Pore-scale wettability distribution characteristics of shale reservoirs before and after fracturing: A case study of shale reservoirs in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China

To address the limitation of conventional macroscopic wettability measurements in characterizing pore-scale wettability distributions in shale, a quantitative method for microscopic wettability characterization was developed. The method improves the conventional Amott method by integrating nuclear magnetic resonance (NMR) and high-pressure mercury intrusion porosimetry. Fluid distributions during spontaneous and pressure-assisted imbibition in oil–water and oil–fracturing-fluid systems were measured, and a model for calculating the pore-size-dependent microscopic wettability index was established. The average microscopic wettability index derived from the proposed method agrees well with those obtained from gravimetric and macroscopic NMR measurements, validating the reliability of the method and revealing mixed-wettability characteristics that may be obscured by macroscopic averaging. The method was applied to interbedded, laminated, and mud-laminated shale cores from Well H181-1 in the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin. The results show that the original macroscopic wettability of the shale is generally oil-wet, whereas the overall water-wetness of all three core types increases markedly after fracturing-fluid interaction. The microscopic wettability responses vary significantly among the three shale types. In the interbedded shale, local oil-wetness increases in small pores, whereas the medium and large pores generally become more water-wet. In the laminated shale, water-wetness increases markedly in small pores, the medium pores exhibit heterogeneous responses, and water-wetness decreases in large pores. In the mud-laminated shale, the small and medium pores shift toward more water-wet conditions, whereas the large pores exhibit heterogeneous wettability changes. These results clarify the pore-scale wettability distribution characteristics of shale reservoirs and provide a theoretical basis for fracturing-fluid selection, stimulation-parameter optimization and enhanced oil recovery.