## 4 Results
### Mercury Injection Porosimetry (MIP)  
The connected porosity of the samples before and after the thermal tests is shown in Table 2 and the variations in the pore radii access distribution in Figure 5. During heating, the differential thermal expansion coefficients of the various mineral grains cause the generation of microcracks. When rocks are held at temperature, cracks generated at this temperature remain closed. During cooling, microstructural modifications can appear [51]. If the microcrack threshold has not been reached, the minerals contracted during cooling without generating microcracks. If the thermal stress has been exceeded, microcracking occurred and increased the porosity [50]. The Chalons chalk showed an initial porosity slightly lower than 40%, which consisted of a homogeneous microporous network with an unimodal distribution centred around 0.26 \(\upmu\)m. During the TT, the porosity increased slightly, between 7.5 and 11%, reflected by an increase in the modal peak intensity. That indicated an augmentation of the pore volume with the same size that the initial rock. After TF, the porosity increased by about 5% with no change in the pore access radii. The Fontainebleau sandstone had a porosity of around 4% in its initial state, with a wide unimodal distribution centred on 1 \(\upmu\)m. The first cycle of the TT already revealed an increase in the porosity of approximately 11%, with also an evident change in the pore access radii larger than 1.5 \(\upmu\)m. The increase was not progressive along the cycles, although the maximal value was found at 130 \({}^{\circ}\)C, with more than 19% increase and a pore access threshold that shifted up to almost 2 \(\upmu\)m. In addition, a new family of bigger pores appeared from the very first heating cycle between 43 and 113 \(\upmu\)m. For the TF cycle 1, the microstructural variations were even more remarkable, with a strong increase of 35% of pore volume and a pore threshold increase up to 3 \(\upmu\)m. The fatigue (TF cycle 5) produced a  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & & **TT** & & & **TF** \\ \hline  & **25 \({}^{\circ}\)C** & **90 \({}^{\circ}\)C** & **100 \({}^{\circ}\)C** & **110 \({}^{\circ}\)C** & **120 \({}^{\circ}\)C** & **130 \({}^{\circ}\)C** & **200(1) \({}^{\circ}\)C** & **200(5) \({}^{\circ}  
slight opening of the microcracks at the end of the test, with an increase of 9% in porosity compared to the initial state and an increase to 2 \(\upmu\)m of the pore threshold. The Gris Alba granite presented an initial porosity volume of 1.05% and showed two main distributions, a wide micropore family with pore access radii below 1 \(\upmu\)m and a mesopore family starting from around 10 \(\upmu\)m. Through the TT, this rock showed a decrease in its pore volume of 17% at 90 \({}^{\circ}\)C and 25% at 130 \({}^{\circ}\)C. Figure 5 exhibited a redistribution and a slight reduction in the micropore family, which could be due to the heating but also to the heterogeneity between the samples. Nevertheless, there was a clear decrease in the mesoporous family for all the heating cycles. The TF test showed a great increase in the porosity, with 51% after the first cycle and 32% at the end. The microporous family  
Figure 5: Pore access size distribution curves of samples before and after the first heating cycle and at the end of the TT (Thermal Threshold test) and TF (Thermal Fatigue test). CC: Chalons chalk; FS: Fontainebleau sandstone; GA: Gris Alba granite; GS: Golden Ski granite. exhibited a variation in the radii access proportion, without being possible to determine an increase or decrease graphically. A great increase in the pore access radii larger than 1 \(\upmu\)m was measured, with the apparition of new peaks and the development of voids around 100 \(\upmu\)m.  
The Golden Ski granite had an initial porosity volume of 3.77%.