Capillary rise in porous media

Capillary rise experiments were performed in columns filled with glass bead s and Berea sandstones, using visual methods to register the advance of the water front. For the glass bead filled columns, early time data are well f itted by the Washburn equation. However, in the experiments, the advancing front exceeded the predicted equilibrium height. For large times, an algebr aic behavior of the velocity of the front is observed (T. Delker et at, Phy s. Rev. Lett. 76, 2902 (1996)), A model for studying the capillary pressure evolution in a regular assembly of spheres is proposed and developed. It i s based on a quasi-static advance of the meniscus with a piston-like motion and allows us to estimate the hydraulic equilibrium height, with values ve ry close to those obtained by fitting early time data to a Washburn equatio n. The change of regime is explained as a transition in the mechanism of ad vance of the meniscus. On the other hand, only the Washburn regime was obse rved for the sandstones. The front velocity was fitted to an algebraical fo rm with an exponent close to 0.5, a value expected from the asymptotic limi t of the Washburn equation. (C) 2001 Academic Press.