Pore Size Distribution of Petroleum Reservoir Rocks

Abstract

Abstract An investigation of equivalent pore entry radii in typical samples of petroleumreservoir rock and the pore volume associated with each value of pore entryradius has been made. Theoretical discussions together with experimentalprocedures for obtaining pore entry radii and the distribution of pore volumewith pore entry radii are presented. Experimental results on a number of coresamples, along with typical distribution curves, are shown. Data on the percent of the pore volume filled by mercury at a pressure of 1500 psi areincluded. The results of re-runs of samples, made possible by regenerativeapparatus, show the repeatability of the data and indicate the amount ofphysical change in the samples by mercury penetration. Theoretical equationsfor calculating permeability from pore size distribution data were derived andthe results of such calculations are compared with measured gas permeabilitydata. The effect of the shape, surface area, and weight of the sample on thepore size distribution of reservoir rocks was investigated experimentally.Mercury capillary pressure curves are compared with those obtained by theporous diaphragm method using gas to displace water. Introduction Standard core analysis techniques, in general, lead to quantities which arestatistical averages of the varying properties of the samples underexamination. Although such statistical information has real value in predictingthe gross performance of porous bodies, it fails to provide fundamentalinformation concerning the properties of the porous medium itself or anyprocesses which may be occurring in it. For example, a gas permeabilitymeasurement on a core sample is an indirect way of determining an average poreradius for that particular sample. Since there are many combinations of poreradii that will give the same radius, and, hence, the same permeability, noinformation is obtained on the pore size distribution. Furthermore, similarvalues in permeability do not imply similarity in other properties of a porousmedium. For instance, it is possible to have two core samples with identicalpermeability which would have different residual oil contents at the end of anair-oil drive, or even different amounts of interstitial water. It has been recognized for some time by the petroleum industry that adetermination of pore size distribution for porous reservoir rocks offeredpromise of increased understanding of fundamental flow processes in the porousmatrix, and therefore of petroleum reservoir performance in general. T.P. 2893