## 5 Conclusions  
A laboratory study on the effect of axial (up to 95 MPa) and radial (up to 60 MPa) stress on the seismic velocity was conducted for a broad range of dry sedimentary and metamorphic rocks that are part of the Kizldere geothermal system. Well-controlled acoustic-assisted unconfined and confined compressive strength experiments yielded the baseline seismic-response characterisation; reflecting the stress-dependence of the seismic velocities. Furthermore, the effect of temperature (up to maximum reservoir temperature of 240 \({}^{\circ}\)C), on seismic velocity, at atmospheric pressure conditions, was investigated as well. This study resulted in the following main conclusions:  
* Thin section analyses showed that the presence of a fracture increases the 2D permeability by roughly a factor 10 for the main reservoir formations, i.e., marble and  
Figure 17: P-wave (continuous lines) and S-wave (dashed lines) velocities as a function of axial stress, and thus depth below the surface, for the (partly) saturated calcschist sample (TD12-CS4) discussed in Section 3.6. Experimental data are shown in black whereas modeled velocities, according to [PERSON] fluid substitution [40], for saturations of 25 and 100% are presented in red and blue, respectively. The equations used (Equations (S1) to (S4)), stress-dependent bulk moduli and densities (Table S2), and a short description of Gassmann’s fluid substitution is presented in the Supplementary Materials. Axial stress–depth relationship is taken from [35]. calcschist. This underlines the importance of the presence of fractures in the Kizildere geothermal reservoir. * Most of the UCS-related acoustic results showed a similar P-wave velocity-stress trend. It includes a sharp increase in velocity due to initial compaction, followed by a moderate increase in velocity since bulk of the pore space was already closed and, finally, just before rock failure, a slight decrease in velocity due to rock damage (i.e., microcracking). * Several limestone and quartzite rock samples revealed the presence of axial splitting patterns after failure occurred. The degree of axial splitting appears to be related to their porosity, where the presence of axial splitting fractures increases with decreasing porosity. All marble, calcschist, quartzschist, mudstone, and siltstone samples investigated showed clear shear failure characteristics after rock failure. * Large variations in static elastic parameters were found among the three limestone samples used for conducting the UCS experiments. Most likely, a high level of heterogeneity, due to diagenesis processes and the presence of fossils, within the limestone yielded different axial stress-strain relationships for the three studied cores. * Radial stress appeared to have a negligible impact on the P-wave velocity, as long as it was higher than atmospheric pressure. Experiments without any additional radial stress applied on the rock, i.e., UCS experiments, showed lower velocities compared to the experiments where a radial stress was imposed on the sample (CCS experiments). This is due to the fact that in the CCS tests (experimental series B-D), overall compaction was more efficient since it also took place in the radial direction. The latter promoted an increased mineral-to-mineral contact area, hence higher velocities. * Most of the studied rock formations showed a reducing P-wave velocity as a function of increasing temperature due to thermal expansion of the constituting minerals, leading to loosening of the rock's internal structure. The marble and calcschist samples showed the largest reduction in P-wave velocity as function of increasing temperature (40-240 \({}^{\circ}\)C): 42% and 36% for the marble and calcschist samples, respectively.