## 3 Results
### Rock-Fluid Interaction  
Sandstone, alongside other common reservoir rocks, has been a dominant research focus through the evolution of TD-NMR wettability applications [13, 14, 28, 35, 37]. The wettability evolution of Berea Sandstone (initially water-wet) was examined here. Figure 10 shows a decrease in the T\({}_{2}\) dominant mode from 31.8 \(\pm\) 0.1 ms on Day 7 to 25.5 \(\pm\) 0.1 ms at day 21. The difference in the main mode/peak location for both day 7 and 21 with respect to untreated glass beds, correlates with the differences in pore sizes and mineral composition. Therefore, the shift in the dominant mode of the T\({}_{2}\) is indicative of the surface becoming increasingly oil wet, as expected for these conditions. Work on silica surfaces shows that polar fractions of the crude oil, for example, polar asphaltenes, adsorb on the surface [56]. This leads to wettability alteration and consequently to changes in relaxation mechanisms in fluid-rock interactions processes. Figure 9: Diffusion coefficient behavior of treated and untreated glass beads for different aging times and at 25 \({}^{\circ}\)C: (**a**) 1% Na\({}_{2}\)SO\({}_{4}\); (**b**) 100% Na\({}_{2}\)SO\({}_{4}\). Compared to the glass beads (Figures 7a and 8a), T\({}_{2}\) distributions of the brine-saturated rock in Figure 11 show a different behavior. Both untreated glass beads and sandstone are considered initially water-wet. However, brine still induces a decrease in the dominant mode location of the T\({}_{2}\) distribution. The increase of the ion exchange between rock surface and the brine, produces this effect. Nevertheless, the decrease is more appreciable for low salinity (from 152.6 \(\pm\) 0.1 ms @ day 7 to 143.1 \(\pm\) 0.1 ms @ day 21) than for high salinity (from 143.1 \(\pm\) 0.1 ms @ day 7 to 139.1 \(\pm\) 0.1 ms @ day 21). This reflects the different effect on wettability alteration offered by low-salinity brine compared to high-salinity. The lower ionic strength of the low-salinity brine allows for expansion of the electric double layer on the surface and consequently the modification of the surface zeta potential. This change in the charge distribution is in part responsible for the change in the interaction and therefore of the surface-induced relaxation. Figure 11: _Cont._  
Figure 10: Relaxation time distribution (T\({}_{2}\)) of Berea Sandstone saturated with crude oil for 21 days at 25 \({}^{\circ}\)C.  
Figure 12a depicts the significant decreases in the diffusion coefficient, from 1.654 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s to 1.460 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s for low-salinity brine and from 1.347 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s to 1.279 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s for high-salinity brine is a verification of the ion exchange on the rock surface. However, this effect has been shown to be beneficial for wettability [6; 55]. On the other hand, Figure 12b shows the diffusion coefficient for the oil-rock interaction, clearly indicates the adsorption process occurring as it decreases over time, assuming that no major chemical reaction occurs between the rock and oil molecules. The T\({}_{2}\) and diffusion coefficient results of the rock-oil or rock-brine systems analyzed together indicate wettability alteration of the surface. However, a new complication arises due to the heterogeneity offered by Berea Sandstone, steering the analysis to focus on trends instead of the specific T\({}_{2}\) peak location and diffusion coefficient values. Effects such as hydration layers around ions can affect the diffusion coefficient dependence on salinity, but could not explain the decrease over time observed in these results. Figure 11: Relaxation time distribution (T\({}_{2}\)) of Berea Sandstone saturated with brine for 21 days at 25 \(\,{}^{\circ}\)C. (a) 1% Na\({}_{2}\)SO\({}_{4}\); (b) 100% Na\({}_{2}\)SO\({}_{4}\). Figure 12: \(Cont\).