Multiscale Single-Phase Flow Mechanisms of Shale Oil Revealed by High-Pressure Nuclear Magnetic Resonance Experiments

To clarify fluid flow mechanisms and establish effective development conditions in continental shale oil reservoirs, a high-temperature, high-pressure steady-state flow system integrated with nuclear magnetic resonance (NMR) technology has been developed. The apparatus combines sample evacuation, rapid pressurization and saturation, and controlled displacement, enabling systematic investigation of single-phase shale oil flow under representative reservoir conditions. Related experiments allow proper quantification of the activation thresholds and relative contributions of different pore types to flow. A movable fluid index (MFI), defined using dual T₂ cutoff values, is introduced accordingly and linked to key flow parameters. The results reveal distinct multi-scale characteristics of single-phase shale oil transport, namely micro-scale graded displacement and macro-scale segmented nonlinear behavior. As the injection–production pressure difference increases, flow pathways are activated progressively, beginning with fractures, followed by large and then smaller macropores, leading to a pronounced enhancement in apparent permeability. Although mesopores and micropores contribute little to direct flow, their indirect influence becomes increasingly important, and apparent permeability gradually approaches a stable limit at higher pressure difference. It is also shown that the MFI exhibits a strong negative correlation with the starting pressure gradient and a positive correlation with apparent permeability, providing a rapid and reliable indicator of shale oil flow capacity. Samples containing through-going fractures display consistently higher MFI values and superior flowability compared with those dominated by laminated fractures, highlighting the pivotal role of well-connected fracture networks generated by large-scale hydraulic fracturing in improving shale oil production.