Scale-dependency Wettability of Tight Sandstone: Insights from an Eocene fluvial sandstone reservoir in the Bohai Bay Basin

In the development of tight oil reservoirs, wettability determines the distribution and flow behavior of oil and water during reservoir development and enhanced oil recovery. However, accurately assessing wettability is highly challenging due to the strong heterogeneity of the mineralogy and pore structures in tight sandstone reservoirs and the complex interactions between minerals and fluids. Traditional studies often focus on the average wettability evaluation at the macro scale; however, such local bulk wettability often overlooks the inherent micro- and nanoscale heterogeneities of tight oil reservoirs. They thus cannot properly represent the true wettability in highly heterogeneous and low-permeable tight sandstones. In this study we employed a multi-scale comprehensive approach to evaluate reservoir wettability of an Eocene tight sandstone reservoirs from the Bohai Bay Basin. An environmental scanning electron microscopy was firstly used to determine wettability at the pore scale through analyzing the condensation patterns of water vapor on pore walls. Contact angle measurements were then employed to quantitatively assess the mesoscopic wettability characteristics of tight sandstone surfaces. Finally, a combination of Nuclear Magnetic Resonance and spontaneous imbibition experiments were carried out to evaluate the distribution characteristics of oil and water across different pore sizes and determine the overall wettability of samples. We have found that significant fractional wettability and mixed wettability are present in the tight oil sandstones. Different minerals and various parts of individual minerals can exhibit distinct wettability characteristics. The fractional wettability of tight sandstone is primarily influenced by clay mineral types and morphology; grain-coating illite and grain-coating chlorite tend to show oil-wet characteristics, while dispersed sheet-like chlorite and rosette chlorite are more likely to be water-wet. The mixed wettability in tight sandstones is mainly controlled by pore sizes: for oil-wet samples, pores larger than 0.1 μm are generally oil-wet, while those smaller than 0.1 μm are predominantly water-wet. For water-wet samples, the pore-size threshold between oil-wet and water-wet pores is around 1 μm. The wettability of tight sandstone reservoirs in the study area is primarily controlled by pore sizes ranging from 0.1 μm to 1 μm. This finding provides critical pore size thresholds for accurately describing reservoir wettability characteristics and is essential for understanding and predicting fluid behavior within tight oil reservoirs. The integrated multi-scale method proposed here allows a more precise and reliable wettability assessment, offering a viable workflow for wettability evaluation of tight oil reservoirs.