-force regulation pathways. Agent 1 mainly modified apparent wettability, increasing the contact angle from 51.0° to 66.1°, whereas Agent 2 reduced the oil–water interfacial tension from 31.85 to 22.12 mN/m while maintaining a favorable apparent contact angle of 49.3°. Time-lapse NMR results showed that oil recovery increased with shut-in time and reached approximately 12–30% after 144 h. Agent 2 generally produced higher recovery than Agent 1, with the optimum response at 0.15 wt%. NMR-derived fractal dimensions ranged mainly from 2.32 to 2.61, confirming the multiscale heterogeneity of the LGS shale pore system. Pore-size-resolved recovery further showed that oil mobilization was dominated by pores larger than 20 nm and microfracture-related spaces, whereas pores smaller than 20 nm contributed only limited bulk recovery. This indicates an apparent nanopore mobilization threshold near 20 nm, controlled by fractal pore complexity, pore-throat connectivity, oil adsorption, capillary pressure, and molecular accessibility of imbibition agents. Visual and permeability evidence further showed that pressurized imbibition can selectively activate connected pore–fracture pathways. Post-imbibition dry-core permeability increased in all tested samples, although the enhancement was highly heterogeneous. These results demonstrate that shut-in imbibition in LGS shale is governed by coupled interfacial regulation, fractal pore heterogeneity, pore-size-dependent oil accessibility, and selective pore–fracture structural modification.