## 3 Results and Discussion
### Acoustic-Assisted CCS Experiments: Varying \(\sigma_{1}\)  
A total of eight CCS experiments, where \(\sigma_{2}\) was kept constant at its respective field-representative stress, and \(\sigma_{1}\) was varied, mimicking the actual depths of the different layers within the Kizldere geothermal field (Table 5), were performed within experimental series D. Proper magnitudes for \(\sigma_{1}\) and \(\sigma_{2}\), that occur in the Kizldere geothermal reservoir (Table 5), were obtained from literature [35]. Figure 8 presents an overview of the obtained P-wave velocities as a function of increasing \(\sigma_{1}\). The S-wave velocity as a function of increasing \(\sigma_{1}\) is shown in Figure S6 (see Supplementary Materials).  
Similar to the observations made for experimental series C (Figure 7), acoustic results related to calcschist, quartzite, quartzschist, micaschist, and marble (Figure 8) show less variation in P-wave velocity, as a function of increasing \(\sigma_{1}\), in comparison to the results related to experimental series A (Figure 5) and B (Figure 6). Once again, this is fully related to the initial conditions of the first measurement point at \(\sigma_{1}=\sigma_{2}\) of 17, 33, 31, 31, and 17 MPa for the calcschist, quartzite, quartzschist, micaschist, and marble, respectively (Table 5). The latter entails that any potential closure of microcracks, leading to an increased grain-to-grain (or mineral-to-mineral) contact area and thus seismic velocity, already occurred prior  
Figure 8: P-wave velocity as a function of axial stress, and thus depth below the surface, for all eight CCS tests conducted within experimental series D. After the samples were loaded until their maximum \(\sigma_{1}\) (Table 5), unloading took place during which several acoustic measurements were taken to check for potential hysteresis effects (circular data points). \(\sigma_{2}\) was kept constant and is presented in Table 5. Axial stress–depth relationship is taken from [35].  
to reaching the abovementioned initial conditions. The loading process, i.e., increasing axial load at a rate of 0.05 kN/s (Section 2.4), seems to take place in the elastic regime as during the unloading cycle similar velocities, that fall well within the loading-related trend, were measured.  
Bulk of the data presented in Figure 8 appear to match the acoustic results of experimental series B (Figure 6) relatively well, suggesting a minimal, or negligible, effect of \(\sigma_{2}\) on the P-wave velocity. In order to verify this, experiments within series A-D, for which exactly the same core sample was used, are presented in a single plot (Figure 9). From the results, one may conclude that \(\sigma_{2}\) appears to have a negligible impact on the P-wave velocity, as long as \(\sigma_{2}\) is higher than atmospheric pressure. The reasons for the latter statement are the deviating trends related to the UCS tests, i.e., experimental series A, for quartzite, quartzschist, limestone, and calcschist (Figure 9). Abovementioned UCS-related results indicate lower velocities compared to the case with a confining pressure, i.e., with \(\sigma_{2}\) (CCS experiments). This is most likely due to the fact that in the CCS experiments (experimental series B-D) overall compaction was more efficient since it also took place in the radial direction (promoting an increased mineral-to-mineral contact area), as a result of the imposed \(\sigma_{2}\), whereas in the UCS experiments (experimental series A) it only occurred in the axial direction. Note that all CCS-related data follow more or less the same trend for each individual rock type investigated.