NMR Analysis of Imbibition and Damage Mechanisms of Fracturing Fluid in Jimsar Shale Oil Reservoirs

Optimizing the shut-in and flowback processes is crucial for improving oil recovery and mitigating formation damage in shale oil development. However, the mechanisms governing fracturing fluid migration and its impact on permeability, particularly across different lithologies, remain poorly understood. This study investigates the spontaneous imbibition behavior of fracturing fluid and the resulting permeability damage in two predominant lithotypes (dolomitic siltstone and argillaceous siltstone) from the Jimsar shale oil reservoir. By integrating low-field nuclear magnetic resonance (NMR) monitoring with core flooding experiments, we dynamically characterize fluid migration and quantitatively evaluate damage rates. The results reveal that lithology exerts a fundamental control on these processes. Dolomitic siltstone, with its higher brittle mineral content and well-connected pore network, facilitates deeper fracturing fluid invasion (30.47 mm) and more efficient oil displacement. In contrast, argillaceous siltstone, which is rich in clay minerals, exhibits stronger capillary trapping and suffers more severe permeability damage (~70%) compared to dolomitic siltstone (~46%), primarily due to the synergistic effects of water blocking and clay swelling. Furthermore, the impact of shut-in time on permeability damage follows a non-monotonic trend, reflecting a dynamic competition between imbibition-driven oil recovery and fluid-induced damage. Flowback analysis on core plugs reveals an economic critical point, beyond which further permeability recovery becomes marginal. This core-scale finding underscores the importance of the initial flowback stage for efficient cleanup and provides a scientific basis for optimizing flowback strategies in the Jimsar shale and similar unconventional reservoirs. These findings offer guidance for designing lithology-specific fracturing fluid systems, optimizing shut-in durations, and tailoring flowback strategies in the Jimsar shale and analogous unconventional reservoirs.