## 4 Results
### NMR Experiments and Data Analysis
#### 4.4.2 Effective Movable Fluid in Pore Space  
The pore fluid in sandstones can be separated into irreducible and movable fluid (including the effective movable fluid and immobile fluid) at the \(T_{\rm 2 cutoff}\) values [29; 57; 58]. The irreducible fluid is considered to be clay- or capillary-bound water retained in pores [47]. NMR analyses show that the \(T_{\rm 2 cutoff}\) values obtained from 20 sandstone samples have a wide range from 2.83-150.41 ms (averaging 42.29 ms), indicating the complexity of movable fluid distribution in the E\({}_{\rm 84}\) sandstone reservoirs (Table 1). Previous studies have shown that the centrifugal component cannot reach zero even when \(T_{2}>T_{\rm 2 cutoff}\) (Figure 9) [40; 41]. There may still be ineffective movable fluid (immobile fluid) presence in the pore throats beyond the \(T_{\rm 2 cutoff}\) values (Figure 9) due to the presence of isolated intergranular pores. The porosity for movable fluids (\(\varphi_{\rm m}\)) is not effective in characterizing movable fluid distribution. As shown in Figure 9, the samples with similar \(\varphi_{\rm m}\) show different pore size distribution and petrophysical properties. Therefore, a new parameter (porosity for effective movable fluids (\(\varphi_{\rm em}\))) was proposed [40; 41], which is the product of the saturation for effective movable fluid and NMR porosity under 100% water-saturated state (Figure 9).  
Statistical analysis shows that the \(\varphi_{\rm em}\) varies from 0.02% to 5.98% (averaging 2.41%) (Table 1), suggesting a poor connectivity of microspore throats and complex occurrence states of fluids in the micro-pore space. The value of \(\varphi_{\rm em}\) exhibits a strong correlation with average pore throat radius (\(R_{\rm ave}\)) (R\({}^{2}\) = 0.76), implying that \(\varphi_{\rm em}\) is closely associated with micro-pore structures (Figure 10). The correlation between \(\varphi_{\rm em}\) and permeability was also investigated, and an excellent exponential correlation (R\({}^{2}\) = 0.90) was obtained (Figure 11A). However, \(\varphi_{\rm em}\) is less strongly correlated with porosity, with a relativity low determination coefficient (R\({}^{2}\) = 0.47; Figure 11B). The movable fluid mainly exists in the large intergranular pores connected by effective pore throats, since the micropores and small secondary dissolution pores are isolated or very poorly connected. Therefore, the permeability is mainly contributed by \(\varphi_{\rm em}\) rather than porosity. Consequently, \(\varphi_{\rm em}\) is more suitable to characterize the reservoir permeability capacity and microspore structures.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Core Data**} & \multicolumn{6}{c}{**NMR Data**} \\ \cline{2-13}
**Sample** & \(\varphi\) & \(K\) & \(\varphi_{\rm em}\) & \(T_{\rm 2 max}\) & \(T_{\rm 2 mid}\) & \(T_{\rm 2 cutoff}\) & \(T_{\rm 2 gm}\) & \(T_{\rm 2 ave}\) & \(T_{\rm 2-peak}\) & \(\varphi_{\rm im,max}\) & \(S_{\rm wj}\) & \(S  
Figure 11: Relationships between porosity for effective movable fluids and petrophysical parameters. (**A**) Effective movable fluids versus permeability; (**B**) Effective movable fluids versus porosity.  
Figure 10: Relationship between porosity for effective movable fluids and average pore throat radius.  
Figure 9: NMR \(T_{2}\) spectra and effective movable fluid distribution obtained from sandstone samples in the E\({}_{\text{s4}}\) reservoirs. (**A**) Sample C141-1, \(\varphi=17.5\%\), K = 11.61 mD; (**B**) Sample C406-1, \(\varphi=13.6\%\), K = 0.76 mD; (**C**) Sample F153-1, \(\varphi=10.9\%\), K = 4.40 mD.