Defining a novel pore-body to pore-throat Morphological Aspect Ratio that scales with residual non-wetting phase capillary trapping in porous media

We introduce a new method for defining a pore-body to pore-throat aspect ratio from segmented 3D image data, based on a connectivity metric applicable to porous media with widely varying pore-space connectivity and porespace morphology. The 'Morphological Aspect Ratio' (MAR) is identified from the pore-space connectivity, using the Euler number (chi) as a function of a pore-space size defined by a morphological opening (erosion and dilation) of the pore space. We show that residual non-wetting phase trapping in porous media resulting from secondary imbibition scales with the MAR. Trapping was investigated in a Bentheimer sandstone core and five columns of partially sintered glass-particle packs with different combinations of glass beads and crushed glass ranging in size from 0.3 to 1.2 mm, resulting in porosity levels of 22-36%. Residual non-wetting phase trapping scales with the MAR, in contrast to the aspect ratio calculated with the traditional Maximum Inscribed Sphere (MIS) algorithm applied after partitioning the pore space into pore bodies and pore throats with a watershed transform followed by a region merging algorithm. This novel aspect ratio is a robust method that is less affected by segmentation errors compared to other methods for calculating aspect ratio and is applicable to residual non-wetting phase trapping resulting from capillary-driven flow of a wetting fluid through water-wet porous media.