## 3 Results
### Glass Beads-Fluid Interaction  
#### 3.2.1 Oil-Surface Interaction  
The oil T\({}_{2}\) distribution for the bulk oil is shown in Figure 4, as a reference for results in the glass beads pack. The crude oil contains several solubility classes. The most polar molecules are usually associated with the asphaltene fraction, which is defined as being insoluble in low-molecular weight alkanes, for example, pentane, but soluble in toluene. Resins are closely related to the asphaltene fraction and can exhibit polar behavior as well. The resins can impact the solubility or stability of the asphaltene class in solution. The asphaltene fraction tends to contain high-molecular weight species, though not always, but they contribute significantly to wettability alteration, due to their affinity for surfaces. Also, short relaxation time is anticipated for this solubility class. Crude oil is complex in molecular composition, so the relaxation structure of the bulk is expected to be more complex than water. As can be seen from the figure, three distinct peaks were found in the T\({}_{2}\) spectrum. To estimate the error in the dominant peak location, the experiment was run five times and the standard deviation of the peak location was used as error metric. However, only one run is shown in the figures. The equivalent oil T\({}_{2}\) distributions contained in untreated and treated glass beads packs saturated with oil and aged for 21 days at 25 \({}^{\circ}\)C are shown in Figure 5. The untreated glass beads (GB) give rise to longer relaxation times (dominant mode location: 40 \(\pm\) 0.1 ms @ Day 1) corresponding to a water-wet (hydrophilic) surface. Over time, the dominant mode (highest peak) of the T\({}_{2}\) distribution shifted to shorter values (dominant mode location: 35.7 \(\pm\) 0.1 ms @ Day 21), indicative of the alteration suffered by the media and corresponding to a more oil-wet surface. Figure 5b shows the T\({}_{2}\) distribution behavior in the treated glass beads (TGB) with oil during the period of 21 days. The slight decrease of the dominant mode position from 35.1 \(\pm\) 0.2 ms to 34.7 \(\pm\) 0.1 ms indicates that the pore surfaces are mildly altered by the interaction with oil, suggesting that after the chemical alteration, aging will still produce further oleophilicity. However, these changes are not significant in comparison to the effect observed for untreated glass beads. It is worth noticing that the other two modes/peaks associated to oil are also affected in similar fashion, namely that the oil residing in the untreated bead pack has all three modes shifted similarly. However, when comparing with the bulk oil response, the relative shifts and broadening of the modes seem to indicate compositional-dependent effects. Figure 4: Oil relaxation time distribution (T\({}_{2}\)) of the bulk oil. Figure 6 shows the diffusion coefficient as a function of aging time. This value in TD-NMR is an average over all molecular groups, so compositional-dependencies are hard to pinpoint. Despite this average behavior, for untreated glass beads, the oil diffusion coefficient decreases significantly during the 21 days from 0.070 \(\pm\) 0.001 to 0.057 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s, in concert with the shifts in T\({}_{2}\). By the same token, the result for treated glass beads shows a much smaller drop in the diffusion coefficient (0.056 \(\pm\) 0.002 \(\times\) 10\({}^{-9}\) m\({}^{2}\)/s), which is also consistent with the small shifts in the T\({}_{2}\) distribution. The analysis of the two aforementioned parameters, that is, T\({}_{2}\) and \(D\), indicates the acquisition of an oleophilic behavior of the surfaces within the porous space, being these chemically treated or aged in oil. In both cases, the alteration is the result of the adsorption process or ionic interaction between oil and the ions present on the surface [12; 38].