Estimating Interstitial Water by the Capillary Pressure Method

Abstract

Abstract Results of the determination of the interstitial water content of several coresamples from oil reservoirs are presented. Data obtained by the capillarypressure method, which has recently been developed, are compared with watersaturations measured in cores cut with oil-base mud and with values calculatedfrom the electrical resistivity of the formation. The agreement between thecapillary pressure and other methods, combined with the fact that similaragreement has been obtained by others, indicates that the capillary methodproperly applied yields results sufficiently accurate for most engineeringpurposes. In some cases, it may be possible to extend the utility of dataobtained with the capillary pressure method by correlation of interstitialwater saturation with more easily measured physical properties of themedia. Introduction Several methods are in use for determining or for estimating the interstitialwater saturation at given points within oil and gas reservoirs. The directmeasurement of the water content of cores obtained with nonaqueous fluid in thehole is believed to be an accurate method under most conditions. The water'saturation may be calculated by use of the resistivity curves of the electriclog, 1 although this method is not applicable under conditions that exist insome reservoirs. Recently, a method has been developed and described whereinthe interstitial water saturation of cores is measured at a capillary pressuresuch that conditions existing in the reservoir are simulated. The purpose ofthis paper is to present data obtained by use of this latter method, and tocompare the results obtained with determinations made by other means. Method The theory underlying the capillary pressure method applied to petroleumreservoirs has been discussed adequately in the literature by Garrison, Leverett, and others. Briefly, and simply, capillary pressure is defined as thedifference in pressure between two contacting fluid phases, such as oil andwater. For the simplified sand, water and oil system illustrated in Fig I, thepressure difference can be stated as follows where p. is the capillary pressurein pounds per square inch, Y is the interfacial tension between oil and waterin pounds per inch, 7, and 72 are the radii of curvature of the interfacebetween oil and water in inches. Capillary pressure varies in a given reservoir with depth because of thegreater density of the water phase as compared with oil or gas. T.P. 2126