An Experimental Study of the Factors Influencing Apparent Wetting Phase Residual Saturation in Dynamically Drained Porous Media

When a porous medium drains, the wetting phase saturation in the medium the fractional volume of wetting phase (often water) in the pores] is typically observed to approach a minimum value, referred to as the wetting phase residual saturation ( $S_{\text {wr}}$ ). While many simulators of unsaturated and multiphase flow assume $S_{\text {wr}}$ to be a single value for a given fluid and porous medium system, there is considerable evidence that $S_{\text {wr}}$ may be a dynamic property, with its value a function of the rate of saturation change. The objective of this work was to study this relationship, with emphasis on understanding the effect of drainage rate on the apparent residual ( $S_{\text {wr-apparent}}$ ) at moderate capillary pressures and over short time frames. Dynamic unsaturated drainage experiments were conducted to explore this relationship. A total of 61 experiments were conducted, involving 363 individual secondary drainages. Experiments covered a total of four different porous media, and three wetting phases. Results indicate that systems that exhibit greater resistance to flow (lower mean grain size and higher kinematic viscosity) also exhibit a greater sensitivity in $S_{\text {wr-apparent}}$ to drainage rate. An imaging experiment was conducted to examine whether the source of the relationship could be observed through continuum-scale imaging. Results of the imaging were able to rule out membrane desaturation artifacts as a cause of the relationship, but were unable to identify significant differences in fluid configurations between fast and slow drainages that might lead to the observed effects.