## 5 Discussion  
### Comparison of Different Methods  
When comparing the macro-scale SI with the meso-scale CA results and the pore-scale ESEM observations, it is noted that there are inconsistencies or sometimes even contradictions in the wetting state of tight sandstone determined by these different methods. Comparing the results from ESEM and CA (Figures 4 and 5), it is evident that there are discernible differences in contact angles between the micro-scale and meso-scale, a phenomenon consistent with observations made by [PERSON] et al. (2023) [35]. Mesoscopic contact angles typically manifest as either water-wet or intermediately wet, whereas at the microscopic level, spatial variations in wettability are readily apparent. Even on the surfaces of the same mineral, the morphology of condensed water can vary (Figure 4), indicating a fractional wettability. This variation is quite challenging to detect at the mesoscopic scale, as the contact angle provided by the sessile drop method is contingent upon the heterogeneity of the solid surface, its roughness, the size of the droplet, and ambient vibrations and thus only characterizes the average wettability of the rock-fluid system [40]. Generally, the surface roughness of individual minerals is less than that of a heterogeneous mineral surface, and a lower roughness leads to a larger contact angle [41; 42], which may be one reason for the larger contact angles at the micro-nano scale compared to that at the meso-scale. These differences in wettability across scales may also stem from mixed mineralologies [35]. Due to the strong heterogeneity in wettability and mineral compositions of tight reservoirs and the fact that the mineral surfaces measured in contact angle assessments do not reflect the actual pore wall minerals, it is difficult to obtain accurate results from mesoscopic contact angle measurements [20; 43; 44].  
The discrepancy on the wettability between SI and CA results may be due to the latter being conducted in a water-air-rock system, where surface roughness can also have a  
Figure 9: Distribution of oil and brine within various pore sizes (radius).  
significant impact [41]. In tight sandstone reservoirs, factors such as mineralogy variations, low permeability, complex pore structures, and the presence of organic matter may further complicate the interpretation of wettability [45]. The spontaneous imbibition characteristics of mixed wettability, influenced by the heterogeneities of mineral compositions and pore structures, require further investigation [20].  
Despite significant variance in wettability measurements across different scales, these methods demonstrate consistency in characterizing their relative strength of wettability. For instance, ESEM observations for Sample C2 show a predominance of spherically condensed water drop and the largest water contact angle among all samples. The sessile drop method yielded the highest water contact angles for Sample C2, albeit hemispherical, aligning with its higher Wetting Affinity Index of Oil compared to its Wetting Affinity Index of Brine. This phenomenon is consistent across all samples, with Sample C2 exhibiting the strongest oil-wetting affinity, as indicated by the highest Wetting Affinity Index of Oil.