Oil-Water Displacements in Microscopic Capillaries

Methods previously developed for the study of liquid displacements in microscopic capillaries (inner diameters of 3 to 40 microns) have been used to investigate oil-water displacements in capillaries initially filled with water. Displacement calculations assuming perfect displacement and no capillary pressure hysteresis yielded oil effective viscosities smaller than the macroscopic viscosities. For a given liquid pair, the oil effective viscosity decreased both with decreasing capillary size and with increasing oil-water viscosity ratio. This behavior can be explained by the existence of an annular water film (20 Å to 260 Å thick) on the capillary wall. When the capillary was first filled with oil, the ratio of the oil effective viscosity to the normal oil viscosity was highest for the first water displacement and decreased with subsequent displacements. Sometimes the oil effective viscosity ratio during the initial water displacement was greater than unity. Introduction In a previous paper, a technique was described for studying air-liquid and liquid-liquid displacements in very small capillaries of uniform diameter, in the hope that such microscopic data would further the understanding of the nature of multiphase fluid flow through porous media. That paper contained comprehensive data for air-liquid displacements in Pyrex capillaries, with a few data for oil-water displacements in capillaries initially filled with water. The purpose of this paper is to present more complete results for oil-water displacements in capillaries initially filled with water, and to describe for the first time such observations in capillaries initially filled with oil. In this way the effect of the wetting history of the system upon the displacement process may be studied. Methods The basic techniques employed were described in the previous paper. The measuring procedure and the working equations will be briefly summarized here and a few modifications will be pointed out. For the present study, temperature control within 0.1 degrees C was obtained by placing the microscope and its immediate accessories in a thermostated air bath made of a steel frame covered with plexiglas.