Predictive model for permeability reduction by small wetting phase saturations

Abstract

Laboratory experiments show that the rate of decrease in gas phase permeability as a function of water saturation can be large at small saturations. Drying out the water saturation during gas injection has been observed to reduce absolute permeability. This paper proposes a quantitative grain‐scale explanation for these observations. We also show why the effect is magnified in rocks having lower porosity. To compute phase geometry and permeability, we use a physically representative network model. The network is extracted from a model rock, built from a dense random packing of spheres modified geometrically to simulate rock‐forming processes. At small saturations the wetting phase exists largely in the form of pendular rings held at grain contacts. The rings decrease the void area available for flowing nonwetting phase. Because the hydraulic conductance of the throat varies with the square of the void area (other factors being equal), the effect on permeability is disproportionate to the volume occupied by the rings. The same approach quantifies the reduction in permeability by salt precipitation during drying.