## 3 Results and Discussion
### Acoustic-Assisted CCS Experiments: Effect of Brine-Saturated Pore Space
Figure 11 presents the seismic velocities, as a function of increasing \(\sigma_{1}\) (which equals \(\sigma_{2}\)), related to the dry- and wet-case experiments as shown in Table 9. Moreover, the acoustic results corresponding to calcschist sample TD12-CS5 in experimental series B (Table 3) are shown as well. Firstly, the effect of a brine-saturated pore space on the seismic velocities will be discussed by comparing the wet-case with the dry-case; both for sample TD12-CS4. Subsequently, an attempt will be made to qualitatively assess the effect of the fracture network in sample TD12-CS4 (Figure 10) on the seismic velocities by comparing its acoustic results with the results obtained for the intact calcschist sample TD12-CS5. For the S-waves, the data related to the dry- and wet-case (TD12-CS4) are almost identical. This is due to the fact that S-waves cannot propagate in fluids, hence the similar velocities observed in both scenarios. On the contrary, a distinct difference, between the dry- and wet-case, is observed for the P-wave velocity. Since P-waves are able to propagate in fluids, and they travel much faster through water compared to air, it is anticipated that the wet-case would show higher velocities compared to the dry-case, for the investigated stress conditions. An attempt was made to reproduce the wet-case results utilising [PERSON]'s theoretical framework for fluid substitution [40]. Unfortunately, it appears that [PERSON]'s methodology is unable to replicate the experimental observations made (Figure 11) as it significantly underestimates the wet-case velocity. Most likely, this is due to [PERSON]'s assumptions (such as a homogeneous, elastic, and isotropic media and a well-connected pore space) since all of these are not applicable to the calcschist sample studied here. By comparing the velocities, related to dry conditions, of TD12-CS4 with the results of TD12-CS5, one may qualify the effect of a fracture network (Figure 10) on the resulting velocities. For both the P- and S-wave velocities, the intact sample TD12-CS5 shows significantly higher velocities compared to the fractured sample TD12-CS4 at \(\sigma_{1}=\sigma_{2}\leq 25\) MPa. This is due to the available pore space, i.e., porosity. While the fractured TD12-CS4 shows a porosity of \(5.93\pm 0.10\) (Table 9), TD12-CS5 reveals a porosity of 2.42 \(\pm\) 0.03 (Table 3), both at atmospheric conditions. This difference in available pore space yields a lower bulk density of TD12-CS4 compared to TD12-CS5, resulting in lower velocities at \(\sigma_{1}=\sigma_{2}\leq 25\) MPa. For stresses of \(\geq 25\) MPa, both samples show roughly identical results in terms of velocities. It appears that most of the cracks present in TD12-CS4 have closed at these elevated stress conditions, leading to similar velocities being observed for both calcschist samples. The comparable velocities are a result of both calcschist samples showing similar elastic properties (Table 8) and densities (Table 2) when intact. Figure 11: P-wave (blue) and S-wave (red) velocity as a function of axial stress, and thus depth below the surface, for a dry and wet (25 \(\pm\) 2% brine) fractured calcschist sample (TD12-CS4). For comparison purposes, results related to intact calcschist sample TD12-CS5 (from experimental series B) are shown as well (purple). Axial stress–depth relationship is taken from [35].