## 4 Results and Interpretation
### Fluid Distributions in Different Pores  
To investigate the fluid distribution characteristics across varying pore scales, this study measured the transverse relaxation time (\(T2\)) distribution of four tight sandstone samples obtained through NMR measurements after the spontaneous imbibition of crude oil and brine. The experimental results demonstrate distinct multimodal \(T2\) spectral patterns (Figure 7), with relaxation times predominantly ranging from 0.01 ms to 10 ms. A comparative analysis of different fluid imbibition experiments reveals distinct patterns: the \(T2\) spectral peaks formed after brine imbibition display an overall broad distribution, whereas the main peak of the \(T2\) spectrum for the crude oil imbibed samples is within the range of 0.01-1 ms. Notably, the peak amplitude of crude oil imbibed samples demonstrates a 40-60% reduction compared to that in the corresponding brine imbibed samples. To investigate the fluid distribution and changes in fluid signals within pores of varying sizes in tight sandstone cores, the NMR \(T2\) spectrum was converted into pore radius. Because the NMR \(T2\) spectrum has similar morphological characteristics to the pore size measured by MICP [38], by analyzing the similarities and differences between the \(T2\) curve of tight sandstone and the mercury injection distribution curve, [PERSON] et al. (2015) found that \(T2\) and \(r\) have a power-exponential relationship [39]:  
\[r=CT_{2}^{n} \tag{3}\]  
where \(r\) is the pore radius, \(\mu\)m; \(T2\) is transverse relaxation time, ms; C = \(\rho_{2}F_{\mathrm{S}}\); \(F_{\mathrm{S}}\) is the shape factor of a single pore, with \(F_{\mathrm{S}}\) = 3 for spherical pores, and \(F_{\mathrm{S}}\) = 2 for columnar pores; \(\rho_{2}\) is the transverse surface relaxation rate. By comparing the cumulative probability curve of the \(T2\) spectrum obtained by NMR experiment with the cumulative probability curve of the pore radius obtained by MICP measurement and determining the conversion coefficient n at the tangent point of the two, the accurate conversion between the \(T2\) spectrum and MICP pore size can be realized. Figure 7: \(T2\) spectrum of four samples after spontaneous imbibition (SI) of brine and crude oil. In Figure 8a,d,g,j, we can observe that the peak of the pore size distribution in the tight sandstone, derived from the MICP experiment, primarily falls within a pore radius range between 0.01 \(\upmu\)m and 1 \(\upmu\)m. Figure 8b,e,h,k, which show the transformed r values derived from \(T2\), also exhibit similar pore size distribution characteristics, thereby demonstrating the validity of converting the NMR \(T2\) spectrum into a pore size distribution curve. Figure 8b,e,h,k reveal the distribution of fluids across different pore radii after spontaneous imbibition of brine and crude oil. Figure 8c,f,i,l display the imbibition amounts of brine and crude oil in different pore radii. The results show that in smaller pores, the average imbibition of brine exceeds that of crude oil, while in larger pores, crude oil imbibition is higher than that of brine. This suggests that smaller pores exhibit stronger water-wet conditions compared to larger pores, which in turn show stronger oil-wet conditions, presenting clear mixed wettability characteristics. A comprehensive analysis of brine and crude oil imbibition across different pore sizes for the four samples (Figure 9) reveals some distinct trends. Samples C1 and C2 exhibit  
Figure 8: NMR experimental results: (**a-c**) MICP pore size distribution, NMR pore size distribution, and oil–water imbibition amount of Sample C1 with different pore sizes; (**d–f**) MICP pore size distribution, NMR pore size distribution, and oil–water imbibition amount of Sample C2 with different pore sizes; (**g–i**) MICP pore size distribution, NMR pore size distribution, and oil-water imbibition amount of Sample C3 with different pore sizes; (**j–l**) MICP pore size distribution, NMR pore size distribution, and oil–water imbibition amount of Sample C4 with different pore sizes.