How to Generate Pore Size Distribution and Characterize Open Versus Closed Pore Networks

Porosity analysis refers to the measure of “void fraction” and the characterization of the empty space. There are numerous application fields for which the analysis of pores is crucial in the understanding of the 3D structure and its mechanical properties. For example, in the field of bone research porosity is a parameter for the overall mechanical strength of the bone, while petroleum engineers are interested in the pore network connectivity to allow for transportation of liquids in oil & gas applications. Porosity analysis refers to the counting of empty space and characterization of their connections. Empty space is either fully surrounded by material on all sides, making it a closed pore, or if the empty space is not fully enclosed but at some point in 3D it can find a connection to the space outside of the object, it is defined as an open pore. The method note "MN059_PorosityAnalysis" explains why the conventional 2D analysis is very different from the 3D analysis where the values are obtained from the entire dataset. In addition, it describes in detail how to quantify the porosity in a dataset, both for open and closed porosity, and how to obtain a size distribution of the pores, as well as a list of all pores and their surface, volume, centroid location,… The method note also demonstrates how to use custom processing for noise removal functions as well as for automatic data analysis in a batch.

Visualization of open and closed porosity is very important when interpreting the data. 3D visualization is a lot more than “pretty pictures” and it is worthwhile to explore the techniques and tools available in the SkyScan software. A visual representation of the calculated characteristic properties can put new insights to the meaning of the numbers that were generated when performing porosity analysis as described in previous section. The image below nicely illustrates how the open porosity (shown in green color) for the sample on the right is a measure of the sample itself, while the reported value for the open porosity for the rock on the left is artificial and is due to the sample drilling or the VOI selection. In fact, the rock on the left actually contains a closed pore structure and no gas or liquid transportation is possible.

CTVox (volume rendering) is by far the easiest and hence most used, well known visualization tool of the SkyScan software package. On the other hand, CTVol (surface rendering), being slightly more challenging, opens up opportunities for visualization which are not possible in CTVox. After calculating a pore size distribution, the techniques discussed in the first Micro-CT academy Newsletter can be applied to generate color-coded images for size using CTVox. In this issue of the Bruker micro-CT Newsletter CTVol is used to visualize color-coded image for open and closed pore networks. The techniques mentioned in the previous section will be very useful when generating surface meshes for the different types of pore network for surface rendering. Even more, they allow restricting analysis to either the open or closed porosity characteristic. A detailed step by step procedure on how to obtain these results is described in "MN060_Extracting open and closed pore networks for analysis and visualisation".