  Pore-network models (PNMs) have become an important tool in the study of
fluid flow in porous media over the last few decades, and the accuracy of their
results highly depends on the extraction of pore networks. Traditional methods
of pore-network extraction are based on pixels and require images with high
quality. Here, a pixel-free method called the flashlight search medial axis
(FSMA) algorithm is proposed for pore-network extraction in a continuous space.
The search domain in a two-dimensional space is a line, whereas a surface
domain is searched in a three-dimensional scenario. Thus, the FSMA algorithm
follows the dimensionality reduction idea; the medial axis can be identified
using only a few points instead of calculating every point in the void space.
In this way, computational complexity of this method is greatly reduced
compared to that of traditional pixel-based extraction methods, thus enabling
large-scale pore-network extraction. Based on cases featuring two- and
three-dimensional porous media, the FSMA algorithm performs well regardless of
the topological structure of the pore network or the positions of the pore and
throat centers. This algorithm can also be used to examine both closed- and
open-boundary cases. Finally, the FSMA algorithm can search dead-end pores,
which is of great significance in the study of multiphase flow in porous media.

Computer Vision and Pattern Recognition;Image and Video Processing