Surface Energy Distribution and Wettability Heterogeneity of Shale Revealed by Finite Concentration Inverse Gas Chromatography

Abstract Wettability heterogeneity in shale reservoirs significantly influences fluid occurrence and flow behavior. However, existing methods make it difficult to quantitatively evaluate wettability heterogeneity through direct measurement of surface energy distributions. This study introduces finite concentration inverse gas chromatography (FC-IGC) to characterize three shale samples with distinct mineralogical compositions, total organic carbon (TOC) contents, and pore structures. The surface energy and its components were measured at 90 °C over a surface coverage range of 0.04–0.16. Based on the van Oss–Chaudhury–Good interfacial tension theory and the methodology proposed by Khoeini et al., the contact angle distributions and work of adhesion ranges for shale reservoir systems were calculated within the experimentally measured coverage range. The results show that the nonpolar component of shale surface energy decreases with increasing coverage, whereas the polar component increases, with the polar component consistently exceeding the nonpolar component. All samples exhibited strong hydrophilicity under oil–water conditions. Significant differences in wettability heterogeneity were observed: Sample A displayed the widest work of adhesion and contact angle ranges and the strongest heterogeneity; Sample C was intermediate; Sample B exhibited the weakest heterogeneity. The differences between the peak maximum method and the peak centroid method for the shale samples showed a systematic correlation with TOC and specific surface area, suggesting a coating effect of organic matter on mineral surfaces. The universal strong hydrophilicity of high-energy shale surfaces can serve as a baseline assumption for reservoir fluid distribution prediction, and the work of adhesion and oil contact angle distributions can provide reference indicators for assessing wettability heterogeneity and water-blocking risk.