## 3 Results
### Drillability Variation with Temperature  
Mean and standard deviation numerical values of SJ and S\({}_{20}\) are depicted in Table 2 and evolution for the different temperatures are represented in Figure 9. The results show significant values of standard deviation at certain temperatures related to the perceived slight variations in the visual appearance of samples (i.e., changes in the grey tone and presence of very thin calcite veins).  
SJ for air-cooled samples showed little variation up to 500 \({}^{\circ}\)C, and then we observed a sudden increase at 600 \({}^{\circ}\)C that tripled the initial mean values of the intact rock. Water-cooled samples showed a constant increase with temperature except for a marked local decrease at 200 \({}^{\circ}\)C. Final mean values at 600 \({}^{\circ}\)C doubled the initial values of the intact rock. Values of S\({}_{20}\) for air-cooled samples were almost constant up to 300 \({}^{\circ}\)C. Values then increased for 400 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, and remained constant at 600 \({}^{\circ}\)C (where a final mean value of 1.3 times that for intact rock was reached). Slight differences were registered between air- and water-cooled samples up to 400 \({}^{\circ}\)C where water-cooled samples showed lower values especially at 200 \({}^{\circ}\)C. For higher temperatures, trends were almost equal for both cooling methods.  
Values of SJ and S\({}_{20}\) were combined to obtain a final DRI value for each temperature (Figure 9c), and this enabled classification of the drillability of Prada limestone (Table 2) according to Bruland [41]. DRI increased with temperature, and this implied a change in the drillability category at 500 \({}^{\circ}\)C (from medium to high) for both cooling methods. Decreases in the DRI could be observed at 200 \({}^{\circ}\)C for water-cooled samples, which is consistent with the recorded variations in SJ and S\({}_{20}\). No relevant differences could be observed between cooling methods for the highest temperatures.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Air-Cooled Samples**} & \multicolumn{4}{c}{**Water-Cooled Samples**} \\
**Temperature (\({}^{\circ}\)C)** & **SJ (1/10 mm)** & **S\({}_{20}\) (\%)** & **DRI** & **Class** & **SJ (1/10 mm)** & **S\({}_{20}\) (\%)** & **DRI** & **Class** \\ \hline
105 & 11.35 \(\pm\) 4.41 & 42.77 \(\pm\) 10.94 & 44 & Medium & 11.35 \(\pm\) 4.41 & 42.77 \(\pm\) 10.94 & 44 & Medium \\
200 & 14.92 \(\pm\) 6.27 & 45.12 \(\pm\) 8.40 & 49 & Medium & 5.73 \(\pm\) 0.71 & 39.23 \(\pm\) 6.02 & 39 & Low \\
300 & 10.30 \(\pm\) 7.05 & 44.23 \(\pm\) 7.41 & 45 & Medium & 14.66 \(\pm\) 5.97 & 41.73 \(\pm\) 7.26 & 45 & Medium \\
400 & 17.29 \(\pm\) 10.66 & 49.45 \(\pm\) 5.67 & 52 & Medium & 14.56 \(\pm\) 9.02 & 48.16 \(\pm\) 2.18 & 50 & Medium \\
500 & 16.34 \(\pm\) 10.70 & 55.84 \(\pm\) 6.07 & 59 & High & 21.88 \(\pm\) 5.60 & 57.71 \(\pm\) 4.74 & 62 & High \\
600 & 30.13 \(\pm\) 11.32 & 56.43 \(\pm\) 4.44 & 62 & High & 23.60 \(\pm\) 9.81 & 55.89 \(\pm\) 9.87 & 62 & High \\ \hline \hline \end{tabular}
\end{table}
Table 2: Variation with temperature of Siever’s J miniature drillability test (S)), brittleness test (S\({}_{20}\)), and drilling rate index (DRI) final value and for air- and water-cooled samples.  
Figure 9: _Cont._