Morphological and Euler characteristics of nonwetting phases in porous media

The geometric structure variation of the nonwetting phase (NWP) in porous media plays a critical role in numerous energy-related processes. This work conducted a series of gas/liquid displacements in three rock cores using high resolution x-ray microcomputed tomography (micro-CT), including two sandstones and a Berea core with different pore architectures. The morphological and topological characteristics of NWP were investigated at the pore scale in a full cycle of primary drainage (PD), main imbibition (MI), and main drainage (MD). It is found that roof and distal snap-off occurred during drainage, resulting in nonwetting (NW) singlets and ganglia. These NW bubbles occupied 10%-20% NWP volume. Both throat snap-off and pore snap-off were observed in MI, which generally occur at a single-pore scale. The effect of viscous force is significant even at a capillary dominant displacement. The trapped NWP during PD can be partially or totally displaced in MI through further snap-off events or viscous remobilization. Due to the temporal and spatial scale diversion in pore scale events, the amplitude of pressure drop fluctuations among PD, MI, and MD is different. The significant difference in NWP characteristics between PD and MD is connectivity, thus leading to hysteresis.