Hydraulic continuity of residual wetting phase in porous media

Abstract

This paper presents results of drainage tests in Berea sandstone, thin glass bead packs, and a two-dimensional glass capillary micromodel, obtained at vanishingly small capillary numbers, using oil and water as the two fluids. The primary objective of the present tests was to investigate the pattern of near-stationary residual wetting fluid saturations reached in Berea sandstone samples at various applied pressure differentials, using a porous plate at the exit face of the sample. Pressures ranging up to about 30 times the threshold pressure of penetration and times on the order of 5 weeks were used. The results do not indicate a lower limit of the wetting phase saturation. The lowest saturation reached is under 10% which is much lower than the “irreducible” saturations reported previously in sandstones. The near-stationary saturation reached under a certain applied pressure differential decreased abruptly when the pressure differential was increased, resulting in a lower near-stationary value after each pressure increase. The performance of the porous plate used in these tests set a limitation to the maximum pressure differential that could be used without running the risk of starting to produce also the displacing nonwetting fluid. Model experiments were performed with thin bead packs contained between two parallel glass plates in an attempt to visualize various displacement mechanisms of water from pore clusters that appear to have been completely surrounded by oil. In these experiments the islands of water which were completely surrounded by oil were displaced and produced through a continuum of oil. When oil breakthrough was prevented by the application of a capillary barrier at the exit, the displacement of water occurred close to the displacement pressure estimated for the island of beads containing the “trapped” water. Without the application of a capillary barrier, however, very much higher pressures and hundreds of pore volumes of throughput of the displacing fluid were necessary for displacement of the “trapped” water. It is believed that the mechanism of displacement of the wetting phase through a continuum of the nonwetting phase consists of transport of the wetting phase on the solid surface in the form of rivulets. Pore wedges or edges, as well as surface capillary grooves which are present on rough surfaces, have been shown to result in surface capillary transport of a wetting fluid. For example, surface capillary rise of wax was observed on ground glass surfaces.