Capillary Pressures - Their Measurement Using Mercury and the Calculation of Permeability Therefrom

Abstract An apparatus is described whereby capillary pressure curves for porous mediamay be determined by a technique that involves forcing mercury under pressureinto the evacuated pores of solids. The data so obtained are compared withcapillary pressure curves determined by the porous diaphragm method, and theadvantages of the mercury injection method are stated. Based upon a simplified working hypothesis, an equation is derived to showthe relationship of the permeability of a porous medium to its porosity andcapillary pressure curve, and experimental data are presented to support itsvalidity. A procedure is outlined whereby an estimate of the permeability of drillcuttings may be made with sufficient accuracy to meet most engineeringrequirements. Introduction The nature of capillary pressures and the role they play in reservoirbehavior have been lucidly discussed by Levrett, Hassler, Brunner, and Deahl, and others. As.a result of these publications the value of determiningcapillary pressure curves for cores has come to be generally recognized withinthe oil industry. While considerable attention has been directed toward thesubject in an effort to provide a reliable method of estimating percentages ofconnate water, it has been recognized that capillary pressure data may prove ofvalue in other equally important applications. This paper describes a method and procedure for determining capillarypressure curves for porous media wherein mercury is forced under pressure intothe evacuated pores of the solids. The pressure-volume relationships obtainedare reasonably similar to capillary pressure curves determined by the generallyaccepted porous diaphragm method. The advantages of the method lie in therapidity with which the experimental data can be obtained and in the fact thatsmall, irregularly shaped samples, e.g., drill cuttings, can be handled in thesame manner as larger pieces of regular shape such as cores or permeabilityplugs. Based upon a simplified working hypothesis, a theoretical equation will bederived which relates the capillary pressure curve to the porosity andpermeability of a porous solid, and experimental data will be presented tosupport its validity. This relationship applied to capillary pressure dataobtained for drill cuttings by the procedure described provides a means forpredicting the permeability of drill cuttings. T.P. 2544