Capillary Pressure Investigations

Abstract This paper presents results of static capillary pressure measurements madeboth by the restored-state and by the mercury-injection methods, and of dynamiccapillary pressure measurements made by the Hassler technique. The results indicate a variation from sample to sample of the factor used toconvert capillary pressure obtained by mercury injection to those applicable toa water-gas system. Static capillary pressures measured by the restored-statemethod were found to be in close agreement with dynamic capillary pressures, indicating that data obtained from static measurements may be utilized withconfidence in the solution of dynamic problems of fluid flow. Experimental data are presented on the use of "J-curves" for thecorrelation of capillary pressure-saturation relations for a number of coresfrom a particular geologic formation. It is shown that the correlation wasimproved in some cases by restricting it to certain lithologicclassifications. Introduction The Meaning and Importance of Capillary Pressure The coexistence of two or more immiscible fluids within the voids of aporous medium, such as reservoir rock, gives rise to capillary forces. Becauseof the interfacial tension existing at the boundary between two immisciblefluids in a pore space, the interface is curved, and there is a pressuredifference across the interface. This pressure difference is termed thecapillary pressure. The magnitude of the capillary pressure between two immiscible fluids whichtogether fill a porous medium depends upon several factors: the texturalproperties of the medium, the wettability of the medium, the interfacialtension between the fluids, the respective saturations of the fluids, and themanner in which these saturations are attained. For a given pair of immisciblefluids in a particular sample of reservoir rock, the capillary pressure is aunique function of the fluid saturation, provided that the saturation of thefluid which wets the rock has previously been decreased unidirectionally froman initially complete saturation. T.P. 3025