Measurement of Capillary Pressures in Small Core Samples

Abstract An apparatus and method is described whereby the relation between saturationand capillary pressure under decreasing saturation can be quickly determinedfor small core samples. The initially saturated core is centrifuged atincreasing rates and the average saturation is measured at each rate with theaid of a stroboscope device. A theory and calculation procedure is givenwhereby the accelerations and saturation values can be converted into a truecurve of capillary pressure versus saturation. The present work differs from previously described use of the centrifuge toobtain capillary-pressure curves of core samples' in that the core iscentrifuged alone, so that the whole range of saturations required by theproperties of the sample and the radially varying centrifugal force occurswithin the sample. The calculation procedure nevertheless secures correctresults from simply obtained values of the average saturation. The apparatus described was developed in connection with studies of capillarypressure and wetting of oil-field rocks, and the paper provides the apparatusbackground for a previous publication dealing more fully with the uses andimplications of the data in petroleum engineering. The method is applicable, however, to general combinations of immiscible fluids and porous or comminutedsolids, and may be useful in other fields. Introduction The importance of capillary phenomena in determining the behavior of liquids inporous media has long been recognized. The concept of capillary pressure wasearly formulated, and in recent years discussions of the application of thelaws of capillarity to oil field problems have appeared. One reason whyprogress in the application of capillary phenomena to the solution of problemsof reservoir mechanics has not been more rapid has been the lack of asatisfactory technique for measuring the capillary pressures in reservoirrocks. It is the principal purpose of this paper to describe a method by whichthe relation between capillary pressure and saturation can be determined forsmall consolidated core samples which have been extracted andresaturated. Capillary Diaphragm Perhaps the most direct way to measure the capillary pressure in a rock is tomake piezometric contact with the wetting liquid through a diaphragm of porousmaterial with high displacement pressure saturated with the liquid in question.The liquid in the diaphragm assumes the pressure of the liquid in the core, which can then be measured with a manometer, while the atmospheric air isprevented by capillarity from entering the diaphragm and affecting the pressuremeasurements as long as the displacement pressure of the diaphragm is notexceeded. T.P. 1817