Laboratory measurement of water imbibition into low-permeability welded tuff

Laboratory imbibition and vapor-diffusion experiments were developed and co nducted to measure accurately water imbibition and vapor condensation into welded tuff of low permeability. Automatically recorded balance readings we re used to quantify the uptake of water into rock cores by imbibition and/o r vapor condensation. The total uptake of water was checked against indepen dent weighings of the sample before and after each experiment. As water was imbibed into the sample: and evaporated from the reservoir, the buoyant fo rce on the sample decreased. Balance readings corrected for the buoyancy ef fect 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 co ntribution to the water uptake into cores during imbibition experiments. Ro ck 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 condens ation contribution. Comparison of the sorptivity values between different c ore treatments shows that a consistent fraction, about 20%, of water uptake actually enters the core by vapor condensation. Overall, methods for separ ating the confounding effects of buoyant-force change and vapor condensatio n result in more accurate measurement of sorptivity, as exhibited by the co nsistent and reproducible results of triplicate measurements. These methods for buoyancy and vapor-condensation correction are expected to be very use ful for measuring imbibition rates on a wide range of porous materials, esp ecially very-low-permeability materials. (C) 2001 Elsevier Science B.V. All rights reserved.