Displacement of Polymers in Waterflooded Porous Media and Its Effects on a Subsequent Micellar Flood
Authors: Dabbous Mahmoud K. 
Published: December 2007

Abstract
Injection of polymers in advance of a micellar fluid slug has been considered to improve reservoir volumetric sweep in a tertiary-mode micellar flood. An investigation was made of the injection of polyacrylamide-type polymers in waterflooded polyacrylamide-type polymers in waterflooded porous media and its effects on a subsequent porous media and its effects on a subsequent micellar flood. It was found that the presence of waterflood residual oil saturations in the porous medium increased the flow resistance and residual resistance factors (2- to 3.5-fold) compared with their corresponding values when the rock was free of residual oil. Inaccessible pore volume to polymer flow also appeared to be larger when waterflood residual oil saturations were present. These effects have been attributed to wettability and phase distribution of fluids in the porous medium. phase distribution of fluids in the porous medium. The study emphasized basic differences in the flow behavior of polymer injected ahead of a micellar slug (to improve sweep) and behind the micellar fluid (to control mobility). Both effects are for improved oil-recovery efficiency. Water mobility was greatly reduced following the displacement of polyacrylamide polymers in the waterflooded cores, yet mobility of the oil-water bank in a subsequent micellar flood was reduced to a lesser degree than the water bank. For a residual resistance factor to water ranging from 2 to 7, mobility control of a subsequent micellar flood could be achieved with a 22- to 39-percent increase in polymer concentration in the mobility buffer bank. This increase is in excess of the concentration required for a flood not preceded with polymer injection. Polymer preinjection had no adverse effects on oil displacement characteristics of the micellar fluid and appeared to reduce surfactant adsorption on the rock for the polymer-micellar system studied. Some experimental data indicated that the oil bank breaks through earlier and at a slightly higher oil cut in linear core floods. Such a result is theoretically feasible if the reduced-mobility water is not completely displaced at the front end (immiscible portion) of the oil-water bank. Oil-bank breakthrough probably would be delayed in the reservoir because of the action of the preinjected polymer to decrease the flow of fluids in polymer to decrease the flow of fluids in high-permeability zones.

Introduction
In a previous paper, preinjection of polymers in advance of a micellar slug was proposed as a means for improving reservoir volumetric sweep and oil recovery by a micellar flood. Increased flooding efficiency should result from reduced interwell permeability contrast in the reservoir following the polymer treatment. Preinjection of polymers also should result in better preflushing polymers also should result in better preflushing efficiency in displacing incompatible formation brines over "conventional" water preflushes. Thus, an improved oil-recovery method designed to increase reservoir volumetric sweep and miscibly recover tertiary oil consists ofpreinjection of a carefully designed slug of preinjection of a carefully designed slug of high-molecular-weight polyacrylamide polymers followed by a water-bank spacer to displace the polymer in the interwell area, andinjection of polymer in the interwell area, andinjection of a surfactant (micellar) slug followed by a polymer mobility buffer bank and chase water.
The fluid banks that are injected or developed during the process are illustrated in Fig. 1. Mixing and process are illustrated in Fig. 1. Mixing and interaction zones at fluid-bank boundaries are not shown in the schematic. The preinjection of a polymer is intended to rectify interwell permeability variation. The polymer is injected in reservoir rock that has waterflood residual oil saturations.
SPEJ
p. 358