Laboratory measurement of water imbibition into low-permeability welded tuff

Abstract

Laboratory imbibition and vapor-diffusion experiments were developed and conducted to measure accurately water imbibition and vapor condensation into welded tuff of low permeability. Automatically recorded balance readings were used to quantify the uptake of water into rock cores by imbibition and/or vapor condensation. The total uptake of water was checked against independent weighings of the sample before and after each experiment. As water was imbibed into the sample and evaporated from the reservoir, the buoyant force on the sample decreased. Balance readings corrected for the buoyancy effect agreed very well with independently measured total uptake. Sorptivity was calculated from the slope of a plot of corrected cumulative imbibition versus square-root of time. Vapor condensation can provide a significant contribution to the water uptake into cores during imbibition experiments. Rock cores were coated with epoxy on the sides and covered on the top (except for a small hole that allowed air to escape) to minimize the vapor condensation contribution. Comparison of the sorptivity values between different core treatments shows that a consistent fraction, about 20%, of water uptake actually enters the core by vapor condensation. Overall, methods for separating the confounding effects of buoyant-force change and vapor condensation result in more accurate measurement of sorptivity, as exhibited by the consistent and reproducible results of triplicate measurements. These methods for buoyancy and vapor-condensation correction are expected to be very useful for measuring imbibition rates on a wide range of porous materials, especially very-low-permeability materials.