Further Experimental Validation Of The External-Drive Technique
Authors: Sarma Hemanta Kumar , Bentsen Ramon G. 
Published: March 2010

Abstract
When making a waterflood prediction, one of the more important properties needed is the water-oil relative permeability characteristics of the reservoir rock. Often the relative permeability correlation/or a particular rock-fluid system is estabilished on the basis of laboratory displacement studies. Moreover, the external drive technique is used frequently to obtain such relative permeability data. However, because the values of saturation and reciprocal mobilily ratio estimated using the external-drive method have never been verified experimentally there remains some uncertainty as to whether the relative permeability curves obtained using the external-drive technique are truly representative of the actual flow phenomena taking place within a core.
The purpose of this study was to validate further the external drive method. This was accomplished by measuring the saturation and pressure gradient at the outlet end of a core and comparing these values with those estimated using external-drive theory. Good agreement was found between the values determined directly and those predicted using the external-drive theory. Moreover, good agreement was found between the dynamic capillary pressure curve recorded during the course of an experiment and the equilibrium capillary pressure curve determined using the same sand-fluid system. Finally, the measured saturation profiles were in good agreement with those predicted using conventional Buckley-Leverett theory, provided the displacement was stable and stabilized.

Introduction
The water-oil relative permeability characteristics of a reservoir rock is one of the more important properties needed to undertake a waterflood prediction. Either steady-state or external-drive techniques maybe used to obtain such information. Because of long time needed to obtain stabilization when using steady-state methods, the external-drive (unsteady-state) methods are usually preferred. Moreover the unsteady-state methods are applicable to displacements where saturation gradients exist, while the steadystate methods are not.
The unsteady-state methods are based on Buckley-Leverett theory (l). As a consequence, displacements used to measure relative permeability mu.st be conducted in systems which are linear and homogeneous. Moreover, it must be permissible to neglect the capillary pressure gradient. In addition, the displacement must be stabilized and one-dimensional (pressure and saturation uniform in any cross section(2). Also, a Lagrangian formulation of the fluid displacement problem must be permissible. That is, it is important that the flow is neither unstabilized nor unstable, and that the saturation profiles are monotonic, when using external-drive methods(3).
In the past, it has been usual to validate the external-drive techniques by comparing the relative permeability curves obtained using these techniques with those obtained using steady-Slate methods(4.5). Good agreement has been found between the curves derived using these two approaches, provided the assumptions underlying the external-drive technique are well met. However, if one of the underlying assumptions is seriously violated, problems can arise. For example, if the sample used for the displacement tests is heterogeneous, the relative permeability curves obtained using the external-drive techniques can differ significantly from those obtained using other techniques(4,6).
Recently, a new approach to measuring relative permeability has been developed at the University of Alberta(7,8). As a consequence, it has become possible to verify more directly the assumptions and theory underlying the