## 3 Result and Discussion
### Comparative Analysis of Sandstone Pore Throat Distributions
When the injection pressure of MICP is lower than the capillary force required to break through the smaller pore throat \(r_{2}\), mercury fluid cannot enter the large pore B. However, when the injection pressure surpasses the capillary force needed for pore throat \(r_{2}\), mercury fluid infiltrates into the large pore B, as depicted in Figure 9b. Although pore B gets filled up at the moment the capillary force needed to penetrate throat \(r_{2}\) is reached, the volume of mercury fluid infiltrated is measured in the MICP method as the mercury volume corresponding to pore throat \(r_{2}\); this could be attributed to ink bottle phenomenon [62]. [PERSON] also indicated in research on cement-based materials that MICP-generated PSD is not reliable [32]. Essentially, as nearly all pores are located internally within the sample, the mercury fluid can only make contact with the majority of these pores through narrow, elongated throats of varying sizes and shapes [32]. Hence, in the pore size and frequency distribution chart (Figure 8) derived from both MICP and NMR, the fluid volume in region 1 detected by NMR is lower than that by MICP, and in region 2, the volume of hydrogen-containing fluid detected by NMR surpasses the mercury volume measured by MICP. Figure 8: A comparison of pore throat size distributions in sandstone S1-1 as determined via NMR and MICP measurements. Consequently, potential inaccuracies might exist in terms of mercury injection volume estimation when using the MICP method, especially in non-uniform reservoirs characterized by numerous large pores linked to smaller pore throats. This could result in the MICP method overestimating the proportion of smaller throats while underestimating the larger ones. Conversely, the NMR technique stands out when delineating the pore size distribution in core samples, with its merits of high precision, speediness in measurement, and an expansive detection scope. Figure 10 compares the pore throat distribution of the Tuha sandstone core samples measured by both NMR and MICP methods, with the distribution trend being largely similar to that of S1-1. The phenomena observed in these eight core samples are consistent with the mismatch between MICP and NMR reported in the literature [29,30,34]. The three core samples are noteworthy: S1-2, S1-3, and S1-4. In the pore size range of 0.001 to 0.0037 \(\upmu\)m, the proportion of the NMR signal distributions reached 5.39%, 9.07%, and 6.67%, respectively, significantly higher than the proportions measured via mercury intrusion in this pore size range. Interestingly, all three of these core samples are conglomerate sandstones. With the increase in permeability, the NMR signal frequency for the pores below 0.0037 \(\upmu\)m also increased. This might suggest the presence of many non-flowing micropores in the gravel, which cannot be detected during high-pressure mercury injection. However, once saturated with formation brine, the micropores in the conglomerate might be filled and can be detected via the NMR method. Additionally, gravel particles in rocks are usually larger, resulting in larger inter-granular spaces. Hence, in most scenarios, the higher the gravel content in conglomerate sandstone, the higher the core permeability. This offers an explanation as to why the NMR signal in the microporous segment of the conglomerate sandstone intensifies with increasing permeability.