Gas pressure gradients in unsaturated porous media and the assumption of infinite gas mobility

Abstract

AbstractThe assumption that gas is infinitely mobile, moving without viscous pressure drops, is common in studies of unsaturated flow in porous media. The objectives of this work were to use experimental measurements to examine that assumption in systems experiencing rapid drainage and to explore the extent to which observed pressure drops could be described by conventional multiphase flow simulation tracking viscous flow in both phases. Because many published studies have used vented columns in an effort to equilibrate pore gas pressures with inlet gas, an additional objective of the work was to use experimental measurements to explore the ability of column vents to equilibrate pore gas with inlet gas during dynamic drainage. Results of the work suggest that gas pressure gradients can be significant, and that the assumption of infinite gas mobility is likely to be unsatisfactory in many systems where moderately rapid saturation change occurs. While vents have the potential to influence flow by providing additional gas inlets, experimental results of this work show almost no impact on pore gas pressures from a vent similar in size to those in other published studies. An equation developed as a part of the work suggests that the spatial slope of gas pressure with distance away from the front during dynamic drainage is proportional to the ratio of outflow Darcy velocity to saturated hydraulic conductivity for vertical columns. As such, systems with more rapid saturation change also have a greater potential to exhibit experimental artifacts related to gas pressure gradients.