## 5 Discussions
### Comparison of NMR \(T_{2}\) Spectral Parameters and MICP Capillary Parameters  
In order to verify the reliability of NMR \(T_{2}\) spectral parameters for pore structure characterization, a comparative analysis between \(T_{2}\) spectral parameters and MICP capillary parameters was carried out for parameters with similar petrophysical proxies. MICP measurement is only effective in measuring pore throat sizes, whereas NMR analysis can better detect the pore sizes. Limited by the maximum mercury injection pressure attainable, some nanopore throats cannot be detected by MICP measurements, but the majority of micro/nano-scale pore throats can be obtained. In addition, influenced by a Haines jump, the mercury cannot enter into tortuous pore networks uniformly, rather in a process of rapid filling [62; 63]. The effect of Haines jump is more significant in the initial stage of mercury intrusion, when most pores are unfilled [63]. This results in that a considerable amount of the large throats has not been accounted for. Therefore, the throat size distribution derived from MICP data is apparently narrower compared with the actual distribution (Figures 13 and 14). As shown in Figure 13, the large pore throats do exist in sandstone samples, and they can be detected by 2D image analysis based on the \"maximal balls\" algorithm [64]. Figure 13: Two-dimensional (2D) pore network model and pore throat size distribution derived from photomicrograph analysis. However, the overall trends in pore sizes revealed by optical photomicrograph analysis, MICP, and NMR are the same (Figures 14 and 15A,B). Regression analysis shows that the \(T_{2}\) relaxation times (\(T_{2\text{max}}\) and \(T_{2\text{mid}}\)) are consistent with the corresponding pore throat radius derived from MICP data. In general, \(T_{2\text{max}}\) and \(T_{2\text{mid}}\) increase with increasing pore throat radius (maximum pore throat radius (\(R_{\text{max}}\)) and median pore throat radius (\(R_{50}\))), with good exponential correlations (Figure 15A,B), confirming the applicability of using NMR \(T_{2}\) relaxation times to calculate pore sizes. Due to the shielding effect of large throats caused by Haines jump, the correlation between the \(T_{2\text{max}}\) and maximum pore throat radius (\(R_{\text{max}}\)) is relatively weak (R\({}^{2}\) = 0.67; Figure 15A). Figure 14: Comparison of throat size distribution derived from MICP and photomicrograph analysis. We also investigated the reliability of using NMR \(T_{2}\) spectral parameters to approximate the pore size distribution by using the sorting coefficient (\(\sigma\)) and skewness (S\({}_{\text{kp}}\)). As shown in Figure 15C, there is a weak to moderate determination correlation (R\({}^{2}\) = 0.53) between \(\sigma\) calculated from NMR and MICP data, but there is no apparent correlation between \(S_{\text{kp}}\) derived from NMR and MICP data (Figure 15D). This may be because MICP is effective in identifying smaller pore throats but is not suitable to detect large pore throats [65], whereas NMR can provide the full-range pore structure information [30]. Thus, the pore size distribution derived from NMR experiments is wider than that from MICP measurements (Figure A3). This leads to some differences in the morphological characteristics and skewness derived from NMR and MICP. As shown in Figure 15E, \(\varphi_{\text{em}}\) derived from NMR experiments is consistent with the displacement pressure computed from MICP measurements, showing a negative relationship (R\({}^{2}\) = 0.78). In addition, for the E\({}_{\text{s4}}\) sandstone reservoirs, the movable fluid saturation (S\({}_{\text{mov}}\)) obtained from NMR appears to be correlated with the maximum mercury saturation from MICP (R\({}^{2}\) = 0.59; Figure 15F). The results indicate that the NMR \(T_{2}\) spectral parameters are effective in estimating the fluid distribution in pore space and microscopic pore connectivity.