UC Berkeley

During drainage, as a voxel of porous material is being actively invaded prior to percolation, the invading nonwetting fluid cannot be transmitted to the neighboring downstream voxel until a critical nonwetting saturation ( S n c ) is reached. This occurs when the invading fluid forms a percolating path across the voxel. We present novel laboratory observations of gravitationally stable, air/water drainage in thin bead packs, with 2D saturation fields monitored at high spatial and temporal resolution using transmitted light. We also present Invasion Percolation simulations. Both show that S n c has a strong dependence on capillary number ( Ca ), Bond number (Bo), and voxel size ( L z ). We propose a percolation model that describes the functional form of S n c ( Bo , Ca , L z ) . We also produce a new theoretical model for the macroscopic capillary pressure function P c ( S w , Bo , Ca , L z ) , where S w is the saturation of wetting fluid in the voxel.