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 is well fi tted by the Washburn equation. However in the experiments, the advancing fr ont exceeded the predicted equilibrium height. For large times, an algebrai c behavior of the velocity of the front is observed (T. Delker, D. Pengra a nd P. Wong, Phys. Rev. Lett. 76 (1996) 2902). A model to study the capillar y pressure evolution in a regular assembly of spheres is proposed and devel oped It is based on a quasi-static advance of the meniscus with a piston-li ke motion, and allows to estimate the hydraulic equilibrium height, with Va lues very close to those obtained by fitting early time data to a Washburn equation. The change of regime is explained as a transition in the mechanis m of advance of the meniscus. On the other hand, only the Washburn regime w as observed for the sandstones. The front velocity was fitted to an algebra ical form with an exponent close to 0.5, a value expected from the asymptot ic limit of the Washburn equation. (C) 2001 Elsevier Science B.V. All right s reserved.