A detailed analysis of geometric parameters of a catalyst pellet for a hydrodemetallation reaction was carried out. A three-dimensional network of interconnected cylindrical pores was taken as a pore model, The change of the pore space with time was analysed by a cluster counting algorithm; It turned out that the reaction continues for a long time after the percolation threshold has passed. In this example, the radii of macropores are the most important parameters. Insights into the importance of different geometric parameters on the catalyst performance could be obtained, which, in principle, cannot be obtained by continuum models.