## 3 Results and Discussion
### Acoustic-Assisted CCS Experiments: Effect of Brine-Saturated Pore Space  
All laboratory results presented so far are related to dry conditions, i.e., pore space solely filled with air. One additional CCS test, where \(\sigma_{1}=\sigma_{2}\) and thus similar to experimental series B (Section 3.3), was carried out for a brine-saturated post-failure calcschist sample (TD12-CS4 in Table 2). A simplified NaCl-based brine, that corresponds to a total ionic strength equal to the total ionic strength of the geothermal water in the Kizildere reservoir, was prepared. The brine contains 0.41 weight percent (wt%) NaCl. The fractured calcschist sample was saturated by placing it in a glass beaker filled with brine, under vacuum conditions, for more than 15 h. Afterwards, by measuring its wet weight and using the known dry weight, the water saturation was estimated. Table 9 presents an overview of the core properties. Figure 9: P-wave velocity as a function of axial stress, and thus depth below the surface, for all experiments within series A (UCS), B (\(\sigma_{1}=\sigma_{2}\)), C (\(\sigma_{\text{diff}}=\text{constant}\)), and D (vary \(\sigma_{1}\)), where the exact same core sample was used. Axial stress–depth relationship is taken from [35]. Prior to discussing the acoustic results, it is instructive to analyse the formed fracture network in the calcschist sample as a result of the performed UCS test (Section 3.2). Figure 10 presents several cross-sections of the post-failure calcschist sample, obtained with X-ray micro-tomography (micro-CT). The images clearly indicate the presence of a relatively dense fracture network in the upper part of the core sample (cross-sections 1 and 2). The dense network consists of multiple fractures with maximum apertures of around 0.1-0.2 mm or smaller. Moving towards the centre part of the calcschist core (cross-sections 3 and 4), the relatively dense network makes place for a localised shear plane in the form of two connected fractures with a maximum aperture of roughly 0.5 mm (red circle in cross-section 3). Near the bottom part of the core (cross-section 5), the two previously connected fractures became disconnected. A network of fractures, as shown in Figure 10, enhances a reservoir's porosity and permeability, and is crucial for the productivity of the Kizildere geothermal reservoir (Section 2.1). Figure 10: Summary of the fracture network in calcschist sample TD12-CS4. Fractures were formed as a result of the UCS experiment conducted (Section 3.2). Cross-sections 1–5 were obtained with micro-CT scanning using an X-ray tube with 160 kV voltage and a current of 240 \(\mu\)A. Pixel size and slice thickness equals 0.06 \(\times\) 0.06 and 0.03 mm, respectively. It yields a resolution of 16.67 pixels per mm. The single scan taken consists of a total of 2054 slices, capturing the entire sample length. The maximum observed fracture aperture equals approximately 0.5 mm (red circle in cross-section 3). \begin{table}
\begin{tabular}{c c c} \hline \hline
**CCS (\(\sigma_{1}=\sigma_{2}\))** & \multicolumn{2}{c}{**Experiment**} \\
**Brine-Saturated vs. Dry** & & \\ \hline Type & Calcschist & Calcschist \\ Code & TD12-CS4 & TD12-CS4 \\ Pore fluid & Air (dry) & Brine \\ Length (mm) & 61.5 \(\pm\) 0.1 & 61.5 \(\pm\) 0.1 \\ Diameter (mm) & 30.1 \(\pm\) 0.1 & 30.1 \(\pm\) 0.1 \\ Porosity (\%) & 5.93 \(\pm\) 0.10 & 5.93 \(\pm\) 0.10 \\ Pore Volume (mm\({}^{3}\)) & 2595 \(\pm\) 66 & 2595 \(\pm\) 66 \\ Matrix density (g/cm\({}^{3}\)) & 2.78 \(\pm\) 0.01 & 2.78 \(\pm\) 0.01 \\ Bulk density (g/cm\({}^{3}\)) & 2.62 \(\pm\) 0.02 & 2.63 \(\pm\) 0.02 \\ Water saturation (\%) & 0 & 25 \(\pm\) 2 \\ \(\sigma_{1}=\sigma_{2}\) (MPa) & 10–50 & 10–50 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Properties of the calcschist sample used to study the effect of a brine-saturated pore space on the seismic velocity. Note that the core sample used here was previously used in experimental series A (Table 2).