Investigation of the residual±funicular nonwetting-phase-saturation relation

The constitutive relation that describes the amount of nonwetting ̄uid phase entrapment is critical to the modeling of multiphase ̄ow in porous media, but it has received insucient attention in the literature. We studied this relation using both experimental and modeling approaches: we used a nondestructive, X-ray monitored, long-column experiment that yielded a rich data set for two di€erent porous media; we also used a quasi-static network model to simulate the experimental data and examine mechanisms a€ecting the relation. The experimental work yielded a signi®cant data set for residual nonwetting phase (NWP) saturation as a function of maximum funicular nonwetting phase saturation. We suggest a functional form that represents the observed data sets accurately. Network model calibration to experimental data yields acceptable model-data agreement and a clear understanding of constraints that should be satis®ed when using such models to avoid physically unrealistic behavior. We found that the pore-throatsize characteristics and the snap-o€ process occurring in pore throats strongly in ̄uence the manifestation of pore-body-size characteristics during imbibition and nonwetting ̄uid phase entrapment. We examined an estimation method proposed by Wardlaw and Taylor for the residual±funicular relation. We observed that the method yields an unrealistic relation for each porous media in the long-column experiments, and we used network modeling to understand the criteria that ensure a realistic estimate. Ó 2000 Elsevier Science Ltd. All rights reserved.