The Influence of Fluid Viscosity, Interfacial Tension, and Flow Velocity on Residual Oil Saturation Left by Waterflood

Abstract Results of short-core flow tests are presented for six different sandstones and one limestone to show the influence of interracial tension, fluid viscosity, and flow velocity on waterflood Sor. To make certain that oil trapping occurred in the presence of the test flood water, the floods were initiated at saturations at which the oil phase was continuous and in contact with the test flood water as the connote water. It was found for strongly water-wet cores (cos 1) that Sor could be described in terms of the Moore and Slobod dimensionless group expanded to include viscosity effects: Normal waterfloods are described by values of the group10 (–6). To reduce Sor, flood-water properties or test conditions had to be modified to increase the value of the group by 100 to 1, 000 times. These increases in the dimensionless group were obtained primarily with increases in flow velocity and water viscosity, although interfacial tensions as low as 1. 5 dynes/cm were used in some floods. Assuming that this dimensionless group will hold for other combinations of the variables, we can estimate reductions in residual oil saturation that can be achieved for selected values of these variables. Introduction This paper presents results of flow experiments on short, water-wet cores showing the influence of fluid viscosity, interfacial tension, and flow velocity on residual oil saturation left by waterflood. Waterfloods with normal water usually leave from one-third to one-half the oil initially present in a reservoir as immobile globules distributed through the pores of the rock. The above mentioned variables can be adjusted to obtain enhanced water, which results in a reduction of the effectiveness of the capillary trapping forces at the flood front. This leads to a reduction of the residual oil saturation left by the waterflood. An understanding of how the capillary trapping forces might be made less effective at the flood front was discussed in 1956 by Moore and Slobod. A basic requirement for the alteration of the capillary forces at the trapping front is that the enhanced water be in contact with the continuous oil at the time of trapping. This is the case investigated in the study. Such data are applicable, for example, to oil banks generated and reduced to residual in the presence of enhanced water. Moore and Slobod recognized the difference that may result from application of the enhanced water at the trapping front and its use to displace oil trapped by normal water. This latter case has been discussed in detail by Taber and Stegemeler and, because of the completeness of their treatments, is not examined further in this paper. The displacement of oil by water from a porous system is considered to be governed by a "competition between viscous and capillary forces. " To illustrate the role of this competition, Moore and Slobod analyzed the behavior of one pair, or doublet, of interconnected cylindrical capillaries with different radii, such as shown in Figs. 1a and 1b. The pore space in a natural rock consists of a three-dimensional network of volume elements with variations in the number of elements that join at each branch point, in the lengths of the elements, and in size and shape along each element. Because of the complexity of the porous network in natural rocks, the simple doublet model's value lies in showing qualitatively how the multiple interconnected paths of different size in a porous system cause oil to become trapped and as a means of obtaining a qualitative sense of how the amount of trapped oil may be expected to vary as the conditions of displacement are changed. SPEJ P. 437^