Relationship of Trapped Oil Saturation to Petrophysical Properties of Porous Media
Authors: Stegemeier George L. 
Published: December 2007

This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., April 22–24, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
An "alternate path" theory is presented which quantitatively relates residual oil saturation to petrophysical properties over a broad range of varying interfacial and viscous conditions. The model allows for intermediate water wetting up to 90 degrees, and discontinuous trapped oil in single, or in interconnected pores. Basic petrophysical measurements, pores. Basic petrophysical measurements, including capillary pressure and relative permeabilities, provide functions related to permeabilities, provide functions related to microscopic pore dimensions. These functions are used to develop a relationship between trapped oil saturation and the displacement conditions of pressure gradient and interfacial tension.

Introduction
Determination of residual oil saturation has been the subject of a great quantity of experimental and some theoretical research for a number of years. Initially, knowledge of residual oil was needed principally for reservoir volumetric calculations in the determination of primary or waterflood recovery. With the advent of tertiary recovery methods, more accurate determination of the remaining oil saturation has become necessary for evaluation of processes which are initiated at low oil saturations. Finally, in the specific tertiary processes which alter interfacial tension or wettability, the conditions necessary for effective process design require an even more thorough understanding of the mechanisms that control release of residual oil.
Because of the complexity of the subject and the wide range of conditions found in various oil fields, a complete treatment of residual oil saturation is not possible. Individual aspects of the problem, including fluid properties, wettability, capillary pressures, and relative permeability have been pressures, and relative permeability have been extensively studied. Fluid properties such as density and viscosity are almost always precisely known. Interfacial tension is not precisely known. Interfacial tension is not as well defined, partly because of its lesser utility in reservoir calculations, and partly because of its greater sensitivity to change with pressure, temperature or minor contaminants. Methods of quantifying wettability have been developed which permit categorization of reservoirs in a fairly precise manner and these results provide insight into imbibition rates and relative permeability. In addition, capillary pressure methods have been used for calculation of pore size distribution and equilibrium oil saturation.