## 3 Results & Discussion
### Imaging Etched Carbonates Pore Casts
#### 3.3.2 Etching Quality  
Micro-CT scanning was also used to understand the effect of dissolving the rock matrix on the epoxy conforming to the pore volume and the overall integrity of the pore cast. CT scans of the pore cast were performed before and after the etching of the rock matrix with a spatial resolution of 5.41 um, as shown in Figure 14. Threshold segmentation was applied to the 8-bit gray-level TIFF images to identify pore volume -epoxy- and rock  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Impregnation Parameters**} \\ \hline Degassing & 80 mbar \\ Degassing time & 30 min \\ Impregnation pressure & 65 bar \\ Curing time & 30 h \\ Resin viscosity & 0.468 Pa-s \\ Temperature & 25 °C \\ \hline \hline \end{tabular}
\end{table}
Table 4: Epoxy pore casting of Indiana limestone.  
Figure 13: The micro-CT image of the Indiana limestone sample impregnated with epoxy. This grayscale image is but one of the 943 slices representing the 5.1-mm sample thickness. The sample is composed mainly of allochems as indicated; however, the microporosity geometry can hardly be identified at the resolution of 5.41 μm per voxel. Defects, such as the highlighted type 2A shrinkage defect, could still be identified in the impregnated sample. They represent only 0.21% of the pore volume. Colors are assigned to different grayscale values to aid visualization. The color code indicates the different elements of the image.  
matrix. Based on the 3D topographical image of 5-mm thick Indiana limestone sample, the total pore volume was calculated before and after acid dissolution and was found to be 0.158 and 0.151, respectively.  
We verified that matrix dissolution would have almost no effect on the epoxy representation of the pore volume from the pore volume calculation. We tested the resilience of epoxy pore casts over a wide range of HCl concentrations from 0.25 to 12 molar, and the pore volume calculation of these samples showed almost no change across the entire range.  
We investigated the dynamics of etching an epoxy impregnated sample by time-series X-ray CT scans, shown in Figure 15. We observed that the etching rate decreased rapidly with the sample thickness, i.e., the etching time of the first mm was 10 h, while it was 50 h for the second mm. The dissolution of calcite in acids is a rapid heterogeneous reaction [60], and [PERSON] et al. [61] showed that, in strong acids like hydrochloric acid (HCl), the dissolution would only be limited by the diffusion of acid to the calcite surface and the reaction at the solid-liquid interface.  
In pore casts, the pore space is filled with epoxy, and thus the sample tortuosity limits the rock surface area exposed to the acid. Moreover, the mass transfer of reactants and/or products slows the etching rate. Hence, the maximum etching rate will be observed at open boundaries of the sample, and the rate will decrease rapidly with the sample  
Figure 14: Two micro-CT scans of resin impregnated sample pre-etching (**left**) and post-etching (**right**) the rock matrix with hydrochloric acid. Color coding based on threshold segmentation was used to identify rock matrix (grey), and resin (green).  
Figure 15: Time-series of micro-CT scans of an epoxy impregnated sample etched with 0.5 M HCl. Only the rock matrix is represented in the 3D images by solid (yellow) to detect the evolution of the acid etching process, while the resin was digitally removed. The scans show that dissolution rate of deeper pads of the impregnated sample was significantly slower compared to the upper parts.  
thickness. To have faster and more homogeneous etching, we mechanically cut slices with a 1-mm thickness where both upper and lower surfaces are not encapsulated by resin. In comparison to a few days for a 5-mm sample, the total etching time of a 1-mm slice was around 3 h.