Impact of Multiscale Pore Structure and Mineralogy on Fracturing Fluid Imbibition in Marine–Continental Transitional Shale Gas Reservoirs

Fracturing fluid invasion in a marine–continental transitional shale gas reservoir could lead to low gas productivity and even groundwater contamination. However, the unique mineral composition and pore structure of transitional shales result in complex fracturing fluid imbibition behavior, which remains insufficiently understood compared with marine shales. In this study, nuclear magnetic resonance (NMR) techniques, including T2 spectrum and T1–T2 2D spectrum, were employed for in situ monitoring of spontaneous/forced imbibition on transitional shales with differing mineral compositions. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used for mineral and pore structure characterization. The results showed that organic pores of transitional shales were underdeveloped, while clay mineral pores dominated and were concentrated in the 10–50 nm range. Kaolinite, a characteristic mineral with a 1:1 layered structure, enhanced pore connectivity through lamellar and intercrystalline fractures, promoted fluid access to 10–100 nm pores, and maintained pore stability due to its low cation exchange capacity and weak hydration swelling. Spontaneous imbibition enriched fluid in 10–50 nm pores, while forced imbibition drove fluid into smaller pores, and induced clay dispersion and migration after 8 h to amplify hydroxyl signals. A 2D NMR fluid occurrence state identification spectrum of transitional shales was established, where 4 signal regions were defined. Near-wellbore forced imbibition, dominated by differential pressure, saturates fractures and continuously supplies the matrix, whereas far-wellbore spontaneous imbibition, controlled by capillary and osmotic forces, maintains unsaturated fractures and dynamic equilibrium. These findings clarify the mechanisms governing fracturing fluid imbibition in transitional shales and provide insights for optimizing hydraulic fracturing designs and mitigating groundwater contamination.